Merge remote-tracking branch 'edolstra/lazy-trees' into lazy-trees

configure.ac

@@ -177,13 +177,16 @@ fi
 PKG_CHECK_MODULES([OPENSSL], [libcrypto], [CXXFLAGS="$OPENSSL_CFLAGS $CXXFLAGS"])
 
 
-# Checks for libarchive
+# Look for libarchive.
 PKG_CHECK_MODULES([LIBARCHIVE], [libarchive >= 3.1.2], [CXXFLAGS="$LIBARCHIVE_CFLAGS $CXXFLAGS"])
 # Workaround until https://github.com/libarchive/libarchive/issues/1446 is fixed
 if test "$shared" != yes; then
     LIBARCHIVE_LIBS+=' -lz'
 fi
 
+# Look for libzip.
+PKG_CHECK_MODULES([LIBZIP], [libzip])
+
 # Look for SQLite, a required dependency.
 PKG_CHECK_MODULES([SQLITE3], [sqlite3 >= 3.6.19], [CXXFLAGS="$SQLITE3_CFLAGS $CXXFLAGS"])
 

doc/manual/generate-manpage.nix

@@ -1,7 +1,7 @@
 { toplevel }:
 
 with builtins;
-with import ./utils.nix;
+with import <nix/utils.nix>;
 
 let
 

doc/manual/local.mk

@@ -24,7 +24,7 @@ dummy-env = env -i \
 	NIX_STATE_DIR=/dummy \
 	NIX_CONFIG='cores = 0'
 
-nix-eval = $(dummy-env) $(bindir)/nix eval --experimental-features nix-command -I nix/corepkgs=corepkgs --store dummy:// --impure --raw
+nix-eval = $(dummy-env) $(bindir)/nix eval --experimental-features nix-command -I nix=doc/manual --store dummy:// --impure --raw
 
 $(d)/%.1: $(d)/src/command-ref/%.md
 	@printf "Title: %s\n\n" "$$(basename $@ .1)" > $^.tmp

doc/manual/src/command-ref/env-common.md

@@ -8,41 +8,10 @@ Most Nix commands interpret the following environment variables:
 
   - [`NIX_PATH`]{#env-NIX_PATH}\
     A colon-separated list of directories used to look up Nix
-    expressions enclosed in angle brackets (i.e., `<path>`). For
-    instance, the value
-
-        /home/eelco/Dev:/etc/nixos
-
-    will cause Nix to look for paths relative to `/home/eelco/Dev` and
-    `/etc/nixos`, in this order. It is also possible to match paths
-    against a prefix. For example, the value
-
-        nixpkgs=/home/eelco/Dev/nixpkgs-branch:/etc/nixos
-
-    will cause Nix to search for `<nixpkgs/path>` in
-    `/home/eelco/Dev/nixpkgs-branch/path` and `/etc/nixos/nixpkgs/path`.
-
-    If a path in the Nix search path starts with `http://` or
-    `https://`, it is interpreted as the URL of a tarball that will be
-    downloaded and unpacked to a temporary location. The tarball must
-    consist of a single top-level directory. For example, setting
-    `NIX_PATH` to
-
-        nixpkgs=https://github.com/NixOS/nixpkgs/archive/master.tar.gz
-
-    tells Nix to download and use the current contents of the
-    `master` branch in the `nixpkgs` repository.
-
-    The URLs of the tarballs from the official nixos.org channels (see
-    [the manual for `nix-channel`](nix-channel.md)) can be abbreviated
-    as `channel:<channel-name>`.  For instance, the following two
-    values of `NIX_PATH` are equivalent:
-
-        nixpkgs=channel:nixos-21.05
-        nixpkgs=https://nixos.org/channels/nixos-21.05/nixexprs.tar.xz
-
-    The Nix search path can also be extended using the `-I` option to
-    many Nix commands, which takes precedence over `NIX_PATH`.
+    expressions enclosed in angle brackets (i.e., `<path>`),
+     e.g. `/home/eelco/Dev:/etc/nixos`. It can be extended using the
+    `-I` option. For more information about the semantics of the Nix
+    search path, see the documentation for `-I`.
 
   - [`NIX_IGNORE_SYMLINK_STORE`]{#env-NIX_IGNORE_SYMLINK_STORE}\
     Normally, the Nix store directory (typically `/nix/store`) is not

doc/manual/src/release-notes/rl-next.md

@@ -5,3 +5,11 @@
   arguments will be ignored and the resulting derivation will have
   `__impure` set to `true`, making it an impure derivation.
 
+* You can now use flake references in the old CLI, e.g.
+
+  ```
+  # nix-build flake:nixpkgs -A hello
+  # nix-build -I nixpkgs=flake:github:NixOS/nixpkgs/nixos-22.05 \
+      '<nixpkgs>' -A hello
+  # NIX_PATH=nixpkgs=flake:nixpkgs nix-build '<nixpkgs>' -A hello
+  ```

flake.nix

@@ -112,6 +112,11 @@
             bzip2 xz brotli editline
             openssl sqlite
             libarchive
+            (libzip.overrideDerivation (old: {
+              # Temporary workaround for https://github.com/NixOS/nixpkgs/pull/178755
+              cmakeFlags = old.cmakeFlags or [] ++ [ "-DBUILD_REGRESS=0" ];
+              patches = [ ./libzip-unix-time.patch ];
+            }))
             boost
             lowdown-nix
             gtest

libzip-unix-time.patch

@@ -0,0 +1,19 @@
+commit 26e8c76ca84999fa5c0e46a9fc3aa7de80be2e9c
+Author: Eelco Dolstra <edolstra@gmail.com>
+Date:   Mon Oct 10 17:12:47 2022 +0200
+
+    Return time_t in the Unix epoch
+
+diff --git a/lib/zip_dirent.c b/lib/zip_dirent.c
+index 7fd2f7ce..5c050b4c 100644
+--- a/lib/zip_dirent.c
++++ b/lib/zip_dirent.c
+@@ -1018,7 +1018,7 @@ _zip_d2u_time(zip_uint16_t dtime, zip_uint16_t ddate) {
+     tm.tm_min = (dtime >> 5) & 63;
+     tm.tm_sec = (dtime << 1) & 62;
+ 
+-    return mktime(&tm);
++    return timegm(&tm);
+ }
+ 
+ 

src/libcmd/command.cc

@@ -208,8 +208,11 @@ void StorePathCommand::run(ref<Store> store, std::vector<StorePath> && storePath
     run(store, *storePaths.begin());
 }
 
-Strings editorFor(const Path & file, uint32_t line)
+Strings editorFor(const SourcePath & file, uint32_t line)
 {
+    auto path = file.getPhysicalPath();
+    if (!path)
+        throw Error("cannot open '%s' in an editor because it has no physical path", file);
     auto editor = getEnv("EDITOR").value_or("cat");
     auto args = tokenizeString<Strings>(editor);
     if (line > 0 && (
@@ -218,7 +221,7 @@ Strings editorFor(const Path & file, uint32_t line)
         editor.find("vim") != std::string::npos ||
         editor.find("kak") != std::string::npos))
         args.push_back(fmt("+%d", line));
-    args.push_back(file);
+    args.push_back(path->abs());
     return args;
 }
 

src/libcmd/command.hh

@@ -238,7 +238,7 @@ static RegisterCommand registerCommand2(std::vector<std::string> && name)
 
 /* Helper function to generate args that invoke $EDITOR on
    filename:lineno. */
-Strings editorFor(const Path & file, uint32_t line);
+Strings editorFor(const SourcePath & file, uint32_t line);
 
 struct MixProfile : virtual StoreCommand
 {

src/libcmd/common-eval-args.cc

@@ -8,6 +8,8 @@
 #include "flake/flakeref.hh"
 #include "store-api.hh"
 #include "command.hh"
+#include "fs-input-accessor.hh"
+#include "tarball.hh"
 
 namespace nix {
 
@@ -32,7 +34,68 @@ MixEvalArgs::MixEvalArgs()
     addFlag({
         .longName = "include",
         .shortName = 'I',
-        .description = "Add *path* to the list of locations used to look up `<...>` file names.",
+        .description = R"(
+  Add *path* to the Nix search path. The Nix search path is
+  initialized from the colon-separated `NIX_PATH` environment
+  variable, and is used to look up Nix expressions enclosed in angle
+  brackets (i.e., `<nixpkgs>`). For instance, if the Nix search path
+  consists of the entries
+
+  ```
+  /home/eelco/Dev
+  /etc/nixos
+  ```
+
+  Nix will look for paths relative to `/home/eelco/Dev` and
+  `/etc/nixos`, in this order. It is also possible to match paths
+  against a prefix. For example, the search path
+
+  ```
+  nixpkgs=/home/eelco/Dev/nixpkgs-branch
+  /etc/nixos
+  ```
+
+  will cause Nix to search for `<nixpkgs/path>` in
+  `/home/eelco/Dev/nixpkgs-branch/path` and `/etc/nixos/nixpkgs/path`.
+
+  If a path in the Nix search path starts with `http://` or `https://`,
+  it is interpreted as the URL of a tarball that will be downloaded and
+  unpacked to a temporary location. The tarball must consist of a single
+  top-level directory. For example, setting `NIX_PATH` to
+
+  ```
+  nixpkgs=https://github.com/NixOS/nixpkgs/archive/master.tar.gz
+  ```
+
+  tells Nix to download and use the current contents of the `master`
+  branch in the `nixpkgs` repository.
+
+  The URLs of the tarballs from the official `nixos.org` channels
+  (see [the manual page for `nix-channel`](nix-channel.md)) can be
+  abbreviated as `channel:<channel-name>`.  For instance, the
+  following two values of `NIX_PATH` are equivalent:
+
+  ```
+  nixpkgs=channel:nixos-21.05
+  nixpkgs=https://nixos.org/channels/nixos-21.05/nixexprs.tar.xz
+  ```
+
+  You can also use refer to source trees looked up in the flake
+  registry. For instance,
+
+  ```
+  nixpkgs=flake:nixpkgs
+  ```
+
+  specifies that the prefix `nixpkgs` shall refer to the source tree
+  downloaded from the `nixpkgs` entry in the flake registry. Similarly,
+
+  ```
+  nixpkgs=flake:github:NixOS/nixpkgs/nixos-22.05
+
+  makes `<nixpkgs>` refer to a particular branch of the
+  `NixOS/nixpkgs` repository on GitHub.
+  ```)",
         .category = category,
         .labels = {"path"},
         .handler = {[&](std::string s) { searchPath.push_back(s); }}
@@ -79,7 +142,7 @@ Bindings * MixEvalArgs::getAutoArgs(EvalState & state)
     for (auto & i : autoArgs) {
         auto v = state.allocValue();
         if (i.second[0] == 'E')
-            state.mkThunk_(*v, state.parseExprFromString(i.second.substr(1), absPath(".")));
+            state.mkThunk_(*v, state.parseExprFromString(i.second.substr(1), state.rootPath(absPath("."))));
         else
             v->mkString(((std::string_view) i.second).substr(1));
         res.insert(state.symbols.create(i.first), v);
@@ -87,17 +150,29 @@ Bindings * MixEvalArgs::getAutoArgs(EvalState & state)
     return res.finish();
 }
 
-Path lookupFileArg(EvalState & state, std::string_view s)
+SourcePath lookupFileArg(EvalState & state, std::string_view s)
 {
-    if (isUri(s)) {
-        return state.store->toRealPath(
-            fetchers::downloadTarball(
-                state.store, resolveUri(s), "source", false).first.storePath);
-    } else if (s.size() > 2 && s.at(0) == '<' && s.at(s.size() - 1) == '>') {
+    if (EvalSettings::isPseudoUrl(s)) {
+        auto storePath = fetchers::downloadTarball(
+            state.store, EvalSettings::resolvePseudoUrl(s), "source", false).first;
+        auto accessor = makeStorePathAccessor(state.store, storePath);
+        state.registerAccessor(accessor);
+        return accessor->root();
+    }
+
+    else if (hasPrefix(s, "flake:")) {
+        auto flakeRef = parseFlakeRef(std::string(s.substr(6)), {}, true, false);
+        auto [accessor, _] = flakeRef.resolve(state.store).lazyFetch(state.store);
+        return accessor->root();
+    }
+
+    else if (s.size() > 2 && s.at(0) == '<' && s.at(s.size() - 1) == '>') {
         Path p(s.substr(1, s.size() - 2));
         return state.findFile(p);
-    } else
-        return absPath(std::string(s));
+    }
+
+    else
+        return state.rootPath(absPath(std::string(s)));
 }
 
 }

src/libcmd/common-eval-args.hh

@@ -7,6 +7,7 @@ namespace nix {
 class Store;
 class EvalState;
 class Bindings;
+struct SourcePath;
 
 struct MixEvalArgs : virtual Args
 {
@@ -24,6 +25,6 @@ private:
     std::map<std::string, std::string> autoArgs;
 };
 
-Path lookupFileArg(EvalState & state, std::string_view s);
+SourcePath lookupFileArg(EvalState & state, std::string_view s);
 
 }

src/libcmd/installables.cc

@@ -275,9 +275,10 @@ void SourceExprCommand::completeInstallable(std::string_view prefix)
 
         evalSettings.pureEval = false;
         auto state = getEvalState();
-        Expr *e = state->parseExprFromFile(
-            resolveExprPath(state->checkSourcePath(lookupFileArg(*state, *file)))
-        );
+        Expr *e =
+            state->parseExprFromFile(
+                resolveExprPath(
+                    lookupFileArg(*state, *file)));
 
         Value root;
         state->eval(e, root);
@@ -635,10 +636,10 @@ ref<eval_cache::EvalCache> openEvalCache(
     EvalState & state,
     std::shared_ptr<flake::LockedFlake> lockedFlake)
 {
-    auto fingerprint = lockedFlake->getFingerprint();
+    auto fingerprint = lockedFlake->getFingerprint(state.store);
     return make_ref<nix::eval_cache::EvalCache>(
         evalSettings.useEvalCache && evalSettings.pureEval
-            ? std::optional { std::cref(fingerprint) }
+            ? fingerprint
             : std::nullopt,
         state,
         [&state, lockedFlake]()
@@ -885,10 +886,11 @@ std::vector<std::shared_ptr<Installable>> SourceExprCommand::parseInstallables(
         if (file == "-") {
             auto e = state->parseStdin();
             state->eval(e, *vFile);
-        } else if (file)
+        }
+        else if (file)
             state->evalFile(lookupFileArg(*state, *file), *vFile);
         else {
-            auto e = state->parseExprFromString(*expr, absPath("."));
+            auto e = state->parseExprFromString(*expr, state->rootPath(absPath(".")));
             state->eval(e, *vFile);
         }
 

src/libcmd/repl.cc

@@ -215,17 +215,15 @@ static std::ostream & showDebugTrace(std::ostream & out, const PosTable & positi
     out << dt.hint.str() << "\n";
 
     // prefer direct pos, but if noPos then try the expr.
-    auto pos = *dt.pos
-        ? *dt.pos
-        : positions[dt.expr.getPos() ? dt.expr.getPos() : noPos];
+    auto pos = dt.pos
+        ? dt.pos
+        : (std::shared_ptr<AbstractPos>) positions[dt.expr.getPos() ? dt.expr.getPos() : noPos];
 
     if (pos) {
-        printAtPos(pos, out);
-
-        auto loc = getCodeLines(pos);
-        if (loc.has_value()) {
+        out << pos;
+        if (auto loc = pos->getCodeLines()) {
             out << "\n";
-            printCodeLines(out, "", pos, *loc);
+            printCodeLines(out, "", *pos, *loc);
             out << "\n";
         }
     }
@@ -584,15 +582,17 @@ bool NixRepl::processLine(std::string line)
         Value v;
         evalString(arg, v);
 
-        const auto [file, line] = [&] () -> std::pair<std::string, uint32_t> {
+        const auto [path, line] = [&] () -> std::pair<SourcePath, uint32_t> {
             if (v.type() == nPath || v.type() == nString) {
                 PathSet context;
-                auto filename = state->coerceToString(noPos, v, context).toOwned();
-                state->symbols.create(filename);
-                return {filename, 0};
+                auto path = state->coerceToPath(noPos, v, context);
+                return {path, 0};
             } else if (v.isLambda()) {
                 auto pos = state->positions[v.lambda.fun->pos];
-                return {pos.file, pos.line};
+                if (auto path = std::get_if<SourcePath>(&pos.origin))
+                    return {*path, pos.line};
+                else
+                    throw Error("'%s' cannot be shown in an editor", pos);
             } else {
                 // assume it's a derivation
                 return findPackageFilename(*state, v, arg);
@@ -600,7 +600,7 @@ bool NixRepl::processLine(std::string line)
         }();
 
         // Open in EDITOR
-        auto args = editorFor(file, line);
+        auto args = editorFor(path, line);
         auto editor = args.front();
         args.pop_front();
 
@@ -782,7 +782,7 @@ void NixRepl::loadFlake(const std::string & flakeRefS)
             flake::LockFlags {
                 .updateLockFile = false,
                 .useRegistries = !evalSettings.pureEval,
-                .allowMutable  = !evalSettings.pureEval,
+                .allowUnlocked = !evalSettings.pureEval,
             }),
         v);
     addAttrsToScope(v);
@@ -859,7 +859,7 @@ void NixRepl::addVarToScope(const Symbol name, Value & v)
 
 Expr * NixRepl::parseString(std::string s)
 {
-    Expr * e = state->parseExprFromString(std::move(s), curDir, staticEnv);
+    Expr * e = state->parseExprFromString(std::move(s), state->rootPath(curDir), staticEnv);
     return e;
 }
 
@@ -917,7 +917,7 @@ std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int m
         break;
 
     case nPath:
-        str << ANSI_GREEN << v.path << ANSI_NORMAL; // !!! escaping?
+        str << ANSI_GREEN << v.path().to_string() << ANSI_NORMAL; // !!! escaping?
         break;
 
     case nNull:

src/libexpr/attr-path.cc

@@ -106,7 +106,7 @@ std::pair<Value *, PosIdx> findAlongAttrPath(EvalState & state, const std::strin
 }
 
 
-std::pair<std::string, uint32_t> findPackageFilename(EvalState & state, Value & v, std::string what)
+std::pair<SourcePath, uint32_t> findPackageFilename(EvalState & state, Value & v, std::string what)
 {
     Value * v2;
     try {
@@ -118,21 +118,25 @@ std::pair<std::string, uint32_t> findPackageFilename(EvalState & state, Value &
 
     // FIXME: is it possible to extract the Pos object instead of doing this
     //        toString + parsing?
-    auto pos = state.forceString(*v2);
+    PathSet context;
+    auto path = state.coerceToPath(noPos, *v2, context);
 
-    auto colon = pos.rfind(':');
-    if (colon == std::string::npos)
-        throw ParseError("cannot parse meta.position attribute '%s'", pos);
+    auto fn = path.path.abs();
+
+    auto fail = [fn]() {
+        throw ParseError("cannot parse 'meta.position' attribute '%s'", fn);
+    };
 
-    std::string filename(pos, 0, colon);
-    unsigned int lineno;
     try {
-        lineno = std::stoi(std::string(pos, colon + 1, std::string::npos));
+        auto colon = fn.rfind(':');
+        if (colon == std::string::npos) fail();
+        std::string filename(fn, 0, colon);
+        auto lineno = std::stoi(std::string(fn, colon + 1, std::string::npos));
+        return {SourcePath{path.accessor, CanonPath(fn.substr(0, colon))}, lineno};
     } catch (std::invalid_argument & e) {
-        throw ParseError("cannot parse line number '%s'", pos);
+        fail();
+        abort();
     }
-
-    return { std::move(filename), lineno };
 }
 
 

src/libexpr/attr-path.hh

@@ -17,7 +17,7 @@ std::pair<Value *, PosIdx> findAlongAttrPath(
     Value & vIn);
 
 /* Heuristic to find the filename and lineno or a nix value. */
-std::pair<std::string, uint32_t> findPackageFilename(EvalState & state, Value & v, std::string what);
+std::pair<SourcePath, uint32_t> findPackageFilename(EvalState & state, Value & v, std::string what);
 
 std::vector<Symbol> parseAttrPath(EvalState & state, std::string_view s);
 

src/libexpr/eval-cache.cc

@@ -442,8 +442,10 @@ Value & AttrCursor::forceValue()
         if (v.type() == nString)
             cachedValue = {root->db->setString(getKey(), v.string.s, v.string.context),
                            string_t{v.string.s, {}}};
-        else if (v.type() == nPath)
-            cachedValue = {root->db->setString(getKey(), v.path), string_t{v.path, {}}};
+        else if (v.type() == nPath) {
+            auto path = v.path().path;
+            cachedValue = {root->db->setString(getKey(), path.abs()), string_t{path.abs(), {}}};
+        }
         else if (v.type() == nBool)
             cachedValue = {root->db->setBool(getKey(), v.boolean), v.boolean};
         else if (v.type() == nInt)
@@ -580,7 +582,7 @@ std::string AttrCursor::getString()
     if (v.type() != nString && v.type() != nPath)
         root->state.debugThrowLastTrace(TypeError("'%s' is not a string but %s", getAttrPathStr(), showType(v.type())));
 
-    return v.type() == nString ? v.string.s : v.path;
+    return v.type() == nString ? v.string.s : v.path().to_string();
 }
 
 string_t AttrCursor::getStringWithContext()
@@ -611,7 +613,7 @@ string_t AttrCursor::getStringWithContext()
     if (v.type() == nString)
         return {v.string.s, v.getContext(*root->state.store)};
     else if (v.type() == nPath)
-        return {v.path, {}};
+        return {v.path().to_string(), {}};
     else
         root->state.debugThrowLastTrace(TypeError("'%s' is not a string but %s", getAttrPathStr(), showType(v.type())));
 }
@@ -645,17 +647,17 @@ NixInt AttrCursor::getInt()
             cachedValue = root->db->getAttr(getKey());
         if (cachedValue && !std::get_if<placeholder_t>(&cachedValue->second)) {
             if (auto i = std::get_if<int_t>(&cachedValue->second)) {
-                debug("using cached Integer attribute '%s'", getAttrPathStr());
+                debug("using cached integer attribute '%s'", getAttrPathStr());
                 return i->x;
             } else
-                throw TypeError("'%s' is not an Integer", getAttrPathStr());
+                throw TypeError("'%s' is not an integer", getAttrPathStr());
         }
     }
 
     auto & v = forceValue();
 
     if (v.type() != nInt)
-        throw TypeError("'%s' is not an Integer", getAttrPathStr());
+        throw TypeError("'%s' is not an integer", getAttrPathStr());
 
     return v.integer;
 }

src/libexpr/eval.cc

@@ -9,6 +9,7 @@
 #include "filetransfer.hh"
 #include "json.hh"
 #include "function-trace.hh"
+#include "fs-input-accessor.hh"
 
 #include <algorithm>
 #include <chrono>
@@ -120,7 +121,7 @@ void Value::print(const SymbolTable & symbols, std::ostream & str,
         str << "\"";
         break;
     case tPath:
-        str << path; // !!! escaping?
+        str << path().to_string(); // !!! escaping?
         break;
     case tNull:
         str << "null";
@@ -404,7 +405,8 @@ static Strings parseNixPath(const std::string & s)
         }
 
         if (*p == ':') {
-            if (isUri(std::string(start2, s.end()))) {
+            auto prefix = std::string(start2, s.end());
+            if (EvalSettings::isPseudoUrl(prefix) || hasPrefix(prefix, "flake:")) {
                 ++p;
                 while (p != s.end() && *p != ':') ++p;
             }
@@ -462,6 +464,28 @@ EvalState::EvalState(
     , sOutputSpecified(symbols.create("outputSpecified"))
     , repair(NoRepair)
     , emptyBindings(0)
+    , rootFS(
+        makeFSInputAccessor(
+            CanonPath::root,
+            evalSettings.restrictEval || evalSettings.pureEval
+            ? std::optional<std::set<CanonPath>>(std::set<CanonPath>())
+            : std::nullopt,
+            [](const CanonPath & path) -> RestrictedPathError {
+                auto modeInformation = evalSettings.pureEval
+                    ? "in pure eval mode (use '--impure' to override)"
+                    : "in restricted mode";
+                throw RestrictedPathError("access to absolute path '%1%' is forbidden %2%", path, modeInformation);
+            }))
+    , corepkgsFS(makeMemoryInputAccessor())
+    , internalFS(makeMemoryInputAccessor())
+    , derivationInternal{corepkgsFS->addFile(
+        CanonPath("derivation-internal.nix"),
+        #include "primops/derivation.nix.gen.hh"
+    )}
+    , callFlakeInternal{internalFS->addFile(
+        CanonPath("call-flake.nix"),
+        #include "flake/call-flake.nix.gen.hh"
+    )}
     , store(store)
     , buildStore(buildStore ? buildStore : store)
     , debugRepl(nullptr)
@@ -479,6 +503,9 @@ EvalState::EvalState(
     , baseEnv(allocEnv(128))
     , staticBaseEnv{std::make_shared<StaticEnv>(false, nullptr)}
 {
+    corepkgsFS->setPathDisplay("<nix", ">");
+    internalFS->setPathDisplay("«nix-internal»", "");
+
     countCalls = getEnv("NIX_COUNT_CALLS").value_or("0") != "0";
 
     assert(gcInitialised);
@@ -491,28 +518,15 @@ EvalState::EvalState(
         for (auto & i : evalSettings.nixPath.get()) addToSearchPath(i);
     }
 
-    if (evalSettings.restrictEval || evalSettings.pureEval) {
-        allowedPaths = PathSet();
+    /* Allow access to all paths in the search path. */
+    if (rootFS->hasAccessControl())
+        for (auto & i : searchPath)
+            resolveSearchPathElem(i, true);
 
-        for (auto & i : searchPath) {
-            auto r = resolveSearchPathElem(i);
-            if (!r.first) continue;
-
-            auto path = r.second;
-
-            if (store->isInStore(r.second)) {
-                try {
-                    StorePathSet closure;
-                    store->computeFSClosure(store->toStorePath(r.second).first, closure);
-                    for (auto & path : closure)
-                        allowPath(path);
-                } catch (InvalidPath &) {
-                    allowPath(r.second);
-                }
-            } else
-                allowPath(r.second);
-        }
-    }
+    corepkgsFS->addFile(
+        CanonPath("fetchurl.nix"),
+        #include "fetchurl.nix.gen.hh"
+    );
 
     createBaseEnv();
 }
@@ -525,14 +539,12 @@ EvalState::~EvalState()
 
 void EvalState::allowPath(const Path & path)
 {
-    if (allowedPaths)
-        allowedPaths->insert(path);
+    rootFS->allowPath(CanonPath(path));
 }
 
 void EvalState::allowPath(const StorePath & storePath)
 {
-    if (allowedPaths)
-        allowedPaths->insert(store->toRealPath(storePath));
+    rootFS->allowPath(CanonPath(store->toRealPath(storePath)));
 }
 
 void EvalState::allowAndSetStorePathString(const StorePath & storePath, Value & v)
@@ -543,52 +555,6 @@ void EvalState::allowAndSetStorePathString(const StorePath & storePath, Value &
     v.mkString(path, PathSet({path}));
 }
 
-Path EvalState::checkSourcePath(const Path & path_)
-{
-    if (!allowedPaths) return path_;
-
-    auto i = resolvedPaths.find(path_);
-    if (i != resolvedPaths.end())
-        return i->second;
-
-    bool found = false;
-
-    /* First canonicalize the path without symlinks, so we make sure an
-     * attacker can't append ../../... to a path that would be in allowedPaths
-     * and thus leak symlink targets.
-     */
-    Path abspath = canonPath(path_);
-
-    if (hasPrefix(abspath, corepkgsPrefix)) return abspath;
-
-    for (auto & i : *allowedPaths) {
-        if (isDirOrInDir(abspath, i)) {
-            found = true;
-            break;
-        }
-    }
-
-    if (!found) {
-        auto modeInformation = evalSettings.pureEval
-            ? "in pure eval mode (use '--impure' to override)"
-            : "in restricted mode";
-        throw RestrictedPathError("access to absolute path '%1%' is forbidden %2%", abspath, modeInformation);
-    }
-
-    /* Resolve symlinks. */
-    debug(format("checking access to '%s'") % abspath);
-    Path path = canonPath(abspath, true);
-
-    for (auto & i : *allowedPaths) {
-        if (isDirOrInDir(path, i)) {
-            resolvedPaths[path_] = path;
-            return path;
-        }
-    }
-
-    throw RestrictedPathError("access to canonical path '%1%' is forbidden in restricted mode", path);
-}
-
 
 void EvalState::checkURI(const std::string & uri)
 {
@@ -609,12 +575,12 @@ void EvalState::checkURI(const std::string & uri)
     /* If the URI is a path, then check it against allowedPaths as
        well. */
     if (hasPrefix(uri, "/")) {
-        checkSourcePath(uri);
+        rootFS->checkAllowed(CanonPath(uri));
         return;
     }
 
     if (hasPrefix(uri, "file://")) {
-        checkSourcePath(std::string(uri, 7));
+        rootFS->checkAllowed(CanonPath(uri.substr(7)));
         return;
     }
 
@@ -824,7 +790,7 @@ void EvalState::runDebugRepl(const Error * error, const Env & env, const Expr &
         ? std::make_unique<DebugTraceStacker>(
             *this,
             DebugTrace {
-                .pos = error->info().errPos ? *error->info().errPos : positions[expr.getPos()],
+                .pos = error->info().errPos ? error->info().errPos : (std::shared_ptr<AbstractPos>) positions[expr.getPos()],
                 .expr = expr,
                 .env = env,
                 .hint = error->info().msg,
@@ -884,7 +850,7 @@ void EvalState::throwEvalError(const PosIdx pos, const Suggestions & suggestions
     }), env, expr);
 }
 
-void EvalState::throwEvalError(const PosIdx pos, const char * s, const std::string & s2)
+void EvalState::throwEvalError(const PosIdx pos, const char * s, std::string_view s2)
 {
     debugThrowLastTrace(EvalError({
         .msg = hintfmt(s, s2),
@@ -1013,7 +979,7 @@ void EvalState::throwMissingArgumentError(const PosIdx pos, const char * s, cons
 
 void EvalState::addErrorTrace(Error & e, const char * s, const std::string & s2) const
 {
-    e.addTrace(std::nullopt, s, s2);
+    e.addTrace(nullptr, s, s2);
 }
 
 void EvalState::addErrorTrace(Error & e, const PosIdx pos, const char * s, const std::string & s2) const
@@ -1025,13 +991,13 @@ static std::unique_ptr<DebugTraceStacker> makeDebugTraceStacker(
     EvalState & state,
     Expr & expr,
     Env & env,
-    std::optional<ErrPos> pos,
+    std::shared_ptr<AbstractPos> && pos,
     const char * s,
     const std::string & s2)
 {
     return std::make_unique<DebugTraceStacker>(state,
         DebugTrace {
-            .pos = pos,
+            .pos = std::move(pos),
             .expr = expr,
             .env = env,
             .hint = hintfmt(s, s2),
@@ -1079,9 +1045,9 @@ void Value::mkStringMove(const char * s, const PathSet & context)
 }
 
 
-void Value::mkPath(std::string_view s)
+void Value::mkPath(const SourcePath & path)
 {
-    mkPath(makeImmutableString(s));
+    mkPath(&*path.accessor, makeImmutableString(path.path.abs()));
 }
 
 
@@ -1137,9 +1103,9 @@ void EvalState::mkThunk_(Value & v, Expr * expr)
 void EvalState::mkPos(Value & v, PosIdx p)
 {
     auto pos = positions[p];
-    if (!pos.file.empty()) {
+    if (auto path = std::get_if<SourcePath>(&pos.origin)) {
         auto attrs = buildBindings(3);
-        attrs.alloc(sFile).mkString(pos.file);
+        attrs.alloc(sFile).mkString(encodePath(*path));
         attrs.alloc(sLine).mkInt(pos.line);
         attrs.alloc(sColumn).mkInt(pos.column);
         v.mkAttrs(attrs);
@@ -1195,17 +1161,15 @@ Value * ExprPath::maybeThunk(EvalState & state, Env & env)
 }
 
 
-void EvalState::evalFile(const Path & path_, Value & v, bool mustBeTrivial)
+void EvalState::evalFile(const SourcePath & path, Value & v, bool mustBeTrivial)
 {
-    auto path = checkSourcePath(path_);
-
     FileEvalCache::iterator i;
     if ((i = fileEvalCache.find(path)) != fileEvalCache.end()) {
         v = i->second;
         return;
     }
 
-    Path resolvedPath = resolveExprPath(path);
+    auto resolvedPath = resolveExprPath(path);
     if ((i = fileEvalCache.find(resolvedPath)) != fileEvalCache.end()) {
         v = i->second;
         return;
@@ -1219,26 +1183,8 @@ void EvalState::evalFile(const Path & path_, Value & v, bool mustBeTrivial)
         e = j->second;
 
     if (!e)
-        e = parseExprFromFile(checkSourcePath(resolvedPath));
+        e = parseExprFromFile(resolvedPath);
 
-    cacheFile(path, resolvedPath, e, v, mustBeTrivial);
-}
-
-
-void EvalState::resetFileCache()
-{
-    fileEvalCache.clear();
-    fileParseCache.clear();
-}
-
-
-void EvalState::cacheFile(
-    const Path & path,
-    const Path & resolvedPath,
-    Expr * e,
-    Value & v,
-    bool mustBeTrivial)
-{
     fileParseCache[resolvedPath] = e;
 
     try {
@@ -1247,8 +1193,8 @@ void EvalState::cacheFile(
                 *this,
                 *e,
                 this->baseEnv,
-                e->getPos() ? std::optional(ErrPos(positions[e->getPos()])) : std::nullopt,
-                "while evaluating the file '%1%':", resolvedPath)
+                e->getPos() ? (std::shared_ptr<AbstractPos>) positions[e->getPos()] : nullptr,
+                "while evaluating the file '%1%':", resolvedPath.to_string())
             : nullptr;
 
         // Enforce that 'flake.nix' is a direct attrset, not a
@@ -1258,7 +1204,7 @@ void EvalState::cacheFile(
             throw EvalError("file '%s' must be an attribute set", path);
         eval(e, v);
     } catch (Error & e) {
-        addErrorTrace(e, "while evaluating the file '%1%':", resolvedPath);
+        addErrorTrace(e, "while evaluating the file '%1%':", resolvedPath.to_string());
         throw;
     }
 
@@ -1267,6 +1213,13 @@ void EvalState::cacheFile(
 }
 
 
+void EvalState::resetFileCache()
+{
+    fileEvalCache.clear();
+    fileParseCache.clear();
+}
+
+
 void EvalState::eval(Expr * e, Value & v)
 {
     e->eval(*this, baseEnv, v);
@@ -1518,10 +1471,13 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
         state.forceValue(*vAttrs, (pos2 ? pos2 : this->pos ) );
 
     } catch (Error & e) {
+        if (pos2) {
             auto pos2r = state.positions[pos2];
-        if (pos2 && pos2r.file != state.derivationNixPath)
+            auto origin = std::get_if<SourcePath>(&pos2r.origin);
+            if (!(origin && *origin == state.derivationInternal))
                 state.addErrorTrace(e, pos2, "while evaluating the attribute '%1%'",
                     showAttrPath(state, env, attrPath));
+        }
         throw;
     }
 
@@ -1661,7 +1617,8 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                         (lambda.name
                             ? concatStrings("'", symbols[lambda.name], "'")
                             : "anonymous lambda"));
-                    addErrorTrace(e, pos, "from call site%s", "");
+                    if (pos != noPos)
+                        addErrorTrace(e, pos, "from call site", "");
                 }
                 throw;
             }
@@ -1996,42 +1953,58 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
 
     Value values[es->size()];
     Value * vTmpP = values;
+    std::shared_ptr<InputAccessor> accessor;
 
     for (auto & [i_pos, i] : *es) {
-        Value & vTmp = *vTmpP++;
-        i->eval(state, env, vTmp);
+        Value * vTmp = vTmpP++;
+        i->eval(state, env, *vTmp);
+
+        if (vTmp->type() == nAttrs) {
+            auto j = vTmp->attrs->find(state.sOutPath);
+            if (j != vTmp->attrs->end())
+                vTmp = j->value;
+        }
 
         /* If the first element is a path, then the result will also
            be a path, we don't copy anything (yet - that's done later,
            since paths are copied when they are used in a derivation),
            and none of the strings are allowed to have contexts. */
         if (first) {
-            firstType = vTmp.type();
+            firstType = vTmp->type();
+            if (vTmp->type() == nPath) {
+                accessor = vTmp->path().accessor;
+                auto part = vTmp->path().path.abs();
+                sSize += part.size();
+                s.emplace_back(std::move(part));
+            }
         }
 
         if (firstType == nInt) {
-            if (vTmp.type() == nInt) {
-                n += vTmp.integer;
-            } else if (vTmp.type() == nFloat) {
+            if (vTmp->type() == nInt) {
+                n += vTmp->integer;
+            } else if (vTmp->type() == nFloat) {
                 // Upgrade the type from int to float;
                 firstType = nFloat;
                 nf = n;
-                nf += vTmp.fpoint;
+                nf += vTmp->fpoint;
             } else
-                state.throwEvalError(i_pos, "cannot add %1% to an integer", showType(vTmp), env, *this);
+                state.throwEvalError(i_pos, "cannot add %1% to an integer", showType(*vTmp), env, *this);
         } else if (firstType == nFloat) {
-            if (vTmp.type() == nInt) {
-                nf += vTmp.integer;
-            } else if (vTmp.type() == nFloat) {
-                nf += vTmp.fpoint;
+            if (vTmp->type() == nInt) {
+                nf += vTmp->integer;
+            } else if (vTmp->type() == nFloat) {
+                nf += vTmp->fpoint;
             } else
-                state.throwEvalError(i_pos, "cannot add %1% to a float", showType(vTmp), env, *this);
+                state.throwEvalError(i_pos, "cannot add %1% to a float", showType(*vTmp), env, *this);
+        } else if (firstType == nPath) {
+            if (!first) {
+                auto part = state.coerceToString(i_pos, *vTmp, context, false, false);
+                sSize += part->size();
+                s.emplace_back(std::move(part));
+            }
         } else {
             if (s.empty()) s.reserve(es->size());
-            /* skip canonization of first path, which would only be not
-            canonized in the first place if it's coming from a ./${foo} type
-            path */
-            auto part = state.coerceToString(i_pos, vTmp, context, false, firstType == nString, !first);
+            auto part = state.coerceToString(i_pos, *vTmp, context, false, firstType == nString);
             sSize += part->size();
             s.emplace_back(std::move(part));
         }
@@ -2046,7 +2019,7 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
     else if (firstType == nPath) {
         if (!context.empty())
             state.throwEvalError(pos, "a string that refers to a store path cannot be appended to a path", env, *this);
-        v.mkPath(canonPath(str()));
+        v.mkPath({ref(accessor), CanonPath(str())});
     } else
         v.mkStringMove(c_str(), context);
 }
@@ -2237,7 +2210,7 @@ std::optional<std::string> EvalState::tryAttrsToString(const PosIdx pos, Value &
 }
 
 BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet & context,
-    bool coerceMore, bool copyToStore, bool canonicalizePath)
+    bool coerceMore, bool copyToStore)
 {
     forceValue(v, pos);
 
@@ -2247,12 +2220,10 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet
     }
 
     if (v.type() == nPath) {
-        BackedStringView path(PathView(v.path));
-        if (canonicalizePath)
-            path = canonPath(*path);
-        if (copyToStore)
-            path = copyPathToStore(context, std::move(path).toOwned());
-        return path;
+        auto path = v.path();
+        return copyToStore
+            ? store->printStorePath(copyPathToStore(context, path))
+            : encodePath(path);
     }
 
     if (v.type() == nAttrs) {
@@ -2294,36 +2265,47 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet
 }
 
 
-std::string EvalState::copyPathToStore(PathSet & context, const Path & path)
+StorePath EvalState::copyPathToStore(PathSet & context, const SourcePath & path)
 {
-    if (nix::isDerivation(path))
-        throwEvalError("file names are not allowed to end in '%1%'", drvExtension);
+    if (nix::isDerivation(path.path.abs()))
+        throw EvalError("file names are not allowed to end in '%s'", drvExtension);
 
-    Path dstPath;
     auto i = srcToStore.find(path);
-    if (i != srcToStore.end())
-        dstPath = store->printStorePath(i->second);
-    else {
-        auto p = settings.readOnlyMode
-            ? store->computeStorePathForPath(std::string(baseNameOf(path)), checkSourcePath(path)).first
-            : store->addToStore(std::string(baseNameOf(path)), checkSourcePath(path), FileIngestionMethod::Recursive, htSHA256, defaultPathFilter, repair);
-        dstPath = store->printStorePath(p);
-        allowPath(p);
-        srcToStore.insert_or_assign(path, std::move(p));
-        printMsg(lvlChatty, "copied source '%1%' -> '%2%'", path, dstPath);
-    }
 
-    context.insert(dstPath);
+    auto dstPath = i != srcToStore.end()
+        ? i->second
+        : [&]() {
+            auto dstPath = path.fetchToStore(store, path.baseName(), nullptr, repair);
+            allowPath(dstPath);
+            srcToStore.insert_or_assign(path, dstPath);
+            printMsg(lvlChatty, "copied source '%1%' -> '%2%'", path, store->printStorePath(dstPath));
+            return dstPath;
+        }();
+
+    context.insert(store->printStorePath(dstPath));
     return dstPath;
 }
 
 
-Path EvalState::coerceToPath(const PosIdx pos, Value & v, PathSet & context)
+SourcePath EvalState::coerceToPath(const PosIdx pos, Value & v, PathSet & context)
 {
-    auto path = coerceToString(pos, v, context, false, false).toOwned();
-    if (path == "" || path[0] != '/')
-        throwEvalError(pos, "string '%1%' doesn't represent an absolute path", path);
-    return path;
+    forceValue(v, pos);
+
+    if (v.type() == nString) {
+        copyContext(v, context);
+        return decodePath(v.str(), pos);
+    }
+
+    if (v.type() == nPath)
+        return v.path();
+
+    if (v.type() == nAttrs) {
+        auto i = v.attrs->find(sOutPath);
+        if (i != v.attrs->end())
+            return coerceToPath(pos, *i->value, context);
+    }
+
+    throwTypeError(pos, "cannot coerce %1% to a path", v);
 }
 
 
@@ -2370,7 +2352,9 @@ bool EvalState::eqValues(Value & v1, Value & v2)
             return strcmp(v1.string.s, v2.string.s) == 0;
 
         case nPath:
-            return strcmp(v1.path, v2.path) == 0;
+            return
+                v1._path.accessor == v2._path.accessor
+                && strcmp(v1._path.path, v2._path.path) == 0;
 
         case nNull:
             return true;
@@ -2508,7 +2492,8 @@ void EvalState::printStats()
                     else
                         obj.attr("name", nullptr);
                     if (auto pos = positions[fun->pos]) {
-                        obj.attr("file", (std::string_view) pos.file);
+                        if (auto path = std::get_if<SourcePath>(&pos.origin))
+                            obj.attr("file", path->to_string());
                         obj.attr("line", pos.line);
                         obj.attr("column", pos.column);
                     }
@@ -2520,7 +2505,8 @@ void EvalState::printStats()
                 for (auto & i : attrSelects) {
                     auto obj = list.object();
                     if (auto pos = positions[i.first]) {
-                        obj.attr("file", (const std::string &) pos.file);
+                        if (auto path = std::get_if<SourcePath>(&pos.origin))
+                            obj.attr("file", path->to_string());
                         obj.attr("line", pos.line);
                         obj.attr("column", pos.column);
                     }
@@ -2585,6 +2571,23 @@ Strings EvalSettings::getDefaultNixPath()
     return res;
 }
 
+bool EvalSettings::isPseudoUrl(std::string_view s)
+{
+    if (s.compare(0, 8, "channel:") == 0) return true;
+    size_t pos = s.find("://");
+    if (pos == std::string::npos) return false;
+    std::string scheme(s, 0, pos);
+    return scheme == "http" || scheme == "https" || scheme == "file" || scheme == "channel" || scheme == "git" || scheme == "s3" || scheme == "ssh";
+}
+
+std::string EvalSettings::resolvePseudoUrl(std::string_view url)
+{
+    if (hasPrefix(url, "channel:"))
+        return "https://nixos.org/channels/" + std::string(url.substr(8)) + "/nixexprs.tar.xz";
+    else
+        return std::string(url);
+}
+
 EvalSettings evalSettings;
 
 static GlobalConfig::Register rEvalSettings(&evalSettings);

src/libexpr/eval.hh

@@ -7,6 +7,7 @@
 #include "symbol-table.hh"
 #include "config.hh"
 #include "experimental-features.hh"
+#include "input-accessor.hh"
 
 #include <map>
 #include <optional>
@@ -19,7 +20,9 @@ namespace nix {
 class Store;
 class EvalState;
 class StorePath;
+struct SourcePath;
 enum RepairFlag : bool;
+struct FSInputAccessor;
 
 
 typedef void (* PrimOpFun) (EvalState & state, const PosIdx pos, Value * * args, Value & v);
@@ -55,16 +58,12 @@ std::unique_ptr<ValMap> mapStaticEnvBindings(const SymbolTable & st, const Stati
 void copyContext(const Value & v, PathSet & context);
 
 
-/* Cache for calls to addToStore(); maps source paths to the store
-   paths. */
-typedef std::map<Path, StorePath> SrcToStore;
-
-
 std::ostream & printValue(const EvalState & state, std::ostream & str, const Value & v);
 std::string printValue(const EvalState & state, const Value & v);
 std::ostream & operator << (std::ostream & os, const ValueType t);
 
 
+// FIXME: maybe change this to an std::variant<SourcePath, URL>.
 typedef std::pair<std::string, std::string> SearchPathElem;
 typedef std::list<SearchPathElem> SearchPath;
 
@@ -78,7 +77,7 @@ struct RegexCache;
 std::shared_ptr<RegexCache> makeRegexCache();
 
 struct DebugTrace {
-    std::optional<ErrPos> pos;
+    std::shared_ptr<AbstractPos> pos;
     const Expr & expr;
     const Env & env;
     hintformat hint;
@@ -93,8 +92,6 @@ public:
     SymbolTable symbols;
     PosTable positions;
 
-    static inline std::string derivationNixPath = "//builtin/derivation.nix";
-
     const Symbol sWith, sOutPath, sDrvPath, sType, sMeta, sName, sValue,
         sSystem, sOverrides, sOutputs, sOutputName, sIgnoreNulls,
         sFile, sLine, sColumn, sFunctor, sToString,
@@ -105,25 +102,31 @@ public:
         sDescription, sSelf, sEpsilon, sStartSet, sOperator, sKey, sPath,
         sPrefix,
         sOutputSpecified;
-    Symbol sDerivationNix;
 
     /* If set, force copying files to the Nix store even if they
        already exist there. */
     RepairFlag repair;
 
-    /* The allowed filesystem paths in restricted or pure evaluation
-       mode. */
-    std::optional<PathSet> allowedPaths;
-
     Bindings emptyBindings;
 
+    const ref<FSInputAccessor> rootFS;
+    const ref<MemoryInputAccessor> corepkgsFS;
+    const ref<MemoryInputAccessor> internalFS;
+
+    const SourcePath derivationInternal;
+
+    const SourcePath callFlakeInternal;
+
+    /* A map keyed by InputAccessor::number that keeps input accessors
+       alive. */
+    std::unordered_map<size_t, ref<InputAccessor>> inputAccessors;
+
     /* Store used to materialise .drv files. */
     const ref<Store> store;
 
     /* Store used to build stuff. */
     const ref<Store> buildStore;
 
-    RootValue vCallFlake = nullptr;
     RootValue vImportedDrvToDerivation = nullptr;
 
     /* Debugger */
@@ -171,30 +174,30 @@ public:
 
 
 private:
-    SrcToStore srcToStore;
+
+    /* Cache for calls to addToStore(); maps source paths to the store
+       paths. */
+    std::map<SourcePath, StorePath> srcToStore;
 
     /* A cache from path names to parse trees. */
 #if HAVE_BOEHMGC
-    typedef std::map<Path, Expr *, std::less<Path>, traceable_allocator<std::pair<const Path, Expr *>>> FileParseCache;
+    typedef std::map<SourcePath, Expr *, std::less<SourcePath>, traceable_allocator<std::pair<const SourcePath, Expr *>>> FileParseCache;
 #else
-    typedef std::map<Path, Expr *> FileParseCache;
+    typedef std::map<SourcePath, Expr *> FileParseCache;
 #endif
     FileParseCache fileParseCache;
 
     /* A cache from path names to values. */
 #if HAVE_BOEHMGC
-    typedef std::map<Path, Value, std::less<Path>, traceable_allocator<std::pair<const Path, Value>>> FileEvalCache;
+    typedef std::map<SourcePath, Value, std::less<SourcePath>, traceable_allocator<std::pair<const SourcePath, Value>>> FileEvalCache;
 #else
-    typedef std::map<Path, Value> FileEvalCache;
+    typedef std::map<SourcePath, Value> FileEvalCache;
 #endif
     FileEvalCache fileEvalCache;
 
     SearchPath searchPath;
 
-    std::map<std::string, std::pair<bool, std::string>> searchPathResolved;
-
-    /* Cache used by checkSourcePath(). */
-    std::unordered_map<Path, Path> resolvedPaths;
+    std::map<std::string, std::optional<SourcePath>> searchPathResolved;
 
     /* Cache used by prim_match(). */
     std::shared_ptr<RegexCache> regexCache;
@@ -219,6 +222,22 @@ public:
 
     SearchPath getSearchPath() { return searchPath; }
 
+    SourcePath rootPath(const Path & path);
+
+    void registerAccessor(ref<InputAccessor> accessor);
+
+    /* Convert a path to a string representation of the format
+       `/__virtual__/<accessor-number>/<path>`. */
+    std::string encodePath(const SourcePath & path);
+
+    /* Decode a path encoded by `encodePath()`. */
+    SourcePath decodePath(std::string_view s, PosIdx pos = noPos);
+
+    /* Decode all virtual paths in a string, i.e. all
+       /__virtual__/... substrings are replaced by the corresponding
+       input accessor. */
+    std::string decodePaths(std::string_view s);
+
     /* Allow access to a path. */
     void allowPath(const Path & path);
 
@@ -229,10 +248,6 @@ public:
     /* Allow access to a store path and return it as a string. */
     void allowAndSetStorePathString(const StorePath & storePath, Value & v);
 
-    /* Check whether access to a path is allowed and throw an error if
-       not. Otherwise return the canonicalised path. */
-    Path checkSourcePath(const Path & path);
-
     void checkURI(const std::string & uri);
 
     /* When using a diverted store and 'path' is in the Nix store, map
@@ -245,36 +260,30 @@ public:
     Path toRealPath(const Path & path, const PathSet & context);
 
     /* Parse a Nix expression from the specified file. */
-    Expr * parseExprFromFile(const Path & path);
-    Expr * parseExprFromFile(const Path & path, std::shared_ptr<StaticEnv> & staticEnv);
+    Expr * parseExprFromFile(const SourcePath & path);
+    Expr * parseExprFromFile(const SourcePath & path, std::shared_ptr<StaticEnv> & staticEnv);
 
     /* Parse a Nix expression from the specified string. */
-    Expr * parseExprFromString(std::string s, const Path & basePath, std::shared_ptr<StaticEnv> & staticEnv);
-    Expr * parseExprFromString(std::string s, const Path & basePath);
+    Expr * parseExprFromString(std::string s, const SourcePath & basePath, std::shared_ptr<StaticEnv> & staticEnv);
+    Expr * parseExprFromString(std::string s, const SourcePath & basePath);
 
     Expr * parseStdin();
 
     /* Evaluate an expression read from the given file to normal
        form. Optionally enforce that the top-level expression is
        trivial (i.e. doesn't require arbitrary computation). */
-    void evalFile(const Path & path, Value & v, bool mustBeTrivial = false);
-
-    /* Like `evalFile`, but with an already parsed expression. */
-    void cacheFile(
-        const Path & path,
-        const Path & resolvedPath,
-        Expr * e,
-        Value & v,
-        bool mustBeTrivial = false);
+    void evalFile(const SourcePath & path, Value & v, bool mustBeTrivial = false);
 
     void resetFileCache();
 
     /* Look up a file in the search path. */
-    Path findFile(const std::string_view path);
-    Path findFile(SearchPath & searchPath, const std::string_view path, const PosIdx pos = noPos);
+    SourcePath findFile(const std::string_view path);
+    SourcePath findFile(SearchPath & searchPath, const std::string_view path, const PosIdx pos = noPos);
 
     /* If the specified search path element is a URI, download it. */
-    std::pair<bool, std::string> resolveSearchPathElem(const SearchPathElem & elem);
+    std::optional<SourcePath> resolveSearchPathElem(
+        const SearchPathElem & elem,
+        bool initAccessControl = false);
 
     /* Evaluate an expression to normal form, storing the result in
        value `v'. */
@@ -323,7 +332,7 @@ public:
     [[gnu::noinline, gnu::noreturn]]
     void throwEvalError(const char * s, const std::string & s2);
     [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const char * s, const std::string & s2);
+    void throwEvalError(const PosIdx pos, const char * s, std::string_view s2);
     [[gnu::noinline, gnu::noreturn]]
     void throwEvalError(const char * s, const std::string & s2,
         Env & env, Expr & expr);
@@ -334,6 +343,8 @@ public:
     void throwEvalError(const char * s, const std::string & s2, const std::string & s3,
         Env & env, Expr & expr);
     [[gnu::noinline, gnu::noreturn]]
+    void throwEvalError(const PosIdx pos, const char * s, std::string_view s2) const;
+    [[gnu::noinline, gnu::noreturn]]
     void throwEvalError(const PosIdx pos, const char * s, const std::string & s2, const std::string & s3,
         Env & env, Expr & expr);
     [[gnu::noinline, gnu::noreturn]]
@@ -397,15 +408,14 @@ public:
        booleans and lists to a string.  If `copyToStore' is set,
        referenced paths are copied to the Nix store as a side effect. */
     BackedStringView coerceToString(const PosIdx pos, Value & v, PathSet & context,
-        bool coerceMore = false, bool copyToStore = true,
-        bool canonicalizePath = true);
+        bool coerceMore = false, bool copyToStore = true);
 
-    std::string copyPathToStore(PathSet & context, const Path & path);
+    StorePath copyPathToStore(PathSet & context, const SourcePath & path);
 
     /* Path coercion.  Converts strings, paths and derivations to a
        path.  The result is guaranteed to be a canonicalised, absolute
        path.  Nothing is copied to the store. */
-    Path coerceToPath(const PosIdx pos, Value & v, PathSet & context);
+    SourcePath coerceToPath(const PosIdx pos, Value & v, PathSet & context);
 
     /* Like coerceToPath, but the result must be a store path. */
     StorePath coerceToStorePath(const PosIdx pos, Value & v, PathSet & context);
@@ -457,8 +467,12 @@ private:
     friend struct ExprAttrs;
     friend struct ExprLet;
 
-    Expr * parse(char * text, size_t length, FileOrigin origin, const PathView path,
-        const PathView basePath, std::shared_ptr<StaticEnv> & staticEnv);
+    Expr * parse(
+        char * text,
+        size_t length,
+        Pos::Origin origin,
+        const SourcePath & basePath,
+        std::shared_ptr<StaticEnv> & staticEnv);
 
 public:
 
@@ -573,7 +587,7 @@ std::string showType(const Value & v);
 NixStringContextElem decodeContext(const Store & store, std::string_view s);
 
 /* If `path' refers to a directory, then append "/default.nix". */
-Path resolveExprPath(Path path);
+SourcePath resolveExprPath(const SourcePath & path);
 
 struct InvalidPathError : EvalError
 {
@@ -590,6 +604,10 @@ struct EvalSettings : Config
 
     static Strings getDefaultNixPath();
 
+    static bool isPseudoUrl(std::string_view s);
+
+    static std::string resolvePseudoUrl(std::string_view url);
+
     Setting<bool> enableNativeCode{this, false, "allow-unsafe-native-code-during-evaluation",
         "Whether builtin functions that allow executing native code should be enabled."};
 
@@ -660,8 +678,6 @@ struct EvalSettings : Config
 
 extern EvalSettings evalSettings;
 
-static const std::string corepkgsPrefix{"/__corepkgs__/"};
-
 }
 
 #include "eval-inline.hh"

src/libexpr/flake/call-flake.nix

@@ -1,27 +1,19 @@
-lockFileStr: rootSrc: rootSubdir:
+# This is a helper to callFlake() to lazily fetch flake inputs.
+
+# The contents of the lock file, in JSON format.
+lockFileStr:
+
+# A mapping of lock file node IDs to { sourceInfo, subdir } attrsets,
+# with sourceInfo.outPath providing an InputAccessor to a previously
+# fetched tree. This is necessary for possibly unlocked inputs, in
+# particular the root input, but also --override-inputs pointing to
+# unlocked trees.
+overrides:
 
 let
 
   lockFile = builtins.fromJSON lockFileStr;
 
-  allNodes =
-    builtins.mapAttrs
-      (key: node:
-        let
-
-          sourceInfo =
-            if key == lockFile.root
-            then rootSrc
-            else fetchTree (node.info or {} // removeAttrs node.locked ["dir"]);
-
-          subdir = if key == lockFile.root then rootSubdir else node.locked.dir or "";
-
-          flake = import (sourceInfo + (if subdir != "" then "/" else "") + subdir + "/flake.nix");
-
-          inputs = builtins.mapAttrs
-            (inputName: inputSpec: allNodes.${resolveInput inputSpec})
-            (node.inputs or {});
-
   # Resolve a input spec into a node name. An input spec is
   # either a node name, or a 'follows' path from the root
   # node.
@@ -41,6 +33,35 @@ let
         (resolveInput lockFile.nodes.${nodeName}.inputs.${builtins.head path})
         (builtins.tail path);
 
+  allNodes =
+    builtins.mapAttrs
+      (key: node:
+        let
+
+          sourceInfo =
+            if overrides ? ${key}
+            then overrides.${key}.sourceInfo
+            else if node.locked.type == "path" && builtins.substring 0 1 node.locked.path != "/"
+            then
+              let
+                parentNode = allNodes.${getInputByPath lockFile.root node.parent};
+              in parentNode.sourceInfo // {
+                outPath = parentNode.sourceInfo.outPath + ("/" + node.locked.path);
+              }
+            else
+              # FIXME: remove obsolete node.info.
+              fetchTree (node.info or {} // removeAttrs node.locked ["dir"]);
+
+          # With overrides, the accessor already points to the right subdirectory.
+          subdir = if overrides ? ${key} then "" else node.locked.dir or "";
+
+          flake =
+            import (sourceInfo.outPath + ((if subdir != "" then "/" else "") + subdir + "/flake.nix"));
+
+          inputs = builtins.mapAttrs
+            (inputName: inputSpec: allNodes.${resolveInput inputSpec})
+            (node.inputs or {});
+
           outputs = flake.outputs (inputs // { self = result; });
 
           result = outputs // sourceInfo // { inherit inputs; inherit outputs; inherit sourceInfo; };

src/libexpr/flake/flake.cc

@@ -14,71 +14,12 @@ using namespace flake;
 
 namespace flake {
 
-typedef std::pair<fetchers::Tree, FlakeRef> FetchedFlake;
-typedef std::vector<std::pair<FlakeRef, FetchedFlake>> FlakeCache;
-
-static std::optional<FetchedFlake> lookupInFlakeCache(
-    const FlakeCache & flakeCache,
-    const FlakeRef & flakeRef)
-{
-    // FIXME: inefficient.
-    for (auto & i : flakeCache) {
-        if (flakeRef == i.first) {
-            debug("mapping '%s' to previously seen input '%s' -> '%s",
-                flakeRef, i.first, i.second.second);
-            return i.second;
-        }
-    }
-
-    return std::nullopt;
-}
-
-static std::tuple<fetchers::Tree, FlakeRef, FlakeRef> fetchOrSubstituteTree(
-    EvalState & state,
-    const FlakeRef & originalRef,
-    bool allowLookup,
-    FlakeCache & flakeCache)
-{
-    auto fetched = lookupInFlakeCache(flakeCache, originalRef);
-    FlakeRef resolvedRef = originalRef;
-
-    if (!fetched) {
-        if (originalRef.input.isDirect()) {
-            fetched.emplace(originalRef.fetchTree(state.store));
-        } else {
-            if (allowLookup) {
-                resolvedRef = originalRef.resolve(state.store);
-                auto fetchedResolved = lookupInFlakeCache(flakeCache, originalRef);
-                if (!fetchedResolved) fetchedResolved.emplace(resolvedRef.fetchTree(state.store));
-                flakeCache.push_back({resolvedRef, *fetchedResolved});
-                fetched.emplace(*fetchedResolved);
-            }
-            else {
-                throw Error("'%s' is an indirect flake reference, but registry lookups are not allowed", originalRef);
-            }
-        }
-        flakeCache.push_back({originalRef, *fetched});
-    }
-
-    auto [tree, lockedRef] = *fetched;
-
-    debug("got tree '%s' from '%s'",
-        state.store->printStorePath(tree.storePath), lockedRef);
-
-    state.allowPath(tree.storePath);
-
-    assert(!originalRef.input.getNarHash() || tree.storePath == originalRef.input.computeStorePath(*state.store));
-
-    return {std::move(tree), resolvedRef, lockedRef};
-}
-
 static void forceTrivialValue(EvalState & state, Value & value, const PosIdx pos)
 {
     if (value.isThunk()) // HACK: always forceValue, even if not trivial
         state.forceValue(value, pos);
 }
 
-
 static void expectType(EvalState & state, ValueType type,
     Value & value, const PosIdx pos)
 {
@@ -89,12 +30,17 @@ static void expectType(EvalState & state, ValueType type,
 }
 
 static std::map<FlakeId, FlakeInput> parseFlakeInputs(
-    EvalState & state, Value * value, const PosIdx pos,
-    const std::optional<Path> & baseDir, InputPath lockRootPath);
+    EvalState & state,
+    Value * value,
+    const PosIdx pos,
+    const InputPath & lockRootPath);
 
-static FlakeInput parseFlakeInput(EvalState & state,
-    const std::string & inputName, Value * value, const PosIdx pos,
-    const std::optional<Path> & baseDir, InputPath lockRootPath)
+static FlakeInput parseFlakeInput(
+    EvalState & state,
+    const std::string & inputName,
+    Value * value,
+    const PosIdx pos,
+    const InputPath & lockRootPath)
 {
     expectType(state, nAttrs, *value, pos);
 
@@ -118,7 +64,7 @@ static FlakeInput parseFlakeInput(EvalState & state,
                 expectType(state, nBool, *attr.value, attr.pos);
                 input.isFlake = attr.value->boolean;
             } else if (attr.name == sInputs) {
-                input.overrides = parseFlakeInputs(state, attr.value, attr.pos, baseDir, lockRootPath);
+                input.overrides = parseFlakeInputs(state, attr.value, attr.pos, lockRootPath);
             } else if (attr.name == sFollows) {
                 expectType(state, nString, *attr.value, attr.pos);
                 auto follows(parseInputPath(attr.value->string.s));
@@ -160,7 +106,7 @@ static FlakeInput parseFlakeInput(EvalState & state,
         if (!attrs.empty())
             throw Error("unexpected flake input attribute '%s', at %s", attrs.begin()->first, state.positions[pos]);
         if (url)
-            input.ref = parseFlakeRef(*url, baseDir, true, input.isFlake);
+            input.ref = parseFlakeRef(*url, {}, true, input.isFlake);
     }
 
     if (!input.follows && !input.ref)
@@ -170,8 +116,10 @@ static FlakeInput parseFlakeInput(EvalState & state,
 }
 
 static std::map<FlakeId, FlakeInput> parseFlakeInputs(
-    EvalState & state, Value * value, const PosIdx pos,
-    const std::optional<Path> & baseDir, InputPath lockRootPath)
+    EvalState & state,
+    Value * value,
+    const PosIdx pos,
+    const InputPath & lockRootPath)
 {
     std::map<FlakeId, FlakeInput> inputs;
 
@@ -183,45 +131,38 @@ static std::map<FlakeId, FlakeInput> parseFlakeInputs(
                 state.symbols[inputAttr.name],
                 inputAttr.value,
                 inputAttr.pos,
-                baseDir,
                 lockRootPath));
     }
 
     return inputs;
 }
 
-static Flake getFlake(
+static Flake readFlake(
     EvalState & state,
     const FlakeRef & originalRef,
-    bool allowLookup,
-    FlakeCache & flakeCache,
-    InputPath lockRootPath)
+    const FlakeRef & resolvedRef,
+    const FlakeRef & lockedRef,
+    const SourcePath & rootDir,
+    const InputPath & lockRootPath)
 {
-    auto [sourceInfo, resolvedRef, lockedRef] = fetchOrSubstituteTree(
-        state, originalRef, allowLookup, flakeCache);
+    CanonPath flakeDir(resolvedRef.subdir);
+    auto flakePath = rootDir + flakeDir + "flake.nix";
 
-    // Guard against symlink attacks.
-    auto flakeDir = canonPath(sourceInfo.actualPath + "/" + lockedRef.subdir, true);
-    auto flakeFile = canonPath(flakeDir + "/flake.nix", true);
-    if (!isInDir(flakeFile, sourceInfo.actualPath))
-        throw Error("'flake.nix' file of flake '%s' escapes from '%s'",
-            lockedRef, state.store->printStorePath(sourceInfo.storePath));
+    if (!flakePath.pathExists())
+        throw Error("file '%s' does not exist", flakePath);
+
+    Value vInfo;
+    state.evalFile(flakePath, vInfo, true);
+
+    expectType(state, nAttrs, vInfo, state.positions.add(Pos::Origin(rootDir), 1, 1));
 
     Flake flake {
         .originalRef = originalRef,
         .resolvedRef = resolvedRef,
         .lockedRef = lockedRef,
-        .sourceInfo = std::make_shared<fetchers::Tree>(std::move(sourceInfo))
+        .path = flakePath,
     };
 
-    if (!pathExists(flakeFile))
-        throw Error("source tree referenced by '%s' does not contain a '%s/flake.nix' file", lockedRef, lockedRef.subdir);
-
-    Value vInfo;
-    state.evalFile(flakeFile, vInfo, true); // FIXME: symlink attack
-
-    expectType(state, nAttrs, vInfo, state.positions.add({flakeFile, foFile}, 0, 0));
-
     if (auto description = vInfo.attrs->get(state.sDescription)) {
         expectType(state, nString, *description->value, description->pos);
         flake.description = description->value->string.s;
@@ -230,7 +171,7 @@ static Flake getFlake(
     auto sInputs = state.symbols.create("inputs");
 
     if (auto inputs = vInfo.attrs->get(sInputs))
-        flake.inputs = parseFlakeInputs(state, inputs->value, inputs->pos, flakeDir, lockRootPath);
+        flake.inputs = parseFlakeInputs(state, inputs->value, inputs->pos, lockRootPath);
 
     auto sOutputs = state.symbols.create("outputs");
 
@@ -247,7 +188,7 @@ static Flake getFlake(
         }
 
     } else
-        throw Error("flake '%s' lacks attribute 'outputs'", lockedRef);
+        throw Error("flake '%s' lacks attribute 'outputs'", resolvedRef);
 
     auto sNixConfig = state.symbols.create("nixConfig");
 
@@ -264,7 +205,7 @@ static Flake getFlake(
                 PathSet emptyContext = {};
                 flake.config.settings.emplace(
                     state.symbols[setting.name],
-                    state.coerceToString(setting.pos, *setting.value, emptyContext, false, true, true) .toOwned());
+                    state.coerceToString(setting.pos, *setting.value, emptyContext, false, true).toOwned());
             }
             else if (setting.value->type() == nInt)
                 flake.config.settings.emplace(
@@ -296,21 +237,51 @@ static Flake getFlake(
             attr.name != sOutputs &&
             attr.name != sNixConfig)
             throw Error("flake '%s' has an unsupported attribute '%s', at %s",
-                lockedRef, state.symbols[attr.name], state.positions[attr.pos]);
+                resolvedRef, state.symbols[attr.name], state.positions[attr.pos]);
     }
 
     return flake;
 }
 
-Flake getFlake(EvalState & state, const FlakeRef & originalRef, bool allowLookup, FlakeCache & flakeCache)
+static FlakeRef maybeResolve(
+    EvalState & state,
+    const FlakeRef & originalRef,
+    bool useRegistries)
 {
-    return getFlake(state, originalRef, allowLookup, flakeCache, {});
+    if (!originalRef.input.isDirect()) {
+        if (!useRegistries)
+            throw Error("'%s' is an indirect flake reference, but registry lookups are not allowed", originalRef);
+        return originalRef.resolve(state.store);
+    } else
+        return originalRef;
 }
 
-Flake getFlake(EvalState & state, const FlakeRef & originalRef, bool allowLookup)
+static Flake getFlake(
+    EvalState & state,
+    const FlakeRef & originalRef,
+    bool useRegistries,
+    const InputPath & lockRootPath)
 {
-    FlakeCache flakeCache;
-    return getFlake(state, originalRef, allowLookup, flakeCache);
+    auto resolvedRef = maybeResolve(state, originalRef, useRegistries);
+
+    auto [accessor, lockedRef] = resolvedRef.lazyFetch(state.store);
+
+    state.registerAccessor(accessor);
+
+    return readFlake(state, originalRef, resolvedRef, lockedRef, SourcePath {accessor, CanonPath::root}, lockRootPath);
+}
+
+Flake getFlake(EvalState & state, const FlakeRef & originalRef, bool useRegistries)
+{
+    return getFlake(state, originalRef, useRegistries, {});
+}
+
+static LockFile readLockFile(const Flake & flake)
+{
+    auto lockFilePath = flake.path.parent() + "flake.lock";
+    return lockFilePath.pathExists()
+        ? LockFile(lockFilePath.readFile(), fmt("%s", lockFilePath))
+        : LockFile();
 }
 
 /* Compute an in-memory lock file for the specified top-level flake,
@@ -322,30 +293,32 @@ LockedFlake lockFlake(
 {
     settings.requireExperimentalFeature(Xp::Flakes);
 
-    FlakeCache flakeCache;
-
     auto useRegistries = lockFlags.useRegistries.value_or(fetchSettings.useRegistries);
 
-    auto flake = getFlake(state, topRef, useRegistries, flakeCache);
+    auto flake = std::make_unique<Flake>(getFlake(state, topRef, useRegistries, {}));
 
     if (lockFlags.applyNixConfig) {
-        flake.config.apply();
+        flake->config.apply();
         state.store->setOptions();
     }
 
     try {
 
-        // FIXME: symlink attack
-        auto oldLockFile = LockFile::read(
-            flake.sourceInfo->actualPath + "/" + flake.lockedRef.subdir + "/flake.lock");
+        auto oldLockFile = readLockFile(*flake);
 
         debug("old lock file: %s", oldLockFile);
 
-        std::map<InputPath, FlakeInput> overrides;
+        std::map<InputPath, std::tuple<FlakeInput, SourcePath, std::optional<InputPath>>> overrides;
         std::set<InputPath> overridesUsed, updatesUsed;
+        std::map<ref<Node>, SourcePath> nodePaths;
 
         for (auto & i : lockFlags.inputOverrides)
-            overrides.insert_or_assign(i.first, FlakeInput { .ref = i.second });
+            overrides.emplace(
+                i.first,
+                std::make_tuple(
+                    FlakeInput { .ref = i.second },
+                    state.rootPath("/"),
+                    std::nullopt));
 
         LockFile newLockFile;
 
@@ -353,21 +326,32 @@ LockedFlake lockFlake(
 
         std::function<void(
             const FlakeInputs & flakeInputs,
-            std::shared_ptr<Node> node,
+            ref<Node> node,
             const InputPath & inputPathPrefix,
             std::shared_ptr<const Node> oldNode,
-            const InputPath & lockRootPath,
-            const Path & parentPath,
+            const InputPath & followsPrefix,
+            const SourcePath & sourcePath,
             bool trustLock)>
             computeLocks;
 
         computeLocks = [&](
+            /* The inputs of this node, either from flake.nix or
+               flake.lock */
             const FlakeInputs & flakeInputs,
-            std::shared_ptr<Node> node,
+            /* The node whose locks are to be updated.*/
+            ref<Node> node,
+            /* The path to this node in the lock file graph. */
             const InputPath & inputPathPrefix,
+            /* The old node, if any, from which locks can be
+               copied. */
             std::shared_ptr<const Node> oldNode,
-            const InputPath & lockRootPath,
-            const Path & parentPath,
+            /* The prefix relative to which 'follows' should be
+               interpreted. When a node is initially locked, it's
+               relative to the node's flake; when it's already locked,
+               it's relative to the root of the lock file. */
+            const InputPath & followsPrefix,
+            /* The source path of this node's flake. */
+            const SourcePath & sourcePath,
             bool trustLock)
         {
             debug("computing lock file node '%s'", printInputPath(inputPathPrefix));
@@ -379,7 +363,8 @@ LockedFlake lockFlake(
                     auto inputPath(inputPathPrefix);
                     inputPath.push_back(id);
                     inputPath.push_back(idOverride);
-                    overrides.insert_or_assign(inputPath, inputOverride);
+                    overrides.emplace(inputPath,
+                        std::make_tuple(inputOverride, sourcePath, inputPathPrefix));
                 }
             }
 
@@ -410,13 +395,18 @@ LockedFlake lockFlake(
                        ancestors? */
                     auto i = overrides.find(inputPath);
                     bool hasOverride = i != overrides.end();
-                    if (hasOverride) {
+                    if (hasOverride)
                         overridesUsed.insert(inputPath);
-                        // Respect the “flakeness” of the input even if we
-                        // override it
-                        i->second.isFlake = input2.isFlake;
-                    }
-                    auto & input = hasOverride ? i->second : input2;
+                    auto input = hasOverride ? std::get<0>(i->second) : input2;
+
+                    /* Resolve relative 'path:' inputs relative to
+                       the source path of the overrider. */
+                    auto overridenSourcePath = hasOverride ? std::get<1>(i->second) : sourcePath;
+
+                    /* Respect the "flakeness" of the input even if we
+                       override it. */
+                    if (hasOverride)
+                        input.isFlake = input2.isFlake;
 
                     /* Resolve 'follows' later (since it may refer to an input
                        path we haven't processed yet. */
@@ -432,6 +422,25 @@ LockedFlake lockFlake(
 
                     assert(input.ref);
 
+                    auto overridenParentPath =
+                        input.ref->input.isRelative()
+                        ? std::optional<InputPath>(hasOverride ? std::get<2>(i->second) : inputPathPrefix)
+                        : std::nullopt;
+
+                    /* Get the input flake, resolve 'path:./...'
+                       flakerefs relative to the parent flake. */
+                    auto getInputFlake = [&]()
+                    {
+                        if (auto relativePath = input.ref->input.isRelative()) {
+                            SourcePath inputSourcePath {
+                                overridenSourcePath.accessor,
+                                CanonPath(*relativePath, *overridenSourcePath.path.parent())
+                            };
+                            return readFlake(state, *input.ref, *input.ref, *input.ref, inputSourcePath, inputPath);
+                        } else
+                            return getFlake(state, *input.ref, useRegistries, inputPath);
+                    };
+
                     /* Do we have an entry in the existing lock file? And we
                        don't have a --update-input flag for this input? */
                     std::shared_ptr<LockedNode> oldLock;
@@ -445,6 +454,7 @@ LockedFlake lockFlake(
 
                     if (oldLock
                         && oldLock->originalRef == *input.ref
+                        && oldLock->parentPath == overridenParentPath
                         && !hasOverride)
                     {
                         debug("keeping existing input '%s'", inputPathS);
@@ -452,8 +462,9 @@ LockedFlake lockFlake(
                         /* Copy the input from the old lock since its flakeref
                            didn't change and there is no override from a
                            higher level flake. */
-                        auto childNode = std::make_shared<LockedNode>(
-                            oldLock->lockedRef, oldLock->originalRef, oldLock->isFlake);
+                        auto childNode = make_ref<LockedNode>(
+                            oldLock->lockedRef, oldLock->originalRef, oldLock->isFlake,
+                            oldLock->parentPath);
 
                         node->inputs.insert_or_assign(id, childNode);
 
@@ -496,7 +507,7 @@ LockedFlake lockFlake(
                                             break;
                                         }
                                     }
-                                    auto absoluteFollows(lockRootPath);
+                                    auto absoluteFollows(followsPrefix);
                                     absoluteFollows.insert(absoluteFollows.end(), follows->begin(), follows->end());
                                     fakeInputs.emplace(i.first, FlakeInput {
                                         .follows = absoluteFollows,
@@ -505,24 +516,26 @@ LockedFlake lockFlake(
                             }
                         }
 
-                        auto localPath(parentPath);
-                        // If this input is a path, recurse it down.
-                        // This allows us to resolve path inputs relative to the current flake.
-                        if ((*input.ref).input.getType() == "path")
-                            localPath = absPath(*input.ref->input.getSourcePath(), parentPath);
-                        computeLocks(
-                            mustRefetch
-                            ? getFlake(state, oldLock->lockedRef, false, flakeCache, inputPath).inputs
-                            : fakeInputs,
-                            childNode, inputPath, oldLock, lockRootPath, parentPath, !mustRefetch);
+                        if (mustRefetch) {
+                            auto inputFlake = getInputFlake();
+                            nodePaths.emplace(childNode, inputFlake.path.parent());
+                            computeLocks(inputFlake.inputs, childNode, inputPath, oldLock, followsPrefix,
+                                inputFlake.path, !mustRefetch);
+                        } else {
+                             // FIXME: sourcePath is wrong here, we
+                             // should pass a lambda that lazily
+                             // fetches the parent flake if needed
+                             // (i.e. getInputFlake()).
+                            computeLocks(fakeInputs, childNode, inputPath, oldLock, followsPrefix, sourcePath, !mustRefetch);
+                        }
 
                     } else {
                         /* We need to create a new lock file entry. So fetch
                            this input. */
                         debug("creating new input '%s'", inputPathS);
 
-                        if (!lockFlags.allowMutable && !input.ref->input.isLocked())
-                            throw Error("cannot update flake input '%s' in pure mode", inputPathS);
+                        if (!lockFlags.allowUnlocked && !input.ref->input.isLocked() && !input.ref->input.isRelative())
+                            throw Error("cannot update unlocked flake input '%s' in pure mode", inputPathS);
 
                         /* Note: in case of an --override-input, we use
                             the *original* ref (input2.ref) for the
@@ -534,17 +547,11 @@ LockedFlake lockFlake(
                         auto ref = input2.ref ? *input2.ref : *input.ref;
 
                         if (input.isFlake) {
-                            Path localPath = parentPath;
-                            FlakeRef localRef = *input.ref;
+                            auto inputFlake = getInputFlake();
 
-                            // If this input is a path, recurse it down.
-                            // This allows us to resolve path inputs relative to the current flake.
-                            if (localRef.input.getType() == "path")
-                                localPath = absPath(*input.ref->input.getSourcePath(), parentPath);
-
-                            auto inputFlake = getFlake(state, localRef, useRegistries, flakeCache, inputPath);
-
-                            auto childNode = std::make_shared<LockedNode>(inputFlake.lockedRef, ref);
+                            auto childNode = make_ref<LockedNode>(
+                                inputFlake.lockedRef, ref, true,
+                                overridenParentPath);
 
                             node->inputs.insert_or_assign(id, childNode);
 
@@ -559,20 +566,27 @@ LockedFlake lockFlake(
                                flake. Also, unless we already have this flake
                                in the top-level lock file, use this flake's
                                own lock file. */
+                            nodePaths.emplace(childNode, inputFlake.path.parent());
                             computeLocks(
                                 inputFlake.inputs, childNode, inputPath,
                                 oldLock
                                 ? std::dynamic_pointer_cast<const Node>(oldLock)
-                                : LockFile::read(
-                                    inputFlake.sourceInfo->actualPath + "/" + inputFlake.lockedRef.subdir + "/flake.lock").root,
-                                oldLock ? lockRootPath : inputPath, localPath, false);
+                                : (std::shared_ptr<Node>) readLockFile(inputFlake).root,
+                                oldLock ? followsPrefix : inputPath,
+                                inputFlake.path,
+                                false);
                         }
 
                         else {
-                            auto [sourceInfo, resolvedRef, lockedRef] = fetchOrSubstituteTree(
-                                state, *input.ref, useRegistries, flakeCache);
-                            node->inputs.insert_or_assign(id,
-                                std::make_shared<LockedNode>(lockedRef, ref, false));
+                            auto resolvedRef = maybeResolve(state, *input.ref, useRegistries);
+
+                            auto [accessor, lockedRef] = resolvedRef.lazyFetch(state.store);
+
+                            auto childNode = make_ref<LockedNode>(lockedRef, ref, false, overridenParentPath);
+
+                            nodePaths.emplace(childNode, accessor->root());
+
+                            node->inputs.insert_or_assign(id, childNode);
                         }
                     }
 
@@ -583,12 +597,16 @@ LockedFlake lockFlake(
             }
         };
 
-        // Bring in the current ref for relative path resolution if we have it
-        auto parentPath = canonPath(flake.sourceInfo->actualPath + "/" + flake.lockedRef.subdir, true);
+        nodePaths.emplace(newLockFile.root, flake->path.parent());
 
         computeLocks(
-            flake.inputs, newLockFile.root, {},
-            lockFlags.recreateLockFile ? nullptr : oldLockFile.root, {}, parentPath, false);
+            flake->inputs,
+            newLockFile.root,
+            {},
+            lockFlags.recreateLockFile ? nullptr : (std::shared_ptr<Node>) oldLockFile.root,
+            {},
+            flake->path,
+            false);
 
         for (auto & i : lockFlags.inputOverrides)
             if (!overridesUsed.count(i.first))
@@ -610,19 +628,16 @@ LockedFlake lockFlake(
             auto diff = LockFile::diff(oldLockFile, newLockFile);
 
             if (lockFlags.writeLockFile) {
-                if (auto sourcePath = topRef.input.getSourcePath()) {
-                    if (!newLockFile.isImmutable()) {
+                if (auto unlockedInput = newLockFile.isUnlocked()) {
                     if (fetchSettings.warnDirty)
-                            warn("will not write lock file of flake '%s' because it has a mutable input", topRef);
+                        warn("will not write lock file of flake '%s' because it has an unlocked input ('%s')", topRef, *unlockedInput);
                 } else {
                     if (!lockFlags.updateLockFile)
                         throw Error("flake '%s' requires lock file changes but they're not allowed due to '--no-update-lock-file'", topRef);
 
-                        auto relPath = (topRef.subdir == "" ? "" : topRef.subdir + "/") + "flake.lock";
+                    auto path = flake->path.parent() + "flake.lock";
 
-                        auto path = *sourcePath + "/" + relPath;
-
-                        bool lockFileExists = pathExists(path);
+                    bool lockFileExists = path.pathExists();
 
                     if (lockFileExists) {
                         auto s = chomp(diff);
@@ -633,59 +648,48 @@ LockedFlake lockFlake(
                     } else
                         warn("creating lock file '%s'", path);
 
-                        newLockFile.write(path);
-
                     std::optional<std::string> commitMessage = std::nullopt;
                     if (lockFlags.commitLockFile) {
                         std::string cm;
 
                         cm = fetchSettings.commitLockFileSummary.get();
 
-                            if (cm == "") {
-                                cm = fmt("%s: %s", relPath, lockFileExists ? "Update" : "Add");
-                            }
+                        if (cm == "")
+                            cm = fmt("%s: %s", path.path.rel(), lockFileExists ? "Update" : "Add");
 
                         cm += "\n\nFlake lock file updates:\n\n";
                         cm += filterANSIEscapes(diff, true);
                         commitMessage = cm;
                     }
 
-                        topRef.input.markChangedFile(
-                            (topRef.subdir == "" ? "" : topRef.subdir + "/") + "flake.lock",
-                            commitMessage);
+                    topRef.input.putFile(path.path, fmt("%s\n", newLockFile), commitMessage);
 
                     /* Rewriting the lockfile changed the top-level
                        repo, so we should re-read it. FIXME: we could
                        also just clear the 'rev' field... */
-                        auto prevLockedRef = flake.lockedRef;
-                        FlakeCache dummyCache;
-                        flake = getFlake(state, topRef, useRegistries, dummyCache);
+                    auto prevLockedRef = flake->lockedRef;
+                    flake = std::make_unique<Flake>(getFlake(state, topRef, useRegistries));
 
                     if (lockFlags.commitLockFile &&
-                            flake.lockedRef.input.getRev() &&
-                            prevLockedRef.input.getRev() != flake.lockedRef.input.getRev())
-                            warn("committed new revision '%s'", flake.lockedRef.input.getRev()->gitRev());
-
-                        /* Make sure that we picked up the change,
-                           i.e. the tree should usually be dirty
-                           now. Corner case: we could have reverted from a
-                           dirty to a clean tree! */
-                        if (flake.lockedRef.input == prevLockedRef.input
-                            && !flake.lockedRef.input.isLocked())
-                            throw Error("'%s' did not change after I updated its 'flake.lock' file; is 'flake.lock' under version control?", flake.originalRef);
+                        flake->lockedRef.input.getRev() &&
+                        prevLockedRef.input.getRev() != flake->lockedRef.input.getRev())
+                        warn("committed new revision '%s'", flake->lockedRef.input.getRev()->gitRev());
                 }
-                } else
-                    throw Error("cannot write modified lock file of flake '%s' (use '--no-write-lock-file' to ignore)", topRef);
             } else {
                 warn("not writing modified lock file of flake '%s':\n%s", topRef, chomp(diff));
-                flake.forceDirty = true;
+                flake->forceDirty = true;
             }
         }
 
-        return LockedFlake { .flake = std::move(flake), .lockFile = std::move(newLockFile) };
+        return LockedFlake {
+            .flake = std::move(*flake),
+            .lockFile = std::move(newLockFile),
+            .nodePaths = std::move(nodePaths)
+        };
 
     } catch (Error & e) {
-        e.addTrace({}, "while updating the lock file of flake '%s'", flake.lockedRef.to_string());
+        if (flake)
+            e.addTrace({}, "while updating the lock file of flake '%s'", flake->lockedRef.to_string());
         throw;
     }
 }
@@ -694,34 +698,42 @@ void callFlake(EvalState & state,
     const LockedFlake & lockedFlake,
     Value & vRes)
 {
-    auto vLocks = state.allocValue();
-    auto vRootSrc = state.allocValue();
-    auto vRootSubdir = state.allocValue();
-    auto vTmp1 = state.allocValue();
-    auto vTmp2 = state.allocValue();
+    auto [lockFileStr, keyMap] = lockedFlake.lockFile.to_string();
 
-    vLocks->mkString(lockedFlake.lockFile.to_string());
+    auto overrides = state.buildBindings(lockedFlake.nodePaths.size());
+
+    for (auto & [node, sourcePath] : lockedFlake.nodePaths) {
+        auto override = state.buildBindings(2);
+
+        auto & vSourceInfo = override.alloc(state.symbols.create("sourceInfo"));
+
+        auto lockedNode = node.dynamic_pointer_cast<const LockedNode>();
 
         emitTreeAttrs(
             state,
-        *lockedFlake.flake.sourceInfo,
-        lockedFlake.flake.lockedRef.input,
-        *vRootSrc,
+            sourcePath,
+            lockedNode ? lockedNode->lockedRef.input : lockedFlake.flake.lockedRef.input,
+            vSourceInfo,
             false,
-        lockedFlake.flake.forceDirty);
+            !lockedNode && lockedFlake.flake.forceDirty);
 
-    vRootSubdir->mkString(lockedFlake.flake.lockedRef.subdir);
+        auto key = keyMap.find(node);
+        assert(key != keyMap.end());
 
-    if (!state.vCallFlake) {
-        state.vCallFlake = allocRootValue(state.allocValue());
-        state.eval(state.parseExprFromString(
-            #include "call-flake.nix.gen.hh"
-            , "/"), **state.vCallFlake);
+        overrides.alloc(state.symbols.create(key->second)).mkAttrs(override);
     }
 
-    state.callFunction(**state.vCallFlake, *vLocks, *vTmp1, noPos);
-    state.callFunction(*vTmp1, *vRootSrc, *vTmp2, noPos);
-    state.callFunction(*vTmp2, *vRootSubdir, vRes, noPos);
+    auto & vOverrides = state.allocValue()->mkAttrs(overrides);
+
+    auto vCallFlake = state.allocValue();
+    state.evalFile(state.callFlakeInternal, *vCallFlake);
+
+    auto vTmp1 = state.allocValue();
+    auto vLocks = state.allocValue();
+    vLocks->mkString(lockFileStr);
+    state.callFunction(*vCallFlake, *vLocks, *vTmp1, noPos);
+
+    state.callFunction(*vTmp1, vOverrides, vRes, noPos);
 }
 
 static void prim_getFlake(EvalState & state, const PosIdx pos, Value * * args, Value & v)
@@ -737,7 +749,7 @@ static void prim_getFlake(EvalState & state, const PosIdx pos, Value * * args, V
                 .updateLockFile = false,
                 .writeLockFile = false,
                 .useRegistries = !evalSettings.pureEval && fetchSettings.useRegistries,
-                .allowMutable  = !evalSettings.pureEval,
+                .allowUnlocked = !evalSettings.pureEval,
             }),
         v);
 }
@@ -769,18 +781,17 @@ static RegisterPrimOp r2({
 
 }
 
-Fingerprint LockedFlake::getFingerprint() const
+std::optional<Fingerprint> LockedFlake::getFingerprint(ref<Store> store) const
 {
+    if (lockFile.isUnlocked()) return std::nullopt;
+
+    auto fingerprint = flake.lockedRef.input.getFingerprint(store);
+    if (!fingerprint) return std::nullopt;
+
     // FIXME: as an optimization, if the flake contains a lock file
     // and we haven't changed it, then it's sufficient to use
     // flake.sourceInfo.storePath for the fingerprint.
-    return hashString(htSHA256,
-        fmt("%s;%s;%d;%d;%s",
-            flake.sourceInfo->storePath.to_string(),
-            flake.lockedRef.subdir,
-            flake.lockedRef.input.getRevCount().value_or(0),
-            flake.lockedRef.input.getLastModified().value_or(0),
-            lockFile));
+    return hashString(htSHA256, fmt("%s;%s;%s", *fingerprint, flake.lockedRef.subdir, lockFile));
 }
 
 Flake::~Flake() { }

src/libexpr/flake/flake.hh

@@ -61,9 +61,9 @@ struct Flake
     FlakeRef originalRef; // the original flake specification (by the user)
     FlakeRef resolvedRef; // registry references and caching resolved to the specific underlying flake
     FlakeRef lockedRef; // the specific local store result of invoking the fetcher
+    SourcePath path;
     bool forceDirty = false; // pretend that 'lockedRef' is dirty
     std::optional<std::string> description;
-    std::shared_ptr<const fetchers::Tree> sourceInfo;
     FlakeInputs inputs;
     ConfigFile config; // 'nixConfig' attribute
     ~Flake();
@@ -79,7 +79,12 @@ struct LockedFlake
     Flake flake;
     LockFile lockFile;
 
-    Fingerprint getFingerprint() const;
+    /* Source tree accessors for nodes that have been fetched in
+       lockFlake(); in particular, the root node and the overriden
+       inputs. */
+    std::map<ref<Node>, SourcePath> nodePaths;
+
+    std::optional<Fingerprint> getFingerprint(ref<Store> store) const;
 };
 
 struct LockFlags
@@ -108,11 +113,11 @@ struct LockFlags
 
     bool applyNixConfig = false;
 
-    /* Whether mutable flake references (i.e. those without a Git
+    /* Whether unlocked flake references (i.e. those without a Git
        revision or similar) without a corresponding lock are
-       allowed. Mutable flake references with a lock are always
+       allowed. Unlocked flake references with a lock are always
        allowed. */
-    bool allowMutable = true;
+    bool allowUnlocked = true;
 
     /* Whether to commit changes to flake.lock. */
     bool commitLockFile = false;
@@ -139,7 +144,7 @@ void callFlake(
 
 void emitTreeAttrs(
     EvalState & state,
-    const fetchers::Tree & tree,
+    const SourcePath & path,
     const fetchers::Input & input,
     Value & v,
     bool emptyRevFallback = false,

src/libexpr/flake/flakeref.cc

@@ -92,6 +92,15 @@ std::pair<FlakeRef, std::string> parseFlakeRefWithFragment(
 
     std::smatch match;
 
+    auto fromParsedURL = [&](ParsedURL && parsedURL)
+    {
+        auto dir = getOr(parsedURL.query, "dir", "");
+        parsedURL.query.erase("dir");
+        std::string fragment;
+        std::swap(fragment, parsedURL.fragment);
+        return std::make_pair(FlakeRef(Input::fromURL(parsedURL), dir), fragment);
+    };
+
     /* Check if 'url' is a flake ID. This is an abbreviated syntax for
        'flake:<flake-id>?ref=<ref>&rev=<rev>'. */
 
@@ -112,6 +121,7 @@ std::pair<FlakeRef, std::string> parseFlakeRefWithFragment(
     else if (std::regex_match(url, match, pathUrlRegex)) {
         std::string path = match[1];
         std::string fragment = percentDecode(match.str(3));
+        auto query = decodeQuery(match[2]);
 
         if (baseDir) {
             /* Check if 'url' is a path (either absolute or relative
@@ -163,7 +173,8 @@ std::pair<FlakeRef, std::string> parseFlakeRefWithFragment(
                             .scheme = "git+file",
                             .authority = "",
                             .path = flakeRoot,
-                            .query = decodeQuery(match[2]),
+                            .query = query,
+                            .fragment = fragment,
                         };
 
                         if (subdir != "") {
@@ -175,9 +186,7 @@ std::pair<FlakeRef, std::string> parseFlakeRefWithFragment(
                         if (pathExists(flakeRoot + "/.git/shallow"))
                             parsedURL.query.insert_or_assign("shallow", "1");
 
-                        return std::make_pair(
-                            FlakeRef(Input::fromURL(parsedURL), getOr(parsedURL.query, "dir", "")),
-                            fragment);
+                        return fromParsedURL(std::move(parsedURL));
                     }
 
                     subdir = std::string(baseNameOf(flakeRoot)) + (subdir.empty() ? "" : "/" + subdir);
@@ -188,29 +197,21 @@ std::pair<FlakeRef, std::string> parseFlakeRefWithFragment(
         } else {
             if (!hasPrefix(path, "/"))
                 throw BadURL("flake reference '%s' is not an absolute path", url);
-            auto query = decodeQuery(match[2]);
-            path = canonPath(path + "/" + getOr(query, "dir", ""));
         }
 
-        fetchers::Attrs attrs;
-        attrs.insert_or_assign("type", "path");
-        attrs.insert_or_assign("path", path);
-
-        return std::make_pair(FlakeRef(Input::fromAttrs(std::move(attrs)), ""), fragment);
+        return fromParsedURL({
+            .url = path, // FIXME
+            .base = path,
+            .scheme = "path",
+            .authority = "",
+            .path = path,
+            .query = query,
+            .fragment = fragment
+        });
     }
 
-    else {
-        auto parsedURL = parseURL(url);
-        std::string fragment;
-        std::swap(fragment, parsedURL.fragment);
-
-        auto input = Input::fromURL(parsedURL);
-        input.parent = baseDir;
-
-        return std::make_pair(
-            FlakeRef(std::move(input), getOr(parsedURL.query, "dir", "")),
-            fragment);
-    }
+    else
+        return fromParsedURL(parseURL(url));
 }
 
 std::optional<std::pair<FlakeRef, std::string>> maybeParseFlakeRefWithFragment(
@@ -232,10 +233,10 @@ FlakeRef FlakeRef::fromAttrs(const fetchers::Attrs & attrs)
         fetchers::maybeGetStrAttr(attrs, "dir").value_or(""));
 }
 
-std::pair<fetchers::Tree, FlakeRef> FlakeRef::fetchTree(ref<Store> store) const
+std::pair<ref<InputAccessor>, FlakeRef> FlakeRef::lazyFetch(ref<Store> store) const
 {
-    auto [tree, lockedInput] = input.fetch(store);
-    return {std::move(tree), FlakeRef(std::move(lockedInput), subdir)};
+    auto [accessor, lockedInput] = input.getAccessor(store);
+    return {accessor, FlakeRef(std::move(lockedInput), subdir)};
 }
 
 std::tuple<FlakeRef, std::string, OutputsSpec> parseFlakeRefWithFragmentAndOutputsSpec(

src/libexpr/flake/flakeref.hh

@@ -35,7 +35,7 @@ typedef std::string FlakeId;
 
 struct FlakeRef
 {
-    /* fetcher-specific representation of the input, sufficient to
+    /* Fetcher-specific representation of the input, sufficient to
        perform the fetch operation. */
     fetchers::Input input;
 
@@ -57,7 +57,7 @@ struct FlakeRef
 
     static FlakeRef fromAttrs(const fetchers::Attrs & attrs);
 
-    std::pair<fetchers::Tree, FlakeRef> fetchTree(ref<Store> store) const;
+    std::pair<ref<InputAccessor>, FlakeRef> lazyFetch(ref<Store> store) const;
 };
 
 std::ostream & operator << (std::ostream & str, const FlakeRef & flakeRef);

src/libexpr/flake/lockfile.cc

@@ -31,30 +31,26 @@ FlakeRef getFlakeRef(
 }
 
 LockedNode::LockedNode(const nlohmann::json & json)
-    : lockedRef(getFlakeRef(json, "locked", "info"))
+    : lockedRef(getFlakeRef(json, "locked", "info")) // FIXME: remove "info"
     , originalRef(getFlakeRef(json, "original", nullptr))
     , isFlake(json.find("flake") != json.end() ? (bool) json["flake"] : true)
+    , parentPath(json.find("parent") != json.end() ? (std::optional<InputPath>) json["parent"] : std::nullopt)
 {
-    if (!lockedRef.input.isLocked())
-        throw Error("lock file contains mutable lock '%s'",
+    if (!lockedRef.input.isLocked() && !lockedRef.input.isRelative())
+        throw Error("lock file contains unlocked input '%s'",
             fetchers::attrsToJSON(lockedRef.input.toAttrs()));
 }
 
-StorePath LockedNode::computeStorePath(Store & store) const
-{
-    return lockedRef.input.computeStorePath(store);
-}
-
 std::shared_ptr<Node> LockFile::findInput(const InputPath & path)
 {
-    auto pos = root;
+    std::shared_ptr<Node> pos = root;
 
     if (!pos) return {};
 
     for (auto & elem : path) {
         if (auto i = get(pos->inputs, elem)) {
             if (auto node = std::get_if<0>(&*i))
-                pos = *node;
+                pos = (std::shared_ptr<LockedNode>) *node;
             else if (auto follows = std::get_if<1>(&*i)) {
                 pos = findInput(*follows);
                 if (!pos) return {};
@@ -66,13 +62,15 @@ std::shared_ptr<Node> LockFile::findInput(const InputPath & path)
     return pos;
 }
 
-LockFile::LockFile(const nlohmann::json & json, const Path & path)
+LockFile::LockFile(std::string_view contents, std::string_view path)
 {
+    auto json = nlohmann::json::parse(contents);
+
     auto version = json.value("version", 0);
     if (version < 5 || version > 7)
         throw Error("lock file '%s' has unsupported version %d", path, version);
 
-    std::unordered_map<std::string, std::shared_ptr<Node>> nodeMap;
+    std::map<std::string, ref<Node>> nodeMap;
 
     std::function<void(Node & node, const nlohmann::json & jsonNode)> getInputs;
 
@@ -93,12 +91,12 @@ LockFile::LockFile(const nlohmann::json & json, const Path & path)
                     auto jsonNode2 = nodes.find(inputKey);
                     if (jsonNode2 == nodes.end())
                         throw Error("lock file references missing node '%s'", inputKey);
-                    auto input = std::make_shared<LockedNode>(*jsonNode2);
+                    auto input = make_ref<LockedNode>(*jsonNode2);
                     k = nodeMap.insert_or_assign(inputKey, input).first;
                     getInputs(*input, *jsonNode2);
                 }
-                if (auto child = std::dynamic_pointer_cast<LockedNode>(k->second))
-                    node.inputs.insert_or_assign(i.key(), child);
+                if (auto child = k->second.dynamic_pointer_cast<LockedNode>())
+                    node.inputs.insert_or_assign(i.key(), ref(child));
                 else
                     // FIXME: replace by follows node
                     throw Error("lock file contains cycle to root node");
@@ -116,15 +114,15 @@ LockFile::LockFile(const nlohmann::json & json, const Path & path)
     // a bit since we don't need to worry about cycles.
 }
 
-nlohmann::json LockFile::toJSON() const
+std::pair<nlohmann::json, LockFile::KeyMap> LockFile::toJSON() const
 {
     nlohmann::json nodes;
-    std::unordered_map<std::shared_ptr<const Node>, std::string> nodeKeys;
+    KeyMap nodeKeys;
     std::unordered_set<std::string> keys;
 
-    std::function<std::string(const std::string & key, std::shared_ptr<const Node> node)> dumpNode;
+    std::function<std::string(const std::string & key, ref<const Node> node)> dumpNode;
 
-    dumpNode = [&](std::string key, std::shared_ptr<const Node> node) -> std::string
+    dumpNode = [&](std::string key, ref<const Node> node) -> std::string
     {
         auto k = nodeKeys.find(node);
         if (k != nodeKeys.end())
@@ -159,10 +157,13 @@ nlohmann::json LockFile::toJSON() const
             n["inputs"] = std::move(inputs);
         }
 
-        if (auto lockedNode = std::dynamic_pointer_cast<const LockedNode>(node)) {
+        if (auto lockedNode = node.dynamic_pointer_cast<const LockedNode>()) {
             n["original"] = fetchers::attrsToJSON(lockedNode->originalRef.toAttrs());
             n["locked"] = fetchers::attrsToJSON(lockedNode->lockedRef.toAttrs());
-            if (!lockedNode->isFlake) n["flake"] = false;
+            if (!lockedNode->isFlake)
+                n["flake"] = false;
+            if (lockedNode->parentPath)
+                n["parent"] = *lockedNode->parentPath;
         }
 
         nodes[key] = std::move(n);
@@ -175,39 +176,28 @@ nlohmann::json LockFile::toJSON() const
     json["root"] = dumpNode("root", root);
     json["nodes"] = std::move(nodes);
 
-    return json;
+    return {json, std::move(nodeKeys)};
 }
 
-std::string LockFile::to_string() const
+std::pair<std::string, LockFile::KeyMap> LockFile::to_string() const
 {
-    return toJSON().dump(2);
-}
-
-LockFile LockFile::read(const Path & path)
-{
-    if (!pathExists(path)) return LockFile();
-    return LockFile(nlohmann::json::parse(readFile(path)), path);
+    auto [json, nodeKeys] = toJSON();
+    return {json.dump(2), std::move(nodeKeys)};
 }
 
 std::ostream & operator <<(std::ostream & stream, const LockFile & lockFile)
 {
-    stream << lockFile.toJSON().dump(2);
+    stream << lockFile.toJSON().first.dump(2);
     return stream;
 }
 
-void LockFile::write(const Path & path) const
+std::optional<FlakeRef> LockFile::isUnlocked() const
 {
-    createDirs(dirOf(path));
-    writeFile(path, fmt("%s\n", *this));
-}
+    std::set<ref<const Node>> nodes;
 
-bool LockFile::isImmutable() const
-{
-    std::unordered_set<std::shared_ptr<const Node>> nodes;
+    std::function<void(ref<const Node> node)> visit;
 
-    std::function<void(std::shared_ptr<const Node> node)> visit;
-
-    visit = [&](std::shared_ptr<const Node> node)
+    visit = [&](ref<const Node> node)
     {
         if (!nodes.insert(node).second) return;
         for (auto & i : node->inputs)
@@ -219,17 +209,20 @@ bool LockFile::isImmutable() const
 
     for (auto & i : nodes) {
         if (i == root) continue;
-        auto lockedNode = std::dynamic_pointer_cast<const LockedNode>(i);
-        if (lockedNode && !lockedNode->lockedRef.input.isLocked()) return false;
+        auto node = i.dynamic_pointer_cast<const LockedNode>();
+        if (node
+            && !node->lockedRef.input.isLocked()
+            && !node->lockedRef.input.isRelative())
+            return node->lockedRef;
     }
 
-    return true;
+    return {};
 }
 
 bool LockFile::operator ==(const LockFile & other) const
 {
     // FIXME: slow
-    return toJSON() == other.toJSON();
+    return toJSON().first == other.toJSON().first;
 }
 
 InputPath parseInputPath(std::string_view s)
@@ -247,12 +240,12 @@ InputPath parseInputPath(std::string_view s)
 
 std::map<InputPath, Node::Edge> LockFile::getAllInputs() const
 {
-    std::unordered_set<std::shared_ptr<Node>> done;
+    std::set<ref<Node>> done;
     std::map<InputPath, Node::Edge> res;
 
-    std::function<void(const InputPath & prefix, std::shared_ptr<Node> node)> recurse;
+    std::function<void(const InputPath & prefix, ref<Node> node)> recurse;
 
-    recurse = [&](const InputPath & prefix, std::shared_ptr<Node> node)
+    recurse = [&](const InputPath & prefix, ref<Node> node)
     {
         if (!done.insert(node).second) return;
 

src/libexpr/flake/lockfile.hh

@@ -20,7 +20,7 @@ struct LockedNode;
    type LockedNode. */
 struct Node : std::enable_shared_from_this<Node>
 {
-    typedef std::variant<std::shared_ptr<LockedNode>, InputPath> Edge;
+    typedef std::variant<ref<LockedNode>, InputPath> Edge;
 
     std::map<FlakeId, Edge> inputs;
 
@@ -33,34 +33,37 @@ struct LockedNode : Node
     FlakeRef lockedRef, originalRef;
     bool isFlake = true;
 
+    /* The node relative to which relative source paths
+       (e.g. 'path:../foo') are interpreted. */
+    std::optional<InputPath> parentPath;
+
     LockedNode(
         const FlakeRef & lockedRef,
         const FlakeRef & originalRef,
-        bool isFlake = true)
-        : lockedRef(lockedRef), originalRef(originalRef), isFlake(isFlake)
+        bool isFlake = true,
+        std::optional<InputPath> parentPath = {})
+        : lockedRef(lockedRef), originalRef(originalRef), isFlake(isFlake), parentPath(parentPath)
     { }
 
     LockedNode(const nlohmann::json & json);
-
-    StorePath computeStorePath(Store & store) const;
 };
 
 struct LockFile
 {
-    std::shared_ptr<Node> root = std::make_shared<Node>();
+    ref<Node> root = make_ref<Node>();
 
     LockFile() {};
-    LockFile(const nlohmann::json & json, const Path & path);
+    LockFile(std::string_view contents, std::string_view path);
 
-    nlohmann::json toJSON() const;
+    typedef std::map<ref<const Node>, std::string> KeyMap;
 
-    std::string to_string() const;
+    std::pair<nlohmann::json, KeyMap> toJSON() const;
 
-    static LockFile read(const Path & path);
+    std::pair<std::string, KeyMap> to_string() const;
 
-    void write(const Path & path) const;
-
-    bool isImmutable() const;
+    /* Check whether this lock file has any unlocked inputs. If so,
+       return one. */
+    std::optional<FlakeRef> isUnlocked() const;
 
     bool operator ==(const LockFile & other) const;
 

src/libexpr/get-drvs.cc

@@ -150,7 +150,7 @@ DrvInfo::Outputs DrvInfo::queryOutputs(bool withPaths, bool onlyOutputsToInstall
         /* Check for `meta.outputsToInstall` and return `outputs` reduced to that. */
         const Value * outTI = queryMeta("outputsToInstall");
         if (!outTI) return outputs;
-        const auto errMsg = Error("this derivation has bad 'meta.outputsToInstall'");
+        auto errMsg = Error("this derivation has bad 'meta.outputsToInstall'");
             /* ^ this shows during `nix-env -i` right under the bad derivation */
         if (!outTI->isList()) throw errMsg;
         Outputs result;

src/libexpr/nixexpr.cc

@@ -8,6 +8,65 @@
 
 namespace nix {
 
+struct SourcePathAdapter : AbstractPos
+{
+    SourcePath path;
+
+    SourcePathAdapter(SourcePath path)
+        : path(std::move(path))
+    {
+    }
+
+    std::optional<std::string> getSource() const override
+    {
+        try {
+            return path.readFile();
+        } catch (Error &) {
+            return std::nullopt;
+        }
+    }
+
+    void print(std::ostream & out) const override
+    {
+        out << path;
+    }
+};
+
+struct StringPosAdapter : AbstractPos
+{
+    void print(std::ostream & out) const override
+    {
+        out << "«string»";
+    }
+};
+
+struct StdinPosAdapter : AbstractPos
+{
+    void print(std::ostream & out) const override
+    {
+        out << "«stdin»";
+    }
+};
+
+Pos::operator std::shared_ptr<AbstractPos>() const
+{
+    std::shared_ptr<AbstractPos> pos;
+
+    if (auto path = std::get_if<SourcePath>(&origin))
+        pos = std::make_shared<SourcePathAdapter>(*path);
+    else if (std::get_if<stdin_tag>(&origin))
+        pos = std::make_shared<StdinPosAdapter>();
+    else if (std::get_if<string_tag>(&origin))
+        pos = std::make_shared<StringPosAdapter>();
+
+    if (pos) {
+        pos->line = line;
+        pos->column = column;
+    }
+
+    return pos;
+}
+
 /* Displaying abstract syntax trees. */
 
 static void showString(std::ostream & str, std::string_view s)
@@ -71,7 +130,7 @@ void ExprString::show(const SymbolTable & symbols, std::ostream & str) const
 
 void ExprPath::show(const SymbolTable & symbols, std::ostream & str) const
 {
-    str << s;
+    str << path;
 }
 
 void ExprVar::show(const SymbolTable & symbols, std::ostream & str) const
@@ -248,24 +307,10 @@ void ExprPos::show(const SymbolTable & symbols, std::ostream & str) const
 
 std::ostream & operator << (std::ostream & str, const Pos & pos)
 {
-    if (!pos)
+    if (auto pos2 = (std::shared_ptr<AbstractPos>) pos) {
+        str << *pos2;
+    } else
         str << "undefined position";
-    else
-    {
-        auto f = format(ANSI_BOLD "%1%" ANSI_NORMAL ":%2%:%3%");
-        switch (pos.origin) {
-            case foFile:
-                f % (const std::string &) pos.file;
-                break;
-            case foStdin:
-            case foString:
-                f % "(string)";
-                break;
-            default:
-                throw Error("unhandled Pos origin!");
-        }
-        str << (f % pos.line % pos.column).str();
-    }
 
     return str;
 }
@@ -289,7 +334,6 @@ std::string showAttrPath(const SymbolTable & symbols, const AttrPath & attrPath)
 }
 
 
-
 /* Computing levels/displacements for variables. */
 
 void Expr::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)

src/libexpr/nixexpr.hh

@@ -20,18 +20,23 @@ MakeError(Abort, EvalError);
 MakeError(TypeError, EvalError);
 MakeError(UndefinedVarError, Error);
 MakeError(MissingArgumentError, EvalError);
-MakeError(RestrictedPathError, Error);
 
 /* Position objects. */
-
 struct Pos
 {
-    std::string file;
-    FileOrigin origin;
     uint32_t line;
     uint32_t column;
 
+    struct stdin_tag {};
+    struct string_tag {};
+
+    typedef std::variant<stdin_tag, string_tag, SourcePath> Origin;
+
+    Origin origin;
+
     explicit operator bool() const { return line > 0; }
+
+    operator std::shared_ptr<AbstractPos>() const;
 };
 
 class PosIdx {
@@ -48,6 +53,10 @@ public:
     explicit operator bool() const { return id > 0; }
 
     bool operator <(const PosIdx other) const { return id < other.id; }
+
+    bool operator ==(const PosIdx other) const { return id == other.id; }
+
+    bool operator !=(const PosIdx other) const { return id != other.id; }
 };
 
 class PosTable
@@ -61,13 +70,13 @@ public:
         // current origins.back() can be reused or not.
         mutable uint32_t idx = std::numeric_limits<uint32_t>::max();
 
-        explicit Origin(uint32_t idx): idx(idx), file{}, origin{} {}
+        // Used for searching in PosTable::[].
+        explicit Origin(uint32_t idx): idx(idx), origin{Pos::stdin_tag()} {}
 
     public:
-        const std::string file;
-        const FileOrigin origin;
+        const Pos::Origin origin;
 
-        Origin(std::string file, FileOrigin origin): file(std::move(file)), origin(origin) {}
+        Origin(Pos::Origin origin): origin(origin) {}
     };
 
     struct Offset {
@@ -107,7 +116,7 @@ public:
             [] (const auto & a, const auto & b) { return a.idx < b.idx; });
         const auto origin = *std::prev(pastOrigin);
         const auto offset = offsets[idx];
-        return {origin.file, origin.origin, offset.line, offset.column};
+        return {offset.line, offset.column, origin.origin};
     }
 };
 
@@ -183,9 +192,13 @@ struct ExprString : Expr
 
 struct ExprPath : Expr
 {
-    std::string s;
+    const SourcePath path;
     Value v;
-    ExprPath(std::string s) : s(std::move(s)) { v.mkPath(this->s.c_str()); };
+    ExprPath(SourcePath && _path)
+        : path(_path)
+    {
+        v.mkPath(&*path.accessor, path.path.abs().data());
+    }
     Value * maybeThunk(EvalState & state, Env & env) override;
     COMMON_METHODS
 };

src/libexpr/parser.y

@@ -31,14 +31,9 @@ namespace nix {
         EvalState & state;
         SymbolTable & symbols;
         Expr * result;
-        Path basePath;
+        SourcePath basePath;
         PosTable::Origin origin;
         std::optional<ErrorInfo> error;
-        ParseData(EvalState & state, PosTable::Origin origin)
-            : state(state)
-            , symbols(state.symbols)
-            , origin(std::move(origin))
-            { };
     };
 
     struct ParserFormals {
@@ -515,11 +510,8 @@ string_parts_interpolated
 
 path_start
   : PATH {
-    Path path(absPath({$1.p, $1.l}, data->basePath));
-    /* add back in the trailing '/' to the first segment */
-    if ($1.p[$1.l-1] == '/' && $1.l > 1)
-      path += "/";
-    $$ = new ExprPath(path);
+    SourcePath path { data->basePath.accessor, CanonPath({$1.p, $1.l}, data->basePath.path) };
+    $$ = new ExprPath(std::move(path));
   }
   | HPATH {
     if (evalSettings.pureEval) {
@@ -529,7 +521,7 @@ path_start
         );
     }
     Path path(getHome() + std::string($1.p + 1, $1.l - 1));
-    $$ = new ExprPath(path);
+    $$ = new ExprPath(data->state.rootPath(path));
   }
   ;
 
@@ -644,30 +636,29 @@ formal
 #include "eval.hh"
 #include "filetransfer.hh"
 #include "fetchers.hh"
+#include "fs-input-accessor.hh"
+#include "tarball.hh"
 #include "store-api.hh"
+#include "flake/flake.hh"
 
 
 namespace nix {
 
 
-Expr * EvalState::parse(char * text, size_t length, FileOrigin origin,
-    const PathView path, const PathView basePath, std::shared_ptr<StaticEnv> & staticEnv)
+Expr * EvalState::parse(
+    char * text,
+    size_t length,
+    Pos::Origin origin,
+    const SourcePath & basePath,
+    std::shared_ptr<StaticEnv> & staticEnv)
 {
     yyscan_t scanner;
-    std::string file;
-    switch (origin) {
-        case foFile:
-            file = path;
-            break;
-        case foStdin:
-        case foString:
-            file = text;
-            break;
-        default:
-            assert(false);
-    }
-    ParseData data(*this, {file, origin});
-    data.basePath = basePath;
+    ParseData data {
+        .state = *this,
+        .symbols = symbols,
+        .basePath = basePath,
+        .origin = {origin},
+    };
 
     yylex_init(&scanner);
     yy_scan_buffer(text, length, scanner);
@@ -682,55 +673,43 @@ Expr * EvalState::parse(char * text, size_t length, FileOrigin origin,
 }
 
 
-Path resolveExprPath(Path path)
+SourcePath resolveExprPath(const SourcePath & path)
 {
-    assert(path[0] == '/');
-
-    unsigned int followCount = 0, maxFollow = 1024;
-
     /* If `path' is a symlink, follow it.  This is so that relative
        path references work. */
-    struct stat st;
-    while (true) {
-        // Basic cycle/depth limit to avoid infinite loops.
-        if (++followCount >= maxFollow)
-            throw Error("too many symbolic links encountered while traversing the path '%s'", path);
-        st = lstat(path);
-        if (!S_ISLNK(st.st_mode)) break;
-        path = absPath(readLink(path), dirOf(path));
-    }
+    auto path2 = path.resolveSymlinks();
 
     /* If `path' refers to a directory, append `/default.nix'. */
-    if (S_ISDIR(st.st_mode))
-        path = canonPath(path + "/default.nix");
+    if (path2.lstat().type == InputAccessor::tDirectory)
+        return path2 + "default.nix";
 
-    return path;
+    return path2;
 }
 
 
-Expr * EvalState::parseExprFromFile(const Path & path)
+Expr * EvalState::parseExprFromFile(const SourcePath & path)
 {
     return parseExprFromFile(path, staticBaseEnv);
 }
 
 
-Expr * EvalState::parseExprFromFile(const Path & path, std::shared_ptr<StaticEnv> & staticEnv)
+Expr * EvalState::parseExprFromFile(const SourcePath & path, std::shared_ptr<StaticEnv> & staticEnv)
 {
-    auto buffer = readFile(path);
-    // readFile should have left some extra space for terminators
+    auto buffer = path.readFile();
+    // readFile hopefully have left some extra space for terminators
     buffer.append("\0\0", 2);
-    return parse(buffer.data(), buffer.size(), foFile, path, dirOf(path), staticEnv);
+    return parse(buffer.data(), buffer.size(), Pos::Origin(path), path.parent(), staticEnv);
 }
 
 
-Expr * EvalState::parseExprFromString(std::string s, const Path & basePath, std::shared_ptr<StaticEnv> & staticEnv)
+Expr * EvalState::parseExprFromString(std::string s, const SourcePath & basePath, std::shared_ptr<StaticEnv> & staticEnv)
 {
     s.append("\0\0", 2);
-    return parse(s.data(), s.size(), foString, "", basePath, staticEnv);
+    return parse(s.data(), s.size(), Pos::string_tag(), basePath, staticEnv);
 }
 
 
-Expr * EvalState::parseExprFromString(std::string s, const Path & basePath)
+Expr * EvalState::parseExprFromString(std::string s, const SourcePath & basePath)
 {
     return parseExprFromString(std::move(s), basePath, staticBaseEnv);
 }
@@ -742,7 +721,7 @@ Expr * EvalState::parseStdin()
     auto buffer = drainFD(0);
     // drainFD should have left some extra space for terminators
     buffer.append("\0\0", 2);
-    return parse(buffer.data(), buffer.size(), foStdin, "", absPath("."), staticBaseEnv);
+    return parse(buffer.data(), buffer.size(), Pos::stdin_tag(), rootPath(absPath(".")), staticBaseEnv);
 }
 
 
@@ -762,13 +741,13 @@ void EvalState::addToSearchPath(const std::string & s)
 }
 
 
-Path EvalState::findFile(const std::string_view path)
+SourcePath EvalState::findFile(const std::string_view path)
 {
     return findFile(searchPath, path);
 }
 
 
-Path EvalState::findFile(SearchPath & searchPath, const std::string_view path, const PosIdx pos)
+SourcePath EvalState::findFile(SearchPath & searchPath, const std::string_view path, const PosIdx pos)
 {
     for (auto & i : searchPath) {
         std::string suffix;
@@ -781,14 +760,14 @@ Path EvalState::findFile(SearchPath & searchPath, const std::string_view path, c
                 continue;
             suffix = path.size() == s ? "" : concatStrings("/", path.substr(s));
         }
-        auto r = resolveSearchPathElem(i);
-        if (!r.first) continue;
-        Path res = r.second + suffix;
-        if (pathExists(res)) return canonPath(res);
+        if (auto path = resolveSearchPathElem(i)) {
+            auto res = *path + CanonPath(suffix);
+            if (res.pathExists()) return res;
+        }
     }
 
     if (hasPrefix(path, "nix/"))
-        return concatStrings(corepkgsPrefix, path.substr(4));
+        return {corepkgsFS, CanonPath(path.substr(3))};
 
     debugThrowLastTrace(ThrownError({
         .msg = hintfmt(evalSettings.pureEval
@@ -800,38 +779,63 @@ Path EvalState::findFile(SearchPath & searchPath, const std::string_view path, c
 }
 
 
-std::pair<bool, std::string> EvalState::resolveSearchPathElem(const SearchPathElem & elem)
+std::optional<SourcePath> EvalState::resolveSearchPathElem(const SearchPathElem & elem, bool initAccessControl)
 {
     auto i = searchPathResolved.find(elem.second);
     if (i != searchPathResolved.end()) return i->second;
 
-    std::pair<bool, std::string> res;
+    std::optional<SourcePath> res;
 
-    if (isUri(elem.second)) {
+    if (EvalSettings::isPseudoUrl(elem.second)) {
         try {
-            res = { true, store->toRealPath(fetchers::downloadTarball(
-                        store, resolveUri(elem.second), "source", false).first.storePath) };
+            auto storePath = fetchers::downloadTarball(
+                store, EvalSettings::resolvePseudoUrl(elem.second), "source", false).first;
+            auto accessor = makeStorePathAccessor(store, storePath);
+            registerAccessor(accessor);
+            res.emplace(accessor->root());
         } catch (FileTransferError & e) {
             logWarning({
                 .msg = hintfmt("Nix search path entry '%1%' cannot be downloaded, ignoring", elem.second)
             });
-            res = { false, "" };
         }
-    } else {
-        auto path = absPath(elem.second);
-        if (pathExists(path))
-            res = { true, path };
+    }
+
+    else if (hasPrefix(elem.second, "flake:")) {
+        auto flakeRef = parseFlakeRef(elem.second.substr(6), {}, true, false);
+        debug("fetching flake search path element '%s''", elem.second);
+        auto [accessor, _] = flakeRef.resolve(store).lazyFetch(store);
+        res.emplace(accessor->root());
+    }
+
+    else {
+        auto path = rootPath(absPath(elem.second));
+
+        /* Allow access to paths in the search path. */
+        if (initAccessControl) {
+            allowPath(path.path.abs());
+            if (store->isInStore(path.path.abs())) {
+                try {
+                    StorePathSet closure;
+                    store->computeFSClosure(store->toStorePath(path.path.abs()).first, closure);
+                    for (auto & p : closure)
+                        allowPath(p);
+                } catch (InvalidPath &) { }
+            }
+        }
+
+        if (path.pathExists())
+            res.emplace(path);
         else {
             logWarning({
                 .msg = hintfmt("Nix search path entry '%1%' does not exist, ignoring", elem.second)
             });
-            res = { false, "" };
         }
     }
 
-    debug(format("resolved search path element '%s' to '%s'") % elem.second % res.second);
+    if (res)
+        debug("resolved search path element '%s' to '%s'", elem.second, *res);
 
-    searchPathResolved[elem.second] = res;
+    searchPathResolved.emplace(elem.second, res);
     return res;
 }
 

src/libexpr/paths.cc

@@ -0,0 +1,96 @@
+#include "eval.hh"
+#include "util.hh"
+#include "fs-input-accessor.hh"
+
+namespace nix {
+
+SourcePath EvalState::rootPath(const Path & path)
+{
+    return {rootFS, CanonPath(path)};
+}
+
+void EvalState::registerAccessor(ref<InputAccessor> accessor)
+{
+    inputAccessors.emplace(accessor->number, accessor);
+}
+
+static constexpr std::string_view marker = "/__virtual__/";
+
+std::string EvalState::encodePath(const SourcePath & path)
+{
+    /* For backward compatibility, return paths in the root FS
+       normally. Encoding any other path is not very reproducible (due
+       to /__virtual__/<N>) and we should depreceate it eventually. So
+       print a warning about use of an encoded path in
+       decodePath(). */
+    return path.accessor == rootFS
+        ? path.path.abs()
+        : std::string(marker) + std::to_string(path.accessor->number) + path.path.abs();
+}
+
+SourcePath EvalState::decodePath(std::string_view s, PosIdx pos)
+{
+    if (!hasPrefix(s, "/"))
+        throwEvalError(pos, "string '%1%' doesn't represent an absolute path", s);
+
+    if (hasPrefix(s, marker)) {
+        auto fail = [s]() {
+            throw Error("cannot decode virtual path '%s'", s);
+        };
+
+        s = s.substr(marker.size());
+
+        try {
+            auto slash = s.find('/');
+            if (slash == std::string::npos) fail();
+            size_t number = std::stoi(std::string(s, 0, slash));
+            s = s.substr(slash);
+
+            auto accessor = inputAccessors.find(number);
+            if (accessor == inputAccessors.end()) fail();
+
+            SourcePath path {accessor->second, CanonPath(s)};
+
+            static bool warned = false;
+            warnOnce(warned, "applying 'toString' to path '%s' and then accessing it is deprecated, at %s", path, positions[pos]);
+
+            return path;
+        } catch (std::invalid_argument & e) {
+            fail();
+            abort();
+        }
+    } else
+        return {rootFS, CanonPath(s)};
+}
+
+std::string EvalState::decodePaths(std::string_view s)
+{
+    std::string res;
+
+    size_t pos = 0;
+
+    while (true) {
+        auto m = s.find(marker, pos);
+        if (m == s.npos) {
+            res.append(s.substr(pos));
+            return res;
+        }
+
+        res.append(s.substr(pos, m - pos));
+
+        auto end = s.find_first_of(" \n\r\t'\"’:", m);
+        if (end == s.npos) end = s.size();
+
+        try {
+            auto path = decodePath(s.substr(m, end - m), noPos);
+            res.append(path.to_string());
+        } catch (...) {
+            throw;
+            res.append(s.substr(pos, end - m));
+        }
+
+        pos = end;
+    }
+}
+
+}

src/libexpr/primops.cc

@@ -11,6 +11,7 @@
 #include "value-to-json.hh"
 #include "value-to-xml.hh"
 #include "primops.hh"
+#include "fs-input-accessor.hh"
 
 #include <boost/container/small_vector.hpp>
 
@@ -83,7 +84,7 @@ StringMap EvalState::realiseContext(const PathSet & context)
 
     /* Add the output of this derivations to the allowed
        paths. */
-    if (allowedPaths) {
+    if (rootFS->hasAccessControl()) {
         for (auto & [_placeholder, outputPath] : res) {
             allowPath(store->toRealPath(outputPath));
         }
@@ -92,12 +93,13 @@ StringMap EvalState::realiseContext(const PathSet & context)
     return res;
 }
 
+// FIXME: remove?
 struct RealisePathFlags {
     // Whether to check that the path is allowed in pure eval mode
     bool checkForPureEval = true;
 };
 
-static Path realisePath(EvalState & state, const PosIdx pos, Value & v, const RealisePathFlags flags = {})
+static SourcePath realisePath(EvalState & state, const PosIdx pos, Value & v, const RealisePathFlags flags = {})
 {
     PathSet context;
 
@@ -112,13 +114,12 @@ static Path realisePath(EvalState & state, const PosIdx pos, Value & v, const Re
     }();
 
     try {
-        StringMap rewrites = state.realiseContext(context);
-
-        auto realPath = state.toRealPath(rewriteStrings(path, rewrites), context);
-
-        return flags.checkForPureEval
-            ? state.checkSourcePath(realPath)
-            : realPath;
+        if (!context.empty()) {
+            auto rewrites = state.realiseContext(context);
+            auto realPath = state.toRealPath(rewriteStrings(path.path.abs(), rewrites), context);
+            return {path.accessor, CanonPath(realPath)};
+        } else
+            return path;
     } catch (Error & e) {
         e.addTrace(state.positions[pos], "while realising the context of path '%s'", path);
         throw;
@@ -162,6 +163,7 @@ static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * v
 {
     auto path = realisePath(state, pos, vPath);
 
+#if 0
     // FIXME
     auto isValidDerivationInStore = [&]() -> std::optional<StorePath> {
         if (!state.store->isStorePath(path))
@@ -201,13 +203,9 @@ static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * v
         state.forceAttrs(v, pos);
     }
 
-    else if (path == corepkgsPrefix + "fetchurl.nix") {
-        state.eval(state.parseExprFromString(
-            #include "fetchurl.nix.gen.hh"
-            , "/"), v);
-    }
-
-    else {
+    else
+#endif
+    {
         if (!vScope)
             state.evalFile(path, v);
         else {
@@ -313,6 +311,9 @@ extern "C" typedef void (*ValueInitializer)(EvalState & state, Value & v);
 /* Load a ValueInitializer from a DSO and return whatever it initializes */
 void prim_importNative(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
+    throw UnimplementedError("importNative");
+
+    #if 0
     auto path = realisePath(state, pos, *args[0]);
 
     std::string sym(state.forceStringNoCtx(*args[1], pos));
@@ -334,6 +335,7 @@ void prim_importNative(EvalState & state, const PosIdx pos, Value * * args, Valu
     (func)(state, v);
 
     /* We don't dlclose because v may be a primop referencing a function in the shared object file */
+    #endif
 }
 
 
@@ -367,8 +369,7 @@ void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v)
     auto output = runProgram(program, true, commandArgs);
     Expr * parsed;
     try {
-        auto base = state.positions[pos];
-        parsed = state.parseExprFromString(std::move(output), base.file);
+        parsed = state.parseExprFromString(std::move(output), state.rootPath("/"));
     } catch (Error & e) {
         e.addTrace(state.positions[pos], "While parsing the output from '%1%'", program);
         throw;
@@ -554,7 +555,8 @@ struct CompareValues
             case nString:
                 return strcmp(v1->string.s, v2->string.s) < 0;
             case nPath:
-                return strcmp(v1->path, v2->path) < 0;
+                // FIXME: handle accessor?
+                return strcmp(v1->_path.path, v2->_path.path) < 0;
             case nList:
                 // Lexicographic comparison
                 for (size_t i = 0;; i++) {
@@ -767,7 +769,7 @@ static RegisterPrimOp primop_abort({
     .fun = [](EvalState & state, const PosIdx pos, Value * * args, Value & v)
     {
         PathSet context;
-        auto s = state.coerceToString(pos, *args[0], context).toOwned();
+        auto s = state.decodePaths(*state.coerceToString(pos, *args[0], context));
         state.debugThrowLastTrace(Abort("evaluation aborted with the following error message: '%1%'", s));
     }
 });
@@ -785,7 +787,7 @@ static RegisterPrimOp primop_throw({
     .fun = [](EvalState & state, const PosIdx pos, Value * * args, Value & v)
     {
       PathSet context;
-      auto s = state.coerceToString(pos, *args[0], context).toOwned();
+      auto s = state.decodePaths(*state.coerceToString(pos, *args[0], context));
       state.debugThrowLastTrace(ThrownError(s));
     }
 });
@@ -797,7 +799,7 @@ static void prim_addErrorContext(EvalState & state, const PosIdx pos, Value * *
         v = *args[1];
     } catch (Error & e) {
         PathSet context;
-        e.addTrace(std::nullopt, state.coerceToString(pos, *args[0], context).toOwned());
+        e.addTrace(nullptr, state.decodePaths(*state.coerceToString(pos, *args[0], context)));
         throw;
     }
 }
@@ -1388,12 +1390,13 @@ static RegisterPrimOp primop_placeholder({
  *************************************************************/
 
 
-/* Convert the argument to a path.  !!! obsolete? */
+/* Convert the argument to a path and then to a string (confusing,
+   eh?).  !!! obsolete? */
 static void prim_toPath(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    Path path = state.coerceToPath(pos, *args[0], context);
-    v.mkString(canonPath(path), context);
+    auto path = state.coerceToPath(pos, *args[0], context);
+    v.mkString(path.path.abs(), context);
 }
 
 static RegisterPrimOp primop_toPath({
@@ -1423,21 +1426,23 @@ static void prim_storePath(EvalState & state, const PosIdx pos, Value * * args,
         }));
 
     PathSet context;
-    Path path = state.checkSourcePath(state.coerceToPath(pos, *args[0], context));
+    // FIXME: check rootPath
+    auto path = state.coerceToPath(pos, *args[0], context).path;
     /* Resolve symlinks in ‘path’, unless ‘path’ itself is a symlink
        directly in the store.  The latter condition is necessary so
        e.g. nix-push does the right thing. */
-    if (!state.store->isStorePath(path)) path = canonPath(path, true);
-    if (!state.store->isInStore(path))
+    if (!state.store->isStorePath(path.abs()))
+        path = CanonPath(canonPath(path.abs(), true));
+    if (!state.store->isInStore(path.abs()))
         state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("path '%1%' is not in the Nix store", path),
             .errPos = state.positions[pos]
         }));
-    auto path2 = state.store->toStorePath(path).first;
+    auto path2 = state.store->toStorePath(path.abs()).first;
     if (!settings.readOnlyMode)
         state.store->ensurePath(path2);
     context.insert(state.store->printStorePath(path2));
-    v.mkString(path, context);
+    v.mkString(path.abs(), context);
 }
 
 static RegisterPrimOp primop_storePath({
@@ -1468,7 +1473,7 @@ static void prim_pathExists(EvalState & state, const PosIdx pos, Value * * args,
     auto path = realisePath(state, pos, *args[0], { .checkForPureEval = false });
 
     try {
-        v.mkBool(pathExists(state.checkSourcePath(path)));
+        v.mkBool(path.pathExists());
     } catch (SysError & e) {
         /* Don't give away info from errors while canonicalising
            ‘path’ in restricted mode. */
@@ -1513,9 +1518,15 @@ static RegisterPrimOp primop_baseNameOf({
 static void prim_dirOf(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
+    state.forceValue(*args[0], pos);
+    if (args[0]->type() == nPath) {
+        auto path = args[0]->path();
+        v.mkPath(path.parent());
+    } else {
         auto path = state.coerceToString(pos, *args[0], context, false, false);
         auto dir = dirOf(*path);
-    if (args[0]->type() == nPath) v.mkPath(dir); else v.mkString(dir, context);
+        v.mkString(dir, context);
+    }
 }
 
 static RegisterPrimOp primop_dirOf({
@@ -1533,16 +1544,14 @@ static RegisterPrimOp primop_dirOf({
 static void prim_readFile(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     auto path = realisePath(state, pos, *args[0]);
-    auto s = readFile(path);
+    auto s = path.readFile();
     if (s.find((char) 0) != std::string::npos)
         state.debugThrowLastTrace(Error("the contents of the file '%1%' cannot be represented as a Nix string", path));
-    StorePathSet refs;
-    if (state.store->isInStore(path)) {
-        try {
-            refs = state.store->queryPathInfo(state.store->toStorePath(path).first)->references;
-        } catch (Error &) { // FIXME: should be InvalidPathError
-        }
-    }
+    // FIXME: only do queryPathInfo if path.accessor is the store accessor
+    auto refs =
+        state.store->isInStore(path.path.abs()) ?
+        state.store->queryPathInfo(state.store->toStorePath(path.path.abs()).first)->references :
+        StorePathSet{};
     auto context = state.store->printStorePathSet(refs);
     v.mkString(s, context);
 }
@@ -1598,7 +1607,7 @@ static void prim_findFile(EvalState & state, const PosIdx pos, Value * * args, V
 
     auto path = state.forceStringNoCtx(*args[1], pos);
 
-    v.mkPath(state.checkSourcePath(state.findFile(searchPath, path, pos)));
+    v.mkPath(state.findFile(searchPath, path, pos));
 }
 
 static RegisterPrimOp primop_findFile(RegisterPrimOp::Info {
@@ -1620,7 +1629,8 @@ static void prim_hashFile(EvalState & state, const PosIdx pos, Value * * args, V
 
     auto path = realisePath(state, pos, *args[1]);
 
-    v.mkString(hashFile(*ht, path).to_string(Base16, false));
+    // FIXME: state.toRealPath(path, context)
+    v.mkString(hashString(*ht, path.readFile()).to_string(Base16, false));
 }
 
 static RegisterPrimOp primop_hashFile({
@@ -1634,23 +1644,30 @@ static RegisterPrimOp primop_hashFile({
     .fun = prim_hashFile,
 });
 
+static std::string_view fileTypeToString(InputAccessor::Type type)
+{
+    return
+        type == InputAccessor::Type::tRegular ? "regular" :
+        type == InputAccessor::Type::tDirectory ? "directory" :
+        type == InputAccessor::Type::tSymlink ? "symlink" :
+        "unknown";
+}
+
 /* Read a directory (without . or ..) */
 static void prim_readDir(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     auto path = realisePath(state, pos, *args[0]);
 
-    DirEntries entries = readDirectory(path);
-
+    auto entries = path.readDirectory();
     auto attrs = state.buildBindings(entries.size());
 
-    for (auto & ent : entries) {
-        if (ent.type == DT_UNKNOWN)
-            ent.type = getFileType(path + "/" + ent.name);
-        attrs.alloc(ent.name).mkString(
-            ent.type == DT_REG ? "regular" :
-            ent.type == DT_DIR ? "directory" :
-            ent.type == DT_LNK ? "symlink" :
-            "unknown");
+    for (auto & [name, type] : entries) {
+        #if 0
+        // FIXME?
+        if (type == InputAccessor::Type::Misc)
+            type = getFileType(path + "/" + name);
+        #endif
+        attrs.alloc(name).mkString(fileTypeToString(type.value_or(InputAccessor::Type::tMisc)));
     }
 
     v.mkAttrs(attrs);
@@ -1956,8 +1973,8 @@ static RegisterPrimOp primop_toFile({
 static void addPath(
     EvalState & state,
     const PosIdx pos,
-    const std::string & name,
-    Path path,
+    std::string_view name,
+    const SourcePath & path,
     Value * filterFun,
     FileIngestionMethod method,
     const std::optional<Hash> expectedHash,
@@ -1965,13 +1982,18 @@ static void addPath(
     const PathSet & context)
 {
     try {
+        // FIXME
+        #if 0
         // FIXME: handle CA derivation outputs (where path needs to
         // be rewritten to the actual output).
         auto rewrites = state.realiseContext(context);
         path = state.toRealPath(rewriteStrings(path, rewrites), context);
+        #endif
 
         StorePathSet refs;
 
+        // FIXME
+        #if 0
         if (state.store->isInStore(path)) {
             try {
                 auto [storePath, subPath] = state.store->toStorePath(path);
@@ -1981,41 +2003,43 @@ static void addPath(
             } catch (Error &) { // FIXME: should be InvalidPathError
             }
         }
+        #endif
 
-        path = evalSettings.pureEval && expectedHash
-            ? path
-            : state.checkSourcePath(path);
+        std::unique_ptr<PathFilter> filter;
+        if (filterFun) filter = std::make_unique<PathFilter>([&](const Path & p) {
+            SourcePath path2{path.accessor, CanonPath(p)};
 
-        PathFilter filter = filterFun ? ([&](const Path & path) {
-            auto st = lstat(path);
+            auto st = path2.lstat();
 
             /* Call the filter function.  The first argument is the path,
                the second is a string indicating the type of the file. */
             Value arg1;
-            arg1.mkString(path);
+            arg1.mkString(path2.path.abs());
 
             Value arg2;
-            arg2.mkString(
-                S_ISREG(st.st_mode) ? "regular" :
-                S_ISDIR(st.st_mode) ? "directory" :
-                S_ISLNK(st.st_mode) ? "symlink" :
-                "unknown" /* not supported, will fail! */);
+            // assert that type is not "unknown"
+            arg2.mkString(fileTypeToString(st.type));
 
             Value * args []{&arg1, &arg2};
             Value res;
             state.callFunction(*filterFun, 2, args, res, pos);
 
             return state.forceBool(res, pos);
-        }) : defaultPathFilter;
+        });
 
         std::optional<StorePath> expectedStorePath;
         if (expectedHash)
             expectedStorePath = state.store->makeFixedOutputPath(method, *expectedHash, name);
 
+        // FIXME: instead of a store path, we could return a
+        // SourcePath that applies the filter lazily and copies to the
+        // store on-demand.
+
         if (!expectedHash || !state.store->isValidPath(*expectedStorePath)) {
-            StorePath dstPath = settings.readOnlyMode
-                ? state.store->computeStorePathForPath(name, path, method, htSHA256, filter).first
-                : state.store->addToStore(name, path, method, htSHA256, filter, state.repair, refs);
+            // FIXME
+            if (method != FileIngestionMethod::Recursive)
+                throw Error("'recursive = false' is not implemented");
+            auto dstPath = path.fetchToStore(state.store, name, filter.get(), state.repair);
             if (expectedHash && expectedStorePath != dstPath)
                 state.debugThrowLastTrace(Error("store path mismatch in (possibly filtered) path added from '%s'", path));
             state.allowAndSetStorePathString(dstPath, v);
@@ -2031,7 +2055,7 @@ static void addPath(
 static void prim_filterSource(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    Path path = state.coerceToPath(pos, *args[1], context);
+    auto path = state.coerceToPath(pos, *args[1], context);
 
     state.forceValue(*args[0], pos);
     if (args[0]->type() != nFunction)
@@ -2042,7 +2066,7 @@ static void prim_filterSource(EvalState & state, const PosIdx pos, Value * * arg
             .errPos = state.positions[pos]
         }));
 
-    addPath(state, pos, std::string(baseNameOf(path)), path, args[0], FileIngestionMethod::Recursive, std::nullopt, v, context);
+    addPath(state, pos, path.baseName(), path, args[0], FileIngestionMethod::Recursive, std::nullopt, v, context);
 }
 
 static RegisterPrimOp primop_filterSource({
@@ -2103,7 +2127,7 @@ static RegisterPrimOp primop_filterSource({
 static void prim_path(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     state.forceAttrs(*args[0], pos);
-    Path path;
+    std::optional<SourcePath> path;
     std::string name;
     Value * filterFun = nullptr;
     auto method = FileIngestionMethod::Recursive;
@@ -2113,7 +2137,7 @@ static void prim_path(EvalState & state, const PosIdx pos, Value * * args, Value
     for (auto & attr : *args[0]->attrs) {
         auto n = state.symbols[attr.name];
         if (n == "path")
-            path = state.coerceToPath(attr.pos, *attr.value, context);
+            path.emplace(state.coerceToPath(attr.pos, *attr.value, context));
         else if (attr.name == state.sName)
             name = state.forceStringNoCtx(*attr.value, attr.pos);
         else if (n == "filter") {
@@ -2129,15 +2153,15 @@ static void prim_path(EvalState & state, const PosIdx pos, Value * * args, Value
                 .errPos = state.positions[attr.pos]
             }));
     }
-    if (path.empty())
+    if (!path)
         state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("'path' required"),
             .errPos = state.positions[pos]
         }));
     if (name.empty())
-        name = baseNameOf(path);
+        name = path->baseName();
 
-    addPath(state, pos, name, path, filterFun, method, expectedHash, v, context);
+    addPath(state, pos, name, *path, filterFun, method, expectedHash, v, context);
 }
 
 static RegisterPrimOp primop_path({
@@ -3994,7 +4018,6 @@ void EvalState::createBaseEnv()
 
     /* Add a wrapper around the derivation primop that computes the
        `drvPath' and `outPath' attributes lazily. */
-    sDerivationNix = symbols.create(derivationNixPath);
     auto vDerivation = allocValue();
     addConstant("derivation", vDerivation);
 
@@ -4006,12 +4029,7 @@ void EvalState::createBaseEnv()
 
     /* Note: we have to initialize the 'derivation' constant *after*
        building baseEnv/staticBaseEnv because it uses 'builtins'. */
-    char code[] =
-        #include "primops/derivation.nix.gen.hh"
-        // the parser needs two NUL bytes as terminators; one of them
-        // is implied by being a C string.
-        "\0";
-    eval(parse(code, sizeof(code), foFile, derivationNixPath, "/", staticBaseEnv), *vDerivation);
+    evalFile(derivationInternal, *vDerivation);
 }
 
 

src/libexpr/primops/fetchMercurial.cc

@@ -68,11 +68,11 @@ static void prim_fetchMercurial(EvalState & state, const PosIdx pos, Value * * a
     auto input = fetchers::Input::fromAttrs(std::move(attrs));
 
     // FIXME: use name
-    auto [tree, input2] = input.fetch(state.store);
+    auto [storePath, input2] = input.fetchToStore(state.store);
 
     auto attrs2 = state.buildBindings(8);
-    auto storePath = state.store->printStorePath(tree.storePath);
-    attrs2.alloc(state.sOutPath).mkString(storePath, {storePath});
+    auto storePath2 = state.store->printStorePath(storePath);
+    attrs2.alloc(state.sOutPath).mkString(storePath2, {storePath2});
     if (input2.getRef())
         attrs2.alloc("branch").mkString(*input2.getRef());
     // Backward compatibility: set 'rev' to
@@ -84,7 +84,7 @@ static void prim_fetchMercurial(EvalState & state, const PosIdx pos, Value * * a
         attrs2.alloc("revCount").mkInt(*revCount);
     v.mkAttrs(attrs2);
 
-    state.allowPath(tree.storePath);
+    state.allowPath(storePath);
 }
 
 static RegisterPrimOp r_fetchMercurial("fetchMercurial", 1, prim_fetchMercurial);

src/libexpr/primops/fetchTree.cc

@@ -4,6 +4,7 @@
 #include "fetchers.hh"
 #include "filetransfer.hh"
 #include "registry.hh"
+#include "tarball.hh"
 
 #include <ctime>
 #include <iomanip>
@@ -11,26 +12,21 @@
 
 namespace nix {
 
-void emitTreeAttrs(
+static void emitTreeAttrs(
     EvalState & state,
-    const fetchers::Tree & tree,
     const fetchers::Input & input,
     Value & v,
+    std::function<void(Value &)> setOutPath,
     bool emptyRevFallback,
     bool forceDirty)
 {
-    assert(input.isLocked());
-
     auto attrs = state.buildBindings(8);
 
-    auto storePath = state.store->printStorePath(tree.storePath);
-
-    attrs.alloc(state.sOutPath).mkString(storePath, {storePath});
+    setOutPath(attrs.alloc(state.sOutPath));
 
     // FIXME: support arbitrary input attributes.
 
-    auto narHash = input.getNarHash();
-    assert(narHash);
+    if (auto narHash = input.getNarHash())
         attrs.alloc("narHash").mkString(narHash->to_string(SRI, true));
 
     if (input.getType() == "git")
@@ -65,6 +61,22 @@ void emitTreeAttrs(
     v.mkAttrs(attrs);
 }
 
+void emitTreeAttrs(
+    EvalState & state,
+    const SourcePath & path,
+    const fetchers::Input & input,
+    Value & v,
+    bool emptyRevFallback,
+    bool forceDirty)
+{
+    emitTreeAttrs(state, input, v,
+        [&](Value & vOutPath) {
+            vOutPath.mkPath(path);
+        },
+        emptyRevFallback,
+        forceDirty);
+}
+
 std::string fixURI(std::string uri, EvalState & state, const std::string & defaultScheme = "file")
 {
     state.checkURI(uri);
@@ -86,6 +98,7 @@ std::string fixURIForGit(std::string uri, EvalState & state)
 struct FetchTreeParams {
     bool emptyRevFallback = false;
     bool allowNameArgument = false;
+    bool returnPath = true; // whether to return a lazily fetched SourcePath or a StorePath
 };
 
 static void fetchTree(
@@ -123,7 +136,9 @@ static void fetchTree(
 
         for (auto & attr : *args[0]->attrs) {
             if (attr.name == state.sType) continue;
+
             state.forceValue(*attr.value, attr.pos);
+
             if (attr.value->type() == nPath || attr.value->type() == nString) {
                 auto s = state.coerceToString(attr.pos, *attr.value, context, false, false).toOwned();
                 attrs.emplace(state.symbols[attr.name],
@@ -169,11 +184,33 @@ static void fetchTree(
     if (evalSettings.pureEval && !input.isLocked())
         state.debugThrowLastTrace(EvalError("in pure evaluation mode, 'fetchTree' requires a locked input, at %s", state.positions[pos]));
 
-    auto [tree, input2] = input.fetch(state.store);
+    if (params.returnPath) {
+        auto [accessor, input2] = input.getAccessor(state.store);
 
-    state.allowPath(tree.storePath);
+        state.registerAccessor(accessor);
 
-    emitTreeAttrs(state, tree, input2, v, params.emptyRevFallback, false);
+        emitTreeAttrs(
+            state,
+            { accessor, CanonPath::root },
+            input2,
+            v,
+            params.emptyRevFallback,
+            false);
+    } else {
+        auto [storePath, input2] = input.fetchToStore(state.store);
+
+        auto storePath2 = state.store->printStorePath(storePath);
+
+        emitTreeAttrs(
+            state, input2, v,
+            [&](Value & vOutPath) {
+                vOutPath.mkString(storePath2, {storePath2});
+            },
+            params.emptyRevFallback,
+            false);
+
+        state.allowPath(storePath);
+    }
 }
 
 static void prim_fetchTree(EvalState & state, const PosIdx pos, Value * * args, Value & v)
@@ -220,8 +257,6 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
     } else
         url = state.forceStringNoCtx(*args[0], pos);
 
-    url = resolveUri(*url);
-
     state.checkURI(*url);
 
     if (name == "")
@@ -247,7 +282,7 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
     //       https://github.com/NixOS/nix/issues/4313
     auto storePath =
         unpack
-        ? fetchers::downloadTarball(state.store, *url, name, (bool) expectedHash).first.storePath
+        ? fetchers::downloadTarball(state.store, *url, name, (bool) expectedHash).first
         : fetchers::downloadFile(state.store, *url, name, (bool) expectedHash).storePath;
 
     if (expectedHash) {
@@ -331,7 +366,13 @@ static RegisterPrimOp primop_fetchTarball({
 
 static void prim_fetchGit(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    fetchTree(state, pos, args, v, "git", FetchTreeParams { .emptyRevFallback = true, .allowNameArgument = true });
+    fetchTree(
+        state, pos, args, v, "git",
+        FetchTreeParams {
+            .emptyRevFallback = true,
+            .allowNameArgument = true,
+            .returnPath = false,
+        });
 }
 
 static RegisterPrimOp primop_fetchGit({

src/libexpr/primops/patch.cc

@@ -0,0 +1,126 @@
+#include "primops.hh"
+
+namespace nix {
+
+static void prim_patch(EvalState & state, const PosIdx pos, Value * * args, Value & v)
+{
+    std::vector<std::string> patches;
+    std::optional<SourcePath> src;
+
+    state.forceAttrs(*args[0], pos);
+
+    for (auto & attr : *args[0]->attrs) {
+        std::string_view n(state.symbols[attr.name]);
+
+        auto check = [&]()
+        {
+            if (!patches.empty())
+                state.debugThrowLastTrace(EvalError({
+                    .msg = hintfmt("'builtins.patch' does not support both 'patches' and 'patchFiles'"),
+                    .errPos = state.positions[attr.pos]
+                }));
+        };
+
+        if (n == "src") {
+            PathSet context;
+            src.emplace(state.coerceToPath(pos, *attr.value, context));
+        }
+
+        else if (n == "patchFiles") {
+            check();
+            state.forceList(*attr.value, attr.pos);
+            for (auto elem : attr.value->listItems()) {
+                // FIXME: use realisePath
+                PathSet context;
+                auto patchFile = state.coerceToPath(attr.pos, *elem, context);
+                patches.push_back(patchFile.readFile());
+            }
+        }
+
+        else if (n == "patches") {
+            check();
+            state.forceList(*attr.value, attr.pos);
+            for (auto elem : attr.value->listItems())
+                patches.push_back(std::string(state.forceStringNoCtx(*elem, attr.pos)));
+        }
+
+        else
+            throw Error({
+                .msg = hintfmt("attribute '%s' isn't supported in call to 'builtins.patch'", n),
+                .errPos = state.positions[pos]
+            });
+    }
+
+    if (!src)
+        state.debugThrowLastTrace(EvalError({
+            .msg = hintfmt("attribute 'src' is missing in call to 'builtins.patch'"),
+            .errPos = state.positions[pos]
+        }));
+
+    if (!src->path.isRoot())
+        throw UnimplementedError("applying patches to a non-root path ('%s') is not yet supported", src->path);
+
+    auto accessor = makePatchingInputAccessor(src->accessor, patches);
+
+    state.registerAccessor(accessor);
+
+    v.mkPath(SourcePath{accessor, src->path});
+}
+
+static RegisterPrimOp primop_patch({
+    .name = "__patch",
+    .args = {"args"},
+    .doc = R"(
+      Apply patches to a source tree. This function has the following required argument:
+
+        - src\
+          The input source tree.
+
+      It also takes one of the following:
+
+        - patchFiles\
+          A list of patch files to be applied to `src`.
+
+        - patches\
+          A list of patches (i.e. strings) to be applied to `src`.
+
+      It returns a source tree that lazily and non-destructively
+      applies the specified patches to `src`.
+
+      Example:
+
+      ```nix
+      let
+        tree = builtins.patch {
+          src = fetchTree {
+            type = "github";
+            owner = "NixOS";
+            repo = "patchelf";
+            rev = "be0cc30a59b2755844bcd48823f6fbc8d97b93a7";
+          };
+          patches = [
+            ''
+              diff --git a/src/patchelf.cc b/src/patchelf.cc
+              index 6882b28..28f511c 100644
+              --- a/src/patchelf.cc
+              +++ b/src/patchelf.cc
+              @@ -1844,6 +1844,8 @@ void showHelp(const std::string & progName)
+
+               int mainWrapped(int argc, char * * argv)
+               {
+              +    printf("Hello!");
+              +
+                   if (argc <= 1) {
+                       showHelp(argv[0]);
+                       return 1;
+
+            ''
+          ];
+        };
+      in builtins.readFile (tree + "/src/patchelf.cc")
+      ```
+    )",
+    .fun = prim_patch,
+});
+
+}

src/libexpr/tests/json.cc

@@ -62,7 +62,7 @@ namespace nix {
     // not supported by store 'dummy'" thrown in the test body.
     TEST_F(JSONValueTest, DISABLED_Path) {
         Value v;
-        v.mkPath("test");
+        v.mkPath(state.rootPath("/test"));
         ASSERT_EQ(getJSONValue(v), "\"/nix/store/g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x\"");
     }
 } /* namespace nix */

src/libexpr/tests/libexprtests.hh

@@ -23,7 +23,7 @@ namespace nix {
             }
             Value eval(std::string input, bool forceValue = true) {
                 Value v;
-                Expr * e = state.parseExprFromString(input, "");
+                Expr * e = state.parseExprFromString(input, state.rootPath("/"));
                 assert(e);
                 state.eval(e, v);
                 if (forceValue)
@@ -102,10 +102,13 @@ namespace nix {
         if (arg.type() != nPath) {
             *result_listener << "Expected a path got " << arg.type();
             return false;
-            } else if (std::string_view(arg.string.s) != p) {
-                *result_listener << "Expected a path that equals \"" << p << "\" but got: " << arg.string.s;
+        } else {
+            auto path = arg.path();
+            if (path.path != CanonPath(p)) {
+                *result_listener << "Expected a path that equals \"" << p << "\" but got: " << path.path;
                 return false;
             }
+        }
         return true;
     }
 

src/libexpr/tests/local.mk

@@ -8,7 +8,7 @@ libexpr-tests_INSTALL_DIR :=
 
 libexpr-tests_SOURCES := $(wildcard $(d)/*.cc)
 
-libexpr-tests_CXXFLAGS += -I src/libexpr -I src/libutil -I src/libstore -I src/libexpr/tests
+libexpr-tests_CXXFLAGS += -I src/libexpr -I src/libutil -I src/libstore -I src/libexpr/tests -I src/libfetchers
 
 libexpr-tests_LIBS = libexpr libutil libstore libfetchers
 

src/libexpr/tests/primops.cc

@@ -148,15 +148,17 @@ namespace nix {
     }
 
     TEST_F(PrimOpTest, unsafeGetAttrPos) {
-        // The `y` attribute is at position
-        const char* expr = "builtins.unsafeGetAttrPos \"y\" { y = \"x\"; }";
+        state.corepkgsFS->addFile(CanonPath("foo.nix"), "{ y = \"x\"; }");
+
+        auto expr = "builtins.unsafeGetAttrPos \"y\" (import <nix/foo.nix>)";
         auto v = eval(expr);
         ASSERT_THAT(v, IsAttrsOfSize(3));
 
         auto file = v.attrs->find(createSymbol("file"));
         ASSERT_NE(file, nullptr);
-        // FIXME: The file when running these tests is the input string?!?
-        ASSERT_THAT(*file->value, IsStringEq(expr));
+        ASSERT_THAT(*file->value, IsString());
+        auto s = baseNameOf(file->value->string.s);
+        ASSERT_EQ(s, "foo.nix");
 
         auto line = v.attrs->find(createSymbol("line"));
         ASSERT_NE(line, nullptr);
@@ -164,7 +166,7 @@ namespace nix {
 
         auto column = v.attrs->find(createSymbol("column"));
         ASSERT_NE(column, nullptr);
-        ASSERT_THAT(*column->value, IsIntEq(33));
+        ASSERT_THAT(*column->value, IsIntEq(3));
     }
 
     TEST_F(PrimOpTest, hasAttr) {

src/libexpr/value-to-json.cc

@@ -2,6 +2,7 @@
 #include "json.hh"
 #include "eval-inline.hh"
 #include "util.hh"
+#include "store-api.hh"
 
 #include <cstdlib>
 #include <iomanip>
@@ -33,9 +34,11 @@ void printValueAsJSON(EvalState & state, bool strict,
 
         case nPath:
             if (copyToStore)
-                out.write(state.copyPathToStore(context, v.path));
+                out.write(
+                    state.store->printStorePath(
+                        state.copyPathToStore(context, v.path())));
             else
-                out.write(v.path);
+                out.write(v.path().path.abs());
             break;
 
         case nNull:

src/libexpr/value-to-xml.cc

@@ -24,7 +24,8 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
 
 static void posToXML(EvalState & state, XMLAttrs & xmlAttrs, const Pos & pos)
 {
-    xmlAttrs["path"] = pos.file;
+    if (auto path = std::get_if<SourcePath>(&pos.origin))
+        xmlAttrs["path"] = path->path.abs();
     xmlAttrs["line"] = (format("%1%") % pos.line).str();
     xmlAttrs["column"] = (format("%1%") % pos.column).str();
 }
@@ -77,7 +78,7 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
             break;
 
         case nPath:
-            doc.writeEmptyElement("path", singletonAttrs("value", v.path));
+            doc.writeEmptyElement("path", singletonAttrs("value", v.path().to_string()));
             break;
 
         case nNull:

src/libexpr/value.hh

@@ -3,6 +3,7 @@
 #include <cassert>
 
 #include "symbol-table.hh"
+#include "input-accessor.hh"
 
 #if HAVE_BOEHMGC
 #include <gc/gc_allocator.h>
@@ -171,7 +172,11 @@ public:
             const char * * context; // must be in sorted order
         } string;
 
+        struct {
+            InputAccessor * accessor;
             const char * path;
+        } _path;
+
         Bindings * attrs;
         struct {
             size_t size;
@@ -251,15 +256,21 @@ public:
 
     void mkStringMove(const char * s, const PathSet & context);
 
-    inline void mkPath(const char * s)
+    inline void mkString(const Symbol & s)
+    {
+        mkString(((const std::string &) s).c_str());
+    }
+
+    void mkPath(const SourcePath & path);
+
+    inline void mkPath(InputAccessor * accessor, const char * path)
     {
         clearValue();
         internalType = tPath;
-        path = s;
+        _path.accessor = accessor;
+        _path.path = path;
     }
 
-    void mkPath(std::string_view s);
-
     inline void mkNull()
     {
         clearValue();
@@ -400,6 +411,21 @@ public:
         auto begin = listElems();
         return ConstListIterable { begin, begin + listSize() };
     }
+
+    SourcePath path() const
+    {
+        assert(internalType == tPath);
+        return SourcePath {
+            .accessor = ref(_path.accessor->shared_from_this()),
+            .path = CanonPath(CanonPath::unchecked_t(), _path.path)
+        };
+    }
+
+    std::string_view str() const
+    {
+        assert(internalType == tString);
+        return std::string_view(string.s);
+    }
 };
 
 

src/libfetchers/cache.cc

@@ -17,14 +17,23 @@ create table if not exists Cache (
     timestamp integer not null,
     primary key (input)
 );
+
+create table if not exists Facts (
+    name     text not null,
+    value    text not null,
+    primary key (name)
+);
 )sql";
 
+// FIXME: we should periodically purge/nuke this cache to prevent it
+// from growing too big.
+
 struct CacheImpl : Cache
 {
     struct State
     {
         SQLite db;
-        SQLiteStmt add, lookup;
+        SQLiteStmt add, lookup, upsertFact, queryFact;
     };
 
     Sync<State> _state;
@@ -33,7 +42,7 @@ struct CacheImpl : Cache
     {
         auto state(_state.lock());
 
-        auto dbPath = getCacheDir() + "/nix/fetcher-cache-v1.sqlite";
+        auto dbPath = getCacheDir() + "/nix/fetcher-cache-v2.sqlite";
         createDirs(dirOf(dbPath));
 
         state->db = SQLite(dbPath);
@@ -45,6 +54,12 @@ struct CacheImpl : Cache
 
         state->lookup.create(state->db,
             "select info, path, immutable, timestamp from Cache where input = ?");
+
+        state->upsertFact.create(state->db,
+            "insert or replace into Facts(name, value) values (?, ?)");
+
+        state->queryFact.create(state->db,
+            "select value from Facts where name = ?");
     }
 
     void add(
@@ -110,6 +125,26 @@ struct CacheImpl : Cache
             .storePath = std::move(storePath)
         };
     }
+
+    void upsertFact(
+        std::string_view key,
+        std::string_view value) override
+    {
+        debug("upserting fact '%s' -> '%s'", key, value);
+        _state.lock()->upsertFact.use()
+            (key)
+            (value).exec();
+    }
+
+    std::optional<std::string> queryFact(std::string_view key) override
+    {
+        auto state(_state.lock());
+
+        auto stmt(state->queryFact.use()(key));
+        if (!stmt.next()) return {};
+
+        return stmt.getStr(0);
+    }
 };
 
 ref<Cache> getCache()

src/libfetchers/cache.hh

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "fetchers.hh"
+#include "path.hh"
 
 namespace nix::fetchers {
 
@@ -29,6 +30,14 @@ struct Cache
     virtual std::optional<Result> lookupExpired(
         ref<Store> store,
         const Attrs & inAttrs) = 0;
+
+    /* A simple key/value store for immutable facts such as the
+       revcount corresponding to a rev. */
+    virtual void upsertFact(
+        std::string_view key,
+        std::string_view value) = 0;
+
+    virtual std::optional<std::string> queryFact(std::string_view key) = 0;
 };
 
 ref<Cache> getCache();

src/libfetchers/fetchers.cc

@@ -1,5 +1,6 @@
 #include "fetchers.hh"
 #include "store-api.hh"
+#include "input-accessor.hh"
 
 #include <nlohmann/json.hpp>
 
@@ -23,12 +24,8 @@ static void fixupInput(Input & input)
     // Check common attributes.
     input.getType();
     input.getRef();
-    if (input.getRev())
-        input.locked = true;
     input.getRevCount();
     input.getLastModified();
-    if (input.getNarHash())
-        input.locked = true;
 }
 
 Input Input::fromURL(const ParsedURL & url)
@@ -87,9 +84,21 @@ Attrs Input::toAttrs() const
     return attrs;
 }
 
-bool Input::hasAllInfo() const
+bool Input::isDirect() const
 {
-    return getNarHash() && scheme && scheme->hasAllInfo(*this);
+    assert(scheme);
+    return !scheme || scheme->isDirect(*this);
+}
+
+bool Input::isLocked() const
+{
+    return scheme && scheme->isLocked(*this);
+}
+
+std::optional<std::string> Input::isRelative() const
+{
+    assert(scheme);
+    return scheme->isRelative(*this);
 }
 
 bool Input::operator ==(const Input & other) const
@@ -107,50 +116,28 @@ bool Input::contains(const Input & other) const
     return false;
 }
 
-std::pair<Tree, Input> Input::fetch(ref<Store> store) const
+std::pair<StorePath, Input> Input::fetchToStore(ref<Store> store) const
 {
-    if (!scheme)
-        throw Error("cannot fetch unsupported input '%s'", attrsToJSON(toAttrs()));
-
-    /* The tree may already be in the Nix store, or it could be
-       substituted (which is often faster than fetching from the
-       original source). So check that. */
-    if (hasAllInfo()) {
-        try {
-            auto storePath = computeStorePath(*store);
-
-            store->ensurePath(storePath);
-
-            debug("using substituted/cached input '%s' in '%s'",
-                to_string(), store->printStorePath(storePath));
-
-            return {Tree { .actualPath = store->toRealPath(storePath), .storePath = std::move(storePath) }, *this};
-        } catch (Error & e) {
-            debug("substitution of input '%s' failed: %s", to_string(), e.what());
-        }
-    }
-
     auto [storePath, input] = [&]() -> std::pair<StorePath, Input> {
         try {
-            return scheme->fetch(store, *this);
+            auto [accessor, input2] = getAccessor(store);
+            auto storePath = accessor->root().fetchToStore(store, input2.getName());
+            return {storePath, input2};
         } catch (Error & e) {
             e.addTrace({}, "while fetching the input '%s'", to_string());
             throw;
         }
     }();
 
-    Tree tree {
-        .actualPath = store->toRealPath(storePath),
-        .storePath = storePath,
-    };
-
-    auto narHash = store->queryPathInfo(tree.storePath)->narHash;
-    input.attrs.insert_or_assign("narHash", narHash.to_string(SRI, true));
+    return {std::move(storePath), input};
+}
 
+void Input::checkLocks(Input & input) const
+{
     if (auto prevNarHash = getNarHash()) {
-        if (narHash != *prevNarHash)
-            throw Error((unsigned int) 102, "NAR hash mismatch in input '%s' (%s), expected '%s', got '%s'",
-                to_string(), tree.actualPath, prevNarHash->to_string(SRI, true), narHash.to_string(SRI, true));
+        if (input.getNarHash() != prevNarHash)
+            throw Error((unsigned int) 102, "NAR hash mismatch in input '%s', expected '%s'",
+                to_string(), prevNarHash->to_string(SRI, true));
     }
 
     if (auto prevLastModified = getLastModified()) {
@@ -164,12 +151,24 @@ std::pair<Tree, Input> Input::fetch(ref<Store> store) const
             throw Error("'revCount' attribute mismatch in input '%s', expected %d",
                 input.to_string(), *prevRevCount);
     }
+}
 
-    input.locked = true;
+std::pair<ref<InputAccessor>, Input> Input::getAccessor(ref<Store> store) const
+{
+    // FIXME: cache the accessor
 
-    assert(input.hasAllInfo());
+    if (!scheme)
+        throw Error("cannot fetch unsupported input '%s'", attrsToJSON(toAttrs()));
 
-    return {std::move(tree), input};
+    try {
+        auto [accessor, final] = scheme->getAccessor(store, *this);
+        accessor->fingerprint = scheme->getFingerprint(store, final);
+        checkLocks(final);
+        return {accessor, std::move(final)};
+    } catch (Error & e) {
+        e.addTrace({}, "while fetching the input '%s'", to_string());
+        throw;
+    }
 }
 
 Input Input::applyOverrides(
@@ -186,18 +185,13 @@ void Input::clone(const Path & destDir) const
     scheme->clone(*this, destDir);
 }
 
-std::optional<Path> Input::getSourcePath() const
-{
-    assert(scheme);
-    return scheme->getSourcePath(*this);
-}
-
-void Input::markChangedFile(
-    std::string_view file,
+void Input::putFile(
+    const CanonPath & path,
+    std::string_view contents,
     std::optional<std::string> commitMsg) const
 {
     assert(scheme);
-    return scheme->markChangedFile(*this, file, commitMsg);
+    return scheme->putFile(*this, path, contents, commitMsg);
 }
 
 std::string Input::getName() const
@@ -205,14 +199,6 @@ std::string Input::getName() const
     return maybeGetStrAttr(attrs, "name").value_or("source");
 }
 
-StorePath Input::computeStorePath(Store & store) const
-{
-    auto narHash = getNarHash();
-    if (!narHash)
-        throw Error("cannot compute store path for unlocked input '%s'", to_string());
-    return store.makeFixedOutputPath(FileIngestionMethod::Recursive, *narHash, getName());
-}
-
 std::string Input::getType() const
 {
     return getStrAttr(attrs, "type");
@@ -266,7 +252,12 @@ std::optional<time_t> Input::getLastModified() const
     return {};
 }
 
-ParsedURL InputScheme::toURL(const Input & input)
+std::optional<std::string> Input::getFingerprint(ref<Store> store) const
+{
+    return scheme ? scheme->getFingerprint(store, *this) : std::nullopt;
+}
+
+ParsedURL InputScheme::toURL(const Input & input) const
 {
     throw Error("don't know how to convert input '%s' to a URL", attrsToJSON(input.attrs));
 }
@@ -274,7 +265,7 @@ ParsedURL InputScheme::toURL(const Input & input)
 Input InputScheme::applyOverrides(
     const Input & input,
     std::optional<std::string> ref,
-    std::optional<Hash> rev)
+    std::optional<Hash> rev) const
 {
     if (ref)
         throw Error("don't know how to set branch/tag name of input '%s' to '%s'", input.to_string(), *ref);
@@ -283,19 +274,26 @@ Input InputScheme::applyOverrides(
     return input;
 }
 
-std::optional<Path> InputScheme::getSourcePath(const Input & input)
+void InputScheme::putFile(
+    const Input & input,
+    const CanonPath & path,
+    std::string_view contents,
+    std::optional<std::string> commitMsg) const
 {
-    return {};
+    throw Error("input '%s' does not support modifying file '%s'", input.to_string(), path);
 }
 
-void InputScheme::markChangedFile(const Input & input, std::string_view file, std::optional<std::string> commitMsg)
-{
-    assert(false);
-}
-
-void InputScheme::clone(const Input & input, const Path & destDir)
+void InputScheme::clone(const Input & input, const Path & destDir) const
 {
     throw Error("do not know how to clone input '%s'", input.to_string());
 }
 
+std::optional<std::string> InputScheme::getFingerprint(ref<Store> store, const Input & input) const
+{
+    if (auto rev = input.getRev())
+        return rev->gitRev();
+    else
+        return std::nullopt;
+}
+
 }

src/libfetchers/fetchers.hh

@@ -2,22 +2,16 @@
 
 #include "types.hh"
 #include "hash.hh"
-#include "path.hh"
+#include "canon-path.hh"
 #include "attrs.hh"
 #include "url.hh"
 
 #include <memory>
 
-namespace nix { class Store; }
+namespace nix { class Store; class StorePath; class InputAccessor; }
 
 namespace nix::fetchers {
 
-struct Tree
-{
-    Path actualPath;
-    StorePath storePath;
-};
-
 struct InputScheme;
 
 /* The Input object is generated by a specific fetcher, based on the
@@ -27,18 +21,12 @@ struct InputScheme;
  * "fromURL()" or "fromAttrs()" static functions which are provided
  * the url or attrset specified in the flake file.
  */
-
 struct Input
 {
     friend struct InputScheme;
 
     std::shared_ptr<InputScheme> scheme; // note: can be null
     Attrs attrs;
-    bool locked = false;
-    bool direct = true;
-
-    /* path of the parent of this input, used for relative path resolution */
-    std::optional<Path> parent;
 
 public:
     static Input fromURL(const std::string & url);
@@ -57,21 +45,28 @@ public:
 
     /* Check whether this is a "direct" input, that is, not
        one that goes through a registry. */
-    bool isDirect() const { return direct; }
+    bool isDirect() const;
 
     /* Check whether this is a "locked" input, that is,
        one that contains a commit hash or content hash. */
-    bool isLocked() const { return locked; }
+    bool isLocked() const;
 
-    bool hasAllInfo() const;
+    /* Only for relative path flakes, i.e. 'path:./foo', returns the
+       relative path, i.e. './foo'. */
+    std::optional<std::string> isRelative() const;
 
     bool operator ==(const Input & other) const;
 
     bool contains(const Input & other) const;
 
-    /* Fetch the input into the Nix store, returning the location in
-       the Nix store and the locked input. */
-    std::pair<Tree, Input> fetch(ref<Store> store) const;
+    /* Fetch the entire input into the Nix store, returning the
+       location in the Nix store and the locked input. */
+    std::pair<StorePath, Input> fetchToStore(ref<Store> store) const;
+
+    /* Return an InputAccessor that allows access to files in the
+       input without copying it to the store. Also return a possibly
+       unlocked input. */
+    std::pair<ref<InputAccessor>, Input> getAccessor(ref<Store> store) const;
 
     Input applyOverrides(
         std::optional<std::string> ref,
@@ -79,16 +74,13 @@ public:
 
     void clone(const Path & destDir) const;
 
-    std::optional<Path> getSourcePath() const;
-
-    void markChangedFile(
-        std::string_view file,
+    void putFile(
+        const CanonPath & path,
+        std::string_view contents,
         std::optional<std::string> commitMsg) const;
 
     std::string getName() const;
 
-    StorePath computeStorePath(Store & store) const;
-
     // Convenience functions for common attributes.
     std::string getType() const;
     std::optional<Hash> getNarHash() const;
@@ -96,8 +88,15 @@ public:
     std::optional<Hash> getRev() const;
     std::optional<uint64_t> getRevCount() const;
     std::optional<time_t> getLastModified() const;
-};
 
+    // For locked inputs, returns a string that uniquely specifies the
+    // content of the input (typically a commit hash or content hash).
+    std::optional<std::string> getFingerprint(ref<Store> store) const;
+
+private:
+
+    void checkLocks(Input & input) const;
+};
 
 /* The InputScheme represents a type of fetcher.  Each fetcher
  * registers with nix at startup time.  When processing an input for a
@@ -107,55 +106,44 @@ public:
  * recognized.  The Input object contains the information the fetcher
  * needs to actually perform the "fetch()" when called.
  */
-
 struct InputScheme
 {
     virtual ~InputScheme()
     { }
 
-    virtual std::optional<Input> inputFromURL(const ParsedURL & url) = 0;
+    virtual std::optional<Input> inputFromURL(const ParsedURL & url) const = 0;
 
-    virtual std::optional<Input> inputFromAttrs(const Attrs & attrs) = 0;
+    virtual std::optional<Input> inputFromAttrs(const Attrs & attrs) const = 0;
 
-    virtual ParsedURL toURL(const Input & input);
-
-    virtual bool hasAllInfo(const Input & input) = 0;
+    virtual ParsedURL toURL(const Input & input) const;
 
     virtual Input applyOverrides(
         const Input & input,
         std::optional<std::string> ref,
-        std::optional<Hash> rev);
+        std::optional<Hash> rev) const;
 
-    virtual void clone(const Input & input, const Path & destDir);
+    virtual void clone(const Input & input, const Path & destDir) const;
 
-    virtual std::optional<Path> getSourcePath(const Input & input);
+    virtual void putFile(
+        const Input & input,
+        const CanonPath & path,
+        std::string_view contents,
+        std::optional<std::string> commitMsg) const;
 
-    virtual void markChangedFile(const Input & input, std::string_view file, std::optional<std::string> commitMsg);
+    virtual std::pair<ref<InputAccessor>, Input> getAccessor(ref<Store> store, const Input & input) const = 0;
 
-    virtual std::pair<StorePath, Input> fetch(ref<Store> store, const Input & input) = 0;
+    virtual bool isDirect(const Input & input) const
+    { return true; }
+
+    virtual bool isLocked(const Input & input) const
+    { return false; }
+
+    virtual std::optional<std::string> isRelative(const Input & input) const
+    { return std::nullopt; }
+
+    virtual std::optional<std::string> getFingerprint(ref<Store> store, const Input & input) const;
 };
 
 void registerInputScheme(std::shared_ptr<InputScheme> && fetcher);
 
-struct DownloadFileResult
-{
-    StorePath storePath;
-    std::string etag;
-    std::string effectiveUrl;
-};
-
-DownloadFileResult downloadFile(
-    ref<Store> store,
-    const std::string & url,
-    const std::string & name,
-    bool locked,
-    const Headers & headers = {});
-
-std::pair<Tree, time_t> downloadTarball(
-    ref<Store> store,
-    const std::string & url,
-    const std::string & name,
-    bool locked,
-    const Headers & headers = {});
-
 }

src/libfetchers/fs-input-accessor.cc

@@ -0,0 +1,140 @@
+#include "fs-input-accessor.hh"
+#include "store-api.hh"
+
+namespace nix {
+
+struct FSInputAccessorImpl : FSInputAccessor
+{
+    CanonPath root;
+    std::optional<std::set<CanonPath>> allowedPaths;
+    MakeNotAllowedError makeNotAllowedError;
+
+    FSInputAccessorImpl(
+        const CanonPath & root,
+        std::optional<std::set<CanonPath>> && allowedPaths,
+        MakeNotAllowedError && makeNotAllowedError)
+        : root(root)
+        , allowedPaths(std::move(allowedPaths))
+        , makeNotAllowedError(std::move(makeNotAllowedError))
+    {
+        displayPrefix = root.isRoot() ? "" : root.abs();
+    }
+
+    std::string readFile(const CanonPath & path) override
+    {
+        auto absPath = makeAbsPath(path);
+        checkAllowed(absPath);
+        return nix::readFile(absPath.abs());
+    }
+
+    bool pathExists(const CanonPath & path) override
+    {
+        auto absPath = makeAbsPath(path);
+        return isAllowed(absPath) && nix::pathExists(absPath.abs());
+    }
+
+    Stat lstat(const CanonPath & path) override
+    {
+        auto absPath = makeAbsPath(path);
+        checkAllowed(absPath);
+        auto st = nix::lstat(absPath.abs());
+        return Stat {
+            .type =
+                S_ISREG(st.st_mode) ? tRegular :
+                S_ISDIR(st.st_mode) ? tDirectory :
+                S_ISLNK(st.st_mode) ? tSymlink :
+                tMisc,
+            .isExecutable = S_ISREG(st.st_mode) && st.st_mode & S_IXUSR
+        };
+    }
+
+    DirEntries readDirectory(const CanonPath & path) override
+    {
+        auto absPath = makeAbsPath(path);
+        checkAllowed(absPath);
+        DirEntries res;
+        for (auto & entry : nix::readDirectory(absPath.abs())) {
+            std::optional<Type> type;
+            switch (entry.type) {
+            case DT_REG: type = Type::tRegular; break;
+            case DT_LNK: type = Type::tSymlink; break;
+            case DT_DIR: type = Type::tDirectory; break;
+            }
+            if (isAllowed(absPath + entry.name))
+                res.emplace(entry.name, type);
+        }
+        return res;
+    }
+
+    std::string readLink(const CanonPath & path) override
+    {
+        auto absPath = makeAbsPath(path);
+        checkAllowed(absPath);
+        return nix::readLink(absPath.abs());
+    }
+
+    CanonPath makeAbsPath(const CanonPath & path)
+    {
+        return root + path;
+    }
+
+    void checkAllowed(const CanonPath & absPath) override
+    {
+        if (!isAllowed(absPath))
+            throw makeNotAllowedError
+                ? makeNotAllowedError(absPath)
+                : RestrictedPathError("access to path '%s' is forbidden", absPath);
+    }
+
+    bool isAllowed(const CanonPath & absPath)
+    {
+        if (!absPath.isWithin(root))
+            return false;
+
+        if (allowedPaths) {
+            auto p = absPath.removePrefix(root);
+            if (!p.isAllowed(*allowedPaths))
+                return false;
+        }
+
+        return true;
+    }
+
+    void allowPath(CanonPath path) override
+    {
+        if (allowedPaths)
+            allowedPaths->insert(std::move(path));
+    }
+
+    bool hasAccessControl() override
+    {
+        return (bool) allowedPaths;
+    }
+
+    std::optional<CanonPath> getPhysicalPath(const CanonPath & path) override
+    {
+        auto absPath = makeAbsPath(path);
+        if (isAllowed(absPath))
+            return absPath;
+        else
+            return std::nullopt;
+    }
+};
+
+ref<FSInputAccessor> makeFSInputAccessor(
+    const CanonPath & root,
+    std::optional<std::set<CanonPath>> && allowedPaths,
+    MakeNotAllowedError && makeNotAllowedError)
+{
+    return make_ref<FSInputAccessorImpl>(root, std::move(allowedPaths), std::move(makeNotAllowedError));
+}
+
+ref<FSInputAccessor> makeStorePathAccessor(
+    ref<Store> store,
+    const StorePath & storePath,
+    MakeNotAllowedError && makeNotAllowedError)
+{
+    return makeFSInputAccessor(CanonPath(store->toRealPath(storePath)), {}, std::move(makeNotAllowedError));
+}
+
+}

src/libfetchers/fs-input-accessor.hh

@@ -0,0 +1,29 @@
+#pragma once
+
+#include "input-accessor.hh"
+
+namespace nix {
+
+class StorePath;
+class Store;
+
+struct FSInputAccessor : InputAccessor
+{
+    virtual void checkAllowed(const CanonPath & absPath) = 0;
+
+    virtual void allowPath(CanonPath path) = 0;
+
+    virtual bool hasAccessControl() = 0;
+};
+
+ref<FSInputAccessor> makeFSInputAccessor(
+    const CanonPath & root,
+    std::optional<std::set<CanonPath>> && allowedPaths = {},
+    MakeNotAllowedError && makeNotAllowedError = {});
+
+ref<FSInputAccessor> makeStorePathAccessor(
+    ref<Store> store,
+    const StorePath & storePath,
+    MakeNotAllowedError && makeNotAllowedError = {});
+
+}

src/libfetchers/git.cc

@@ -7,6 +7,7 @@
 #include "pathlocks.hh"
 #include "util.hh"
 #include "git.hh"
+#include "fs-input-accessor.hh"
 
 #include "fetch-settings.hh"
 
@@ -18,6 +19,7 @@
 using namespace std::string_literals;
 
 namespace nix::fetchers {
+
 namespace {
 
 // Explicit initial branch of our bare repo to suppress warnings from new version of git.
@@ -26,12 +28,12 @@ namespace {
 // old version of git, which will ignore unrecognized `-c` options.
 const std::string gitInitialBranch = "__nix_dummy_branch";
 
-bool isCacheFileWithinTtl(const time_t now, const struct stat & st)
+bool isCacheFileWithinTtl(time_t now, const struct stat & st)
 {
     return st.st_mtime + settings.tarballTtl > now;
 }
 
-bool touchCacheFile(const Path& path, const time_t& touch_time)
+bool touchCacheFile(const Path & path, time_t touch_time)
 {
     struct timeval times[2];
     times[0].tv_sec = touch_time;
@@ -42,7 +44,7 @@ bool touchCacheFile(const Path& path, const time_t& touch_time)
     return lutimes(path.c_str(), times) == 0;
 }
 
-Path getCachePath(std::string key)
+Path getCachePath(std::string_view key)
 {
     return getCacheDir() + "/nix/gitv3/" +
         hashString(htSHA256, key).to_string(Base32, false);
@@ -57,13 +59,12 @@ Path getCachePath(std::string key)
 //   ...
 std::optional<std::string> readHead(const Path & path)
 {
-    auto [exit_code, output] = runProgram(RunOptions {
+    auto [status, output] = runProgram(RunOptions {
         .program = "git",
+        // FIXME: use 'HEAD' to avoid returning all refs
         .args = {"ls-remote", "--symref", path},
     });
-    if (exit_code != 0) {
-        return std::nullopt;
-    }
+    if (status != 0) return std::nullopt;
 
     std::string_view line = output;
     line = line.substr(0, line.find("\n"));
@@ -85,9 +86,8 @@ std::optional<std::string> readHead(const Path & path)
 bool storeCachedHead(const std::string & actualUrl, const std::string & headRef)
 {
     Path cacheDir = getCachePath(actualUrl);
-    auto gitDir = ".";
     try {
-        runProgram("git", true, { "-C", cacheDir, "--git-dir", gitDir, "symbolic-ref", "--", "HEAD", headRef });
+        runProgram("git", true, { "-C", cacheDir, "--git-dir", ".", "symbolic-ref", "--", "HEAD", headRef });
     } catch (ExecError &e) {
         if (!WIFEXITED(e.status)) throw;
         return false;
@@ -110,16 +110,15 @@ std::optional<std::string> readHeadCached(const std::string& actualUrl)
         cachedRef = readHead(cacheDir);
         if (cachedRef != std::nullopt &&
             *cachedRef != gitInitialBranch &&
-            isCacheFileWithinTtl(now, st)) {
+            isCacheFileWithinTtl(now, st))
+        {
             debug("using cached HEAD ref '%s' for repo '%s'", *cachedRef, actualUrl);
             return cachedRef;
         }
     }
 
     auto ref = readHead(actualUrl);
-    if (ref) {
-        return ref;
-    }
+    if (ref) return ref;
 
     if (cachedRef) {
         // If the cached git ref is expired in fetch() below, and the 'git fetch'
@@ -138,119 +137,11 @@ bool isNotDotGitDirectory(const Path & path)
     return baseNameOf(path) != ".git";
 }
 
-struct WorkdirInfo
-{
-    bool clean = false;
-    bool hasHead = false;
-};
-
-// Returns whether a git workdir is clean and has commits.
-WorkdirInfo getWorkdirInfo(const Input & input, const Path & workdir)
-{
-    const bool submodules = maybeGetBoolAttr(input.attrs, "submodules").value_or(false);
-    std::string gitDir(".git");
-
-    auto env = getEnv();
-    // Set LC_ALL to C: because we rely on the error messages from git rev-parse to determine what went wrong
-    // that way unknown errors can lead to a failure instead of continuing through the wrong code path
-    env["LC_ALL"] = "C";
-
-    /* Check whether HEAD points to something that looks like a commit,
-       since that is the refrence we want to use later on. */
-    auto result = runProgram(RunOptions {
-        .program = "git",
-        .args = { "-C", workdir, "--git-dir", gitDir, "rev-parse", "--verify", "--no-revs", "HEAD^{commit}" },
-        .environment = env,
-        .mergeStderrToStdout = true
-    });
-    auto exitCode = WEXITSTATUS(result.first);
-    auto errorMessage = result.second;
-
-    if (errorMessage.find("fatal: not a git repository") != std::string::npos) {
-        throw Error("'%s' is not a Git repository", workdir);
-    } else if (errorMessage.find("fatal: Needed a single revision") != std::string::npos) {
-        // indicates that the repo does not have any commits
-        // we want to proceed and will consider it dirty later
-    } else if (exitCode != 0) {
-        // any other errors should lead to a failure
-        throw Error("getting the HEAD of the Git tree '%s' failed with exit code %d:\n%s", workdir, exitCode, errorMessage);
-    }
-
-    bool clean = false;
-    bool hasHead = exitCode == 0;
-
-    try {
-        if (hasHead) {
-            // Using git diff is preferrable over lower-level operations here,
-            // because its conceptually simpler and we only need the exit code anyways.
-            auto gitDiffOpts = Strings({ "-C", workdir, "--git-dir", gitDir, "diff", "HEAD", "--quiet"});
-            if (!submodules) {
-                // Changes in submodules should only make the tree dirty
-                // when those submodules will be copied as well.
-                gitDiffOpts.emplace_back("--ignore-submodules");
-            }
-            gitDiffOpts.emplace_back("--");
-            runProgram("git", true, gitDiffOpts);
-
-            clean = true;
-        }
-    } catch (ExecError & e) {
-        if (!WIFEXITED(e.status) || WEXITSTATUS(e.status) != 1) throw;
-    }
-
-    return WorkdirInfo { .clean = clean, .hasHead = hasHead };
-}
-
-std::pair<StorePath, Input> fetchFromWorkdir(ref<Store> store, Input & input, const Path & workdir, const WorkdirInfo & workdirInfo)
-{
-    const bool submodules = maybeGetBoolAttr(input.attrs, "submodules").value_or(false);
-    auto gitDir = ".git";
-
-    if (!fetchSettings.allowDirty)
-        throw Error("Git tree '%s' is dirty", workdir);
-
-    if (fetchSettings.warnDirty)
-        warn("Git tree '%s' is dirty", workdir);
-
-    auto gitOpts = Strings({ "-C", workdir, "--git-dir", gitDir, "ls-files", "-z" });
-    if (submodules)
-        gitOpts.emplace_back("--recurse-submodules");
-
-    auto files = tokenizeString<std::set<std::string>>(
-        runProgram("git", true, gitOpts), "\0"s);
-
-    Path actualPath(absPath(workdir));
-
-    PathFilter filter = [&](const Path & p) -> bool {
-        assert(hasPrefix(p, actualPath));
-        std::string file(p, actualPath.size() + 1);
-
-        auto st = lstat(p);
-
-        if (S_ISDIR(st.st_mode)) {
-            auto prefix = file + "/";
-            auto i = files.lower_bound(prefix);
-            return i != files.end() && hasPrefix(*i, prefix);
-        }
-
-        return files.count(file);
-    };
-
-    auto storePath = store->addToStore(input.getName(), actualPath, FileIngestionMethod::Recursive, htSHA256, filter);
-
-    // FIXME: maybe we should use the timestamp of the last
-    // modified dirty file?
-    input.attrs.insert_or_assign(
-        "lastModified",
-        workdirInfo.hasHead ? std::stoull(runProgram("git", true, { "-C", actualPath, "--git-dir", gitDir, "log", "-1", "--format=%ct", "--no-show-signature", "HEAD" })) : 0);
-
-    return {std::move(storePath), input};
-}
 }  // end namespace
 
 struct GitInputScheme : InputScheme
 {
-    std::optional<Input> inputFromURL(const ParsedURL & url) override
+    std::optional<Input> inputFromURL(const ParsedURL & url) const override
     {
         if (url.scheme != "git" &&
             url.scheme != "git+http" &&
@@ -280,7 +171,7 @@ struct GitInputScheme : InputScheme
         return inputFromAttrs(attrs);
     }
 
-    std::optional<Input> inputFromAttrs(const Attrs & attrs) override
+    std::optional<Input> inputFromAttrs(const Attrs & attrs) const override
     {
         if (maybeGetStrAttr(attrs, "type") != "git") return {};
 
@@ -303,7 +194,7 @@ struct GitInputScheme : InputScheme
         return input;
     }
 
-    ParsedURL toURL(const Input & input) override
+    ParsedURL toURL(const Input & input) const override
     {
         auto url = parseURL(getStrAttr(input.attrs, "url"));
         if (url.scheme != "git") url.scheme = "git+" + url.scheme;
@@ -314,19 +205,10 @@ struct GitInputScheme : InputScheme
         return url;
     }
 
-    bool hasAllInfo(const Input & input) override
-    {
-        bool maybeDirty = !input.getRef();
-        bool shallow = maybeGetBoolAttr(input.attrs, "shallow").value_or(false);
-        return
-            maybeGetIntAttr(input.attrs, "lastModified")
-            && (shallow || maybeDirty || maybeGetIntAttr(input.attrs, "revCount"));
-    }
-
     Input applyOverrides(
         const Input & input,
         std::optional<std::string> ref,
-        std::optional<Hash> rev) override
+        std::optional<Hash> rev) const override
     {
         auto res(input);
         if (rev) res.attrs.insert_or_assign("rev", rev->gitRev());
@@ -336,13 +218,13 @@ struct GitInputScheme : InputScheme
         return res;
     }
 
-    void clone(const Input & input, const Path & destDir) override
+    void clone(const Input & input, const Path & destDir) const override
     {
-        auto [isLocal, actualUrl] = getActualUrl(input);
+        auto repoInfo = getRepoInfo(input);
 
         Strings args = {"clone"};
 
-        args.push_back(actualUrl);
+        args.push_back(repoInfo.url);
 
         if (auto ref = input.getRef()) {
             args.push_back("--branch");
@@ -356,30 +238,91 @@ struct GitInputScheme : InputScheme
         runProgram("git", true, args);
     }
 
-    std::optional<Path> getSourcePath(const Input & input) override
+    void putFile(
+        const Input & input,
+        const CanonPath & path,
+        std::string_view contents,
+        std::optional<std::string> commitMsg) const
     {
-        auto url = parseURL(getStrAttr(input.attrs, "url"));
-        if (url.scheme == "file" && !input.getRef() && !input.getRev())
-            return url.path;
-        return {};
-    }
+        auto repoInfo = getRepoInfo(input);
+        if (!repoInfo.isLocal)
+            throw Error("cannot commit '%s' to Git repository '%s' because it's not a working tree", path, input.to_string());
 
-    void markChangedFile(const Input & input, std::string_view file, std::optional<std::string> commitMsg) override
-    {
-        auto sourcePath = getSourcePath(input);
-        assert(sourcePath);
-        auto gitDir = ".git";
+        auto absPath = CanonPath(repoInfo.url) + path;
+
+        // FIXME: make sure that absPath is not a symlink that escapes
+        // the repo.
+        writeFile(absPath.abs(), contents);
 
         runProgram("git", true,
-            { "-C", *sourcePath, "--git-dir", gitDir, "add", "--intent-to-add", "--", std::string(file) });
+            { "-C", repoInfo.url, "--git-dir", repoInfo.gitDir, "add", "--intent-to-add", "--", std::string(path.rel()) });
 
         if (commitMsg)
             runProgram("git", true,
-                { "-C", *sourcePath, "--git-dir", gitDir, "commit", std::string(file), "-m", *commitMsg });
+                { "-C", repoInfo.url, "--git-dir", repoInfo.gitDir, "commit", std::string(path.rel()), "-m", *commitMsg });
     }
 
-    std::pair<bool, std::string> getActualUrl(const Input & input) const
+    struct RepoInfo
     {
+        bool shallow = false;
+        bool submodules = false;
+        bool allRefs = false;
+
+        std::string cacheType;
+
+        /* Whether this is a local, non-bare repository. */
+        bool isLocal = false;
+
+        /* Whether this is a local, non-bare, dirty repository. */
+        bool isDirty = false;
+
+        /* Whether this repository has any commits. */
+        bool hasHead = true;
+
+        /* URL of the repo, or its path if isLocal. */
+        std::string url;
+
+        void warnDirty() const
+        {
+            if (isDirty) {
+                if (!fetchSettings.allowDirty)
+                    throw Error("Git tree '%s' is dirty", url);
+
+                if (fetchSettings.warnDirty)
+                    warn("Git tree '%s' is dirty", url);
+            }
+        }
+
+        std::string gitDir = ".git";
+    };
+
+    bool getSubmodulesAttr(const Input & input) const
+    {
+        return maybeGetBoolAttr(input.attrs, "submodules").value_or(false);
+    }
+
+    RepoInfo getRepoInfo(const Input & input) const
+    {
+        auto checkHashType = [&](const std::optional<Hash> & hash)
+        {
+            if (hash.has_value() && !(hash->type == htSHA1 || hash->type == htSHA256))
+                throw Error("Hash '%s' is not supported by Git. Supported types are sha1 and sha256.", hash->to_string(Base16, true));
+        };
+
+        if (auto rev = input.getRev())
+            checkHashType(rev);
+
+        RepoInfo repoInfo {
+            .shallow = maybeGetBoolAttr(input.attrs, "shallow").value_or(false),
+            .submodules = getSubmodulesAttr(input),
+            .allRefs = maybeGetBoolAttr(input.attrs, "allRefs").value_or(false)
+        };
+
+        repoInfo.cacheType = "git";
+        if (repoInfo.shallow) repoInfo.cacheType += "-shallow";
+        if (repoInfo.submodules) repoInfo.cacheType += "-submodules";
+        if (repoInfo.allRefs) repoInfo.cacheType += "-all-refs";
+
         // file:// URIs are normally not cloned (but otherwise treated the
         // same as remote URIs, i.e. we don't use the working tree or
         // HEAD). Exception: If _NIX_FORCE_HTTP is set, or the repo is a bare git
@@ -387,52 +330,194 @@ struct GitInputScheme : InputScheme
         static bool forceHttp = getEnv("_NIX_FORCE_HTTP") == "1"; // for testing
         auto url = parseURL(getStrAttr(input.attrs, "url"));
         bool isBareRepository = url.scheme == "file" && !pathExists(url.path + "/.git");
-        bool isLocal = url.scheme == "file" && !forceHttp && !isBareRepository;
-        return {isLocal, isLocal ? url.path : url.base};
+        repoInfo.isLocal = url.scheme == "file" && !forceHttp && !isBareRepository;
+        repoInfo.url = repoInfo.isLocal ? url.path : url.base;
+
+        // If this is a local directory and no ref or revision is
+        // given, then allow the use of an unclean working tree.
+        if (!input.getRef() && !input.getRev() && repoInfo.isLocal) {
+            repoInfo.isDirty = true;
+
+            auto env = getEnv();
+            /* Set LC_ALL to C: because we rely on the error messages
+               from git rev-parse to determine what went wrong that
+               way unknown errors can lead to a failure instead of
+               continuing through the wrong code path. */
+            env["LC_ALL"] = "C";
+
+            /* Check whether HEAD points to something that looks like
+               a commit, since that is the ref we want to use later
+               on. */
+            auto result = runProgram(RunOptions {
+                .program = "git",
+                .args = { "-C", repoInfo.url, "--git-dir", repoInfo.gitDir, "rev-parse", "--verify", "--no-revs", "HEAD^{commit}" },
+                .environment = env,
+                .mergeStderrToStdout = true
+            });
+            auto exitCode = WEXITSTATUS(result.first);
+            auto errorMessage = result.second;
+
+            if (errorMessage.find("fatal: not a git repository") != std::string::npos) {
+                throw Error("'%s' is not a Git repository", repoInfo.url);
+            } else if (errorMessage.find("fatal: Needed a single revision") != std::string::npos) {
+                // indicates that the repo does not have any commits
+                // we want to proceed and will consider it dirty later
+            } else if (exitCode != 0) {
+                // any other errors should lead to a failure
+                throw Error("getting the HEAD of the Git tree '%s' failed with exit code %d:\n%s", repoInfo.url, exitCode, errorMessage);
             }
 
-    std::pair<StorePath, Input> fetch(ref<Store> store, const Input & _input) override
+            repoInfo.hasHead = exitCode == 0;
+
+            try {
+                if (repoInfo.hasHead) {
+                    // Using git diff is preferrable over lower-level operations here,
+                    // because it's conceptually simpler and we only need the exit code anyways.
+                    auto gitDiffOpts = Strings({ "-C", repoInfo.url, "--git-dir", repoInfo.gitDir, "diff", "HEAD", "--quiet"});
+                    if (!repoInfo.submodules) {
+                        // Changes in submodules should only make the tree dirty
+                        // when those submodules will be copied as well.
+                        gitDiffOpts.emplace_back("--ignore-submodules");
+                    }
+                    gitDiffOpts.emplace_back("--");
+                    runProgram("git", true, gitDiffOpts);
+
+                    repoInfo.isDirty = false;
+                }
+            } catch (ExecError & e) {
+                if (!WIFEXITED(e.status) || WEXITSTATUS(e.status) != 1) throw;
+            }
+        }
+
+        return repoInfo;
+    }
+
+    std::set<CanonPath> listFiles(const RepoInfo & repoInfo) const
     {
-        Input input(_input);
-        auto gitDir = ".git";
+        auto gitOpts = Strings({ "-C", repoInfo.url, "--git-dir", repoInfo.gitDir, "ls-files", "-z" });
+        if (repoInfo.submodules)
+            gitOpts.emplace_back("--recurse-submodules");
+
+        std::set<CanonPath> res;
+
+        for (auto & p : tokenizeString<std::set<std::string>>(
+                runProgram("git", true, gitOpts), "\0"s))
+            res.insert(CanonPath(p));
+
+        return res;
+    }
+
+    Hash updateRev(Input & input, const RepoInfo & repoInfo, const std::string & ref) const
+    {
+        if (auto r = input.getRev())
+            return *r;
+        else {
+            auto rev = Hash::parseAny(chomp(runProgram("git", true, { "-C", repoInfo.url, "--git-dir", repoInfo.gitDir, "rev-parse", ref })), htSHA1);
+            input.attrs.insert_or_assign("rev", rev.gitRev());
+            return rev;
+        }
+    }
+
+    uint64_t getLastModified(const RepoInfo & repoInfo, const std::string & repoDir, const std::string & ref) const
+    {
+        return
+            repoInfo.hasHead
+            ? std::stoull(
+                runProgram("git", true,
+                    { "-C", repoDir, "--git-dir", repoInfo.gitDir, "log", "-1", "--format=%ct", "--no-show-signature", ref }))
+            : 0;
+    }
+
+    uint64_t getLastModified(const RepoInfo & repoInfo, const std::string & repoDir, const Hash & rev) const
+    {
+        if (!repoInfo.hasHead) return 0;
+
+        auto key = fmt("git-%s-last-modified", rev.gitRev());
+
+        auto cache = getCache();
+
+        if (auto lastModifiedS = cache->queryFact(key)) {
+            if (auto lastModified = string2Int<uint64_t>(*lastModifiedS))
+                return *lastModified;
+        }
+
+        auto lastModified = getLastModified(repoInfo, repoDir, rev.gitRev());
+
+        cache->upsertFact(key, std::to_string(lastModified));
+
+        return lastModified;
+    }
+
+    uint64_t getRevCount(const RepoInfo & repoInfo, const std::string & repoDir, const Hash & rev) const
+    {
+        if (!repoInfo.hasHead) return 0;
+
+        auto key = fmt("git-%s-revcount", rev.gitRev());
+
+        auto cache = getCache();
+
+        if (auto revCountS = cache->queryFact(key)) {
+            if (auto revCount = string2Int<uint64_t>(*revCountS))
+                return *revCount;
+        }
+
+        Activity act(*logger, lvlChatty, actUnknown, fmt("getting Git revision count of '%s'", repoInfo.url));
+
+        auto revCount = std::stoull(
+            runProgram("git", true,
+                { "-C", repoDir, "--git-dir", repoInfo.gitDir, "rev-list", "--count", rev.gitRev() }));
+
+        cache->upsertFact(key, std::to_string(revCount));
+
+        return revCount;
+    }
+
+    std::string getDefaultRef(const RepoInfo & repoInfo) const
+    {
+        auto head = repoInfo.isLocal
+            ? readHead(repoInfo.url)
+            : readHeadCached(repoInfo.url);
+        if (!head) {
+            warn("could not read HEAD ref from repo at '%s', using 'master'", repoInfo.url);
+            return "master";
+        }
+        return *head;
+    }
+
+    StorePath fetchToStore(
+        ref<Store> store,
+        RepoInfo & repoInfo,
+        Input & input) const
+    {
+        assert(!repoInfo.isDirty);
+
+        auto origRev = input.getRev();
 
         std::string name = input.getName();
 
-        bool shallow = maybeGetBoolAttr(input.attrs, "shallow").value_or(false);
-        bool submodules = maybeGetBoolAttr(input.attrs, "submodules").value_or(false);
-        bool allRefs = maybeGetBoolAttr(input.attrs, "allRefs").value_or(false);
-
-        std::string cacheType = "git";
-        if (shallow) cacheType += "-shallow";
-        if (submodules) cacheType += "-submodules";
-        if (allRefs) cacheType += "-all-refs";
-
-        auto checkHashType = [&](const std::optional<Hash> & hash)
-        {
-            if (hash.has_value() && !(hash->type == htSHA1 || hash->type == htSHA256))
-                throw Error("Hash '%s' is not supported by Git. Supported types are sha1 and sha256.", hash->to_string(Base16, true));
-        };
-
         auto getLockedAttrs = [&]()
         {
-            checkHashType(input.getRev());
-
             return Attrs({
-                {"type", cacheType},
+                {"type", repoInfo.cacheType},
                 {"name", name},
                 {"rev", input.getRev()->gitRev()},
             });
         };
 
-        auto makeResult = [&](const Attrs & infoAttrs, StorePath && storePath)
-            -> std::pair<StorePath, Input>
+        auto makeResult = [&](const Attrs & infoAttrs, const StorePath & storePath) -> StorePath
         {
             assert(input.getRev());
-            assert(!_input.getRev() || _input.getRev() == input.getRev());
-            if (!shallow)
+            assert(!origRev || origRev == input.getRev());
+            if (!repoInfo.shallow)
                 input.attrs.insert_or_assign("revCount", getIntAttr(infoAttrs, "revCount"));
             input.attrs.insert_or_assign("lastModified", getIntAttr(infoAttrs, "lastModified"));
-            return {std::move(storePath), input};
+
+            // FIXME: remove?
+            //input.attrs.erase("narHash");
+            auto narHash = store->queryPathInfo(storePath)->narHash;
+            input.attrs.insert_or_assign("narHash", narHash.to_string(SRI, true));
+
+            return storePath;
         };
 
         if (input.getRev()) {
@@ -440,54 +525,23 @@ struct GitInputScheme : InputScheme
                 return makeResult(res->first, std::move(res->second));
         }
 
-        auto [isLocal, actualUrl_] = getActualUrl(input);
-        auto actualUrl = actualUrl_; // work around clang bug
-
-        /* If this is a local directory and no ref or revision is given,
-           allow fetching directly from a dirty workdir. */
-        if (!input.getRef() && !input.getRev() && isLocal) {
-            auto workdirInfo = getWorkdirInfo(input, actualUrl);
-            if (!workdirInfo.clean) {
-                return fetchFromWorkdir(store, input, actualUrl, workdirInfo);
-            }
-        }
+        auto originalRef = input.getRef();
+        auto ref = originalRef ? *originalRef : getDefaultRef(repoInfo);
+        input.attrs.insert_or_assign("ref", ref);
 
         Attrs unlockedAttrs({
-            {"type", cacheType},
+            {"type", repoInfo.cacheType},
             {"name", name},
-            {"url", actualUrl},
+            {"url", repoInfo.url},
+            {"ref", ref},
         });
 
         Path repoDir;
 
-        if (isLocal) {
-            if (!input.getRef()) {
-                auto head = readHead(actualUrl);
-                if (!head) {
-                    warn("could not read HEAD ref from repo at '%s', using 'master'", actualUrl);
-                    head = "master";
-                }
-                input.attrs.insert_or_assign("ref", *head);
-                unlockedAttrs.insert_or_assign("ref", *head);
-            }
-
-            if (!input.getRev())
-                input.attrs.insert_or_assign("rev",
-                    Hash::parseAny(chomp(runProgram("git", true, { "-C", actualUrl, "--git-dir", gitDir, "rev-parse", *input.getRef() })), htSHA1).gitRev());
-
-            repoDir = actualUrl;
+        if (repoInfo.isLocal) {
+            updateRev(input, repoInfo, ref);
+            repoDir = repoInfo.url;
         } else {
-            const bool useHeadRef = !input.getRef();
-            if (useHeadRef) {
-                auto head = readHeadCached(actualUrl);
-                if (!head) {
-                    warn("could not read HEAD ref from repo at '%s', using 'master'", actualUrl);
-                    head = "master";
-                }
-                input.attrs.insert_or_assign("ref", *head);
-                unlockedAttrs.insert_or_assign("ref", *head);
-            }
-
             if (auto res = getCache()->lookup(store, unlockedAttrs)) {
                 auto rev2 = Hash::parseAny(getStrAttr(res->first, "rev"), htSHA1);
                 if (!input.getRev() || input.getRev() == rev2) {
@@ -496,9 +550,9 @@ struct GitInputScheme : InputScheme
                 }
             }
 
-            Path cacheDir = getCachePath(actualUrl);
+            Path cacheDir = getCachePath(repoInfo.url);
             repoDir = cacheDir;
-            gitDir = ".";
+            repoInfo.gitDir = ".";
 
             createDirs(dirOf(cacheDir));
             PathLocks cacheDirLock({cacheDir + ".lock"});
@@ -508,9 +562,9 @@ struct GitInputScheme : InputScheme
             }
 
             Path localRefFile =
-                input.getRef()->compare(0, 5, "refs/") == 0
-                ? cacheDir + "/" + *input.getRef()
-                : cacheDir + "/refs/heads/" + *input.getRef();
+                ref.compare(0, 5, "refs/") == 0
+                ? cacheDir + "/" + ref
+                : cacheDir + "/refs/heads/" + ref;
 
             bool doFetch;
             time_t now = time(0);
@@ -519,7 +573,7 @@ struct GitInputScheme : InputScheme
                repo. */
             if (input.getRev()) {
                 try {
-                    runProgram("git", true, { "-C", repoDir, "--git-dir", gitDir, "cat-file", "-e", input.getRev()->gitRev() });
+                    runProgram("git", true, { "-C", repoDir, "--git-dir", repoInfo.gitDir, "cat-file", "-e", input.getRev()->gitRev() });
                     doFetch = false;
                 } catch (ExecError & e) {
                     if (WIFEXITED(e.status)) {
@@ -529,7 +583,7 @@ struct GitInputScheme : InputScheme
                     }
                 }
             } else {
-                if (allRefs) {
+                if (repoInfo.allRefs) {
                     doFetch = true;
                 } else {
                     /* If the local ref is older than ‘tarball-ttl’ seconds, do a
@@ -541,29 +595,37 @@ struct GitInputScheme : InputScheme
             }
 
             if (doFetch) {
-                Activity act(*logger, lvlTalkative, actUnknown, fmt("fetching Git repository '%s'", actualUrl));
+                Activity act(*logger, lvlTalkative, actUnknown, fmt("fetching Git repository '%s'", repoInfo.url));
 
                 // FIXME: git stderr messes up our progress indicator, so
                 // we're using --quiet for now. Should process its stderr.
                 try {
-                    auto ref = input.getRef();
-                    auto fetchRef = allRefs
+                    auto fetchRef = repoInfo.allRefs
                         ? "refs/*"
-                        : ref->compare(0, 5, "refs/") == 0
-                            ? *ref
+                        : ref.compare(0, 5, "refs/") == 0
+                            ? ref
                             : ref == "HEAD"
-                                ? *ref
-                                : "refs/heads/" + *ref;
-                    runProgram("git", true, { "-C", repoDir, "--git-dir", gitDir, "fetch", "--quiet", "--force", "--", actualUrl, fmt("%s:%s", fetchRef, fetchRef) });
+                                ? ref
+                                : "refs/heads/" + ref;
+                    runProgram("git", true,
+                        { "-C", repoDir,
+                          "--git-dir", repoInfo.gitDir,
+                          "fetch",
+                          "--quiet",
+                          "--force",
+                          "--",
+                          repoInfo.url,
+                          fmt("%s:%s", fetchRef, fetchRef)
+                        });
                 } catch (Error & e) {
                     if (!pathExists(localRefFile)) throw;
-                    warn("could not update local clone of Git repository '%s'; continuing with the most recent version", actualUrl);
+                    warn("could not update local clone of Git repository '%s'; continuing with the most recent version", repoInfo.url);
                 }
 
                 if (!touchCacheFile(localRefFile, now))
                     warn("could not update mtime for file '%s': %s", localRefFile, strerror(errno));
-                if (useHeadRef && !storeCachedHead(actualUrl, *input.getRef()))
-                    warn("could not update cached head '%s' for '%s'", *input.getRef(), actualUrl);
+                if (!originalRef && !storeCachedHead(repoInfo.url, ref))
+                    warn("could not update cached head '%s' for '%s'", ref, repoInfo.url);
             }
 
             if (!input.getRev())
@@ -572,14 +634,14 @@ struct GitInputScheme : InputScheme
             // cache dir lock is removed at scope end; we will only use read-only operations on specific revisions in the remainder
         }
 
-        bool isShallow = chomp(runProgram("git", true, { "-C", repoDir, "--git-dir", gitDir, "rev-parse", "--is-shallow-repository" })) == "true";
+        bool isShallow = chomp(runProgram("git", true, { "-C", repoDir, "--git-dir", repoInfo.gitDir, "rev-parse", "--is-shallow-repository" })) == "true";
 
-        if (isShallow && !shallow)
-            throw Error("'%s' is a shallow Git repository, but shallow repositories are only allowed when `shallow = true;` is specified.", actualUrl);
+        if (isShallow && !repoInfo.shallow)
+            throw Error("'%s' is a shallow Git repository, but shallow repositories are only allowed when `shallow = true;` is specified", repoInfo.url);
 
         // FIXME: check whether rev is an ancestor of ref.
 
-        printTalkative("using revision %s of repo '%s'", input.getRev()->gitRev(), actualUrl);
+        printTalkative("using revision %s of repo '%s'", input.getRev()->gitRev(), repoInfo.url);
 
         /* Now that we know the ref, check again whether we have it in
            the store. */
@@ -592,7 +654,7 @@ struct GitInputScheme : InputScheme
 
         auto result = runProgram(RunOptions {
             .program = "git",
-            .args = { "-C", repoDir, "--git-dir", gitDir, "cat-file", "commit", input.getRev()->gitRev() },
+            .args = { "-C", repoDir, "--git-dir", repoInfo.gitDir, "cat-file", "commit", input.getRev()->gitRev() },
             .mergeStderrToStdout = true
         });
         if (WEXITSTATUS(result.first) == 128
@@ -604,12 +666,14 @@ struct GitInputScheme : InputScheme
                 ANSI_BOLD "ref" ANSI_NORMAL " you've specified or add " ANSI_BOLD
                 "allRefs = true;" ANSI_NORMAL " to " ANSI_BOLD "fetchGit" ANSI_NORMAL ".",
                 input.getRev()->gitRev(),
-                *input.getRef(),
-                actualUrl
+                ref,
+                repoInfo.url
             );
         }
 
-        if (submodules) {
+        Activity act(*logger, lvlChatty, actUnknown, fmt("copying Git tree '%s' to the store", input.to_string()));
+
+        if (repoInfo.submodules) {
             Path tmpGitDir = createTempDir();
             AutoDelete delTmpGitDir(tmpGitDir, true);
 
@@ -621,7 +685,7 @@ struct GitInputScheme : InputScheme
                                       "--update-head-ok", "--", repoDir, "refs/*:refs/*" });
 
             runProgram("git", true, { "-C", tmpDir, "checkout", "--quiet", input.getRev()->gitRev() });
-            runProgram("git", true, { "-C", tmpDir, "remote", "add", "origin", actualUrl });
+            runProgram("git", true, { "-C", tmpDir, "remote", "add", "origin", repoInfo.url });
             runProgram("git", true, { "-C", tmpDir, "submodule", "--quiet", "update", "--init", "--recursive" });
 
             filter = isNotDotGitDirectory;
@@ -631,7 +695,7 @@ struct GitInputScheme : InputScheme
             auto source = sinkToSource([&](Sink & sink) {
                 runProgram2({
                     .program = "git",
-                    .args = { "-C", repoDir, "--git-dir", gitDir, "archive", input.getRev()->gitRev() },
+                    .args = { "-C", repoDir, "--git-dir", repoInfo.gitDir, "archive", input.getRev()->gitRev() },
                     .standardOut = &sink
                 });
             });
@@ -641,18 +705,18 @@ struct GitInputScheme : InputScheme
 
         auto storePath = store->addToStore(name, tmpDir, FileIngestionMethod::Recursive, htSHA256, filter);
 
-        auto lastModified = std::stoull(runProgram("git", true, { "-C", repoDir, "--git-dir", gitDir, "log", "-1", "--format=%ct", "--no-show-signature", input.getRev()->gitRev() }));
+        auto rev = *input.getRev();
 
         Attrs infoAttrs({
-            {"rev", input.getRev()->gitRev()},
-            {"lastModified", lastModified},
+            {"rev", rev.gitRev()},
+            {"lastModified", getLastModified(repoInfo, repoDir, rev)},
         });
 
-        if (!shallow)
+        if (!repoInfo.shallow)
             infoAttrs.insert_or_assign("revCount",
-                std::stoull(runProgram("git", true, { "-C", repoDir, "--git-dir", gitDir, "rev-list", "--count", input.getRev()->gitRev() })));
+                getRevCount(repoInfo, repoDir, rev));
 
-        if (!_input.getRev())
+        if (!origRev)
             getCache()->add(
                 store,
                 unlockedAttrs,
@@ -669,6 +733,70 @@ struct GitInputScheme : InputScheme
 
         return makeResult(infoAttrs, std::move(storePath));
     }
+
+    std::pair<ref<InputAccessor>, Input> getAccessor(ref<Store> store, const Input & _input) const override
+    {
+        Input input(_input);
+
+        auto repoInfo = getRepoInfo(input);
+
+        auto makeNotAllowedError = [url{repoInfo.url}](const CanonPath & path) -> RestrictedPathError
+        {
+            if (nix::pathExists(path.abs()))
+                return RestrictedPathError("access to path '%s' is forbidden because it is not under Git control; maybe you should 'git add' it to the repository '%s'?", path, url);
+            else
+                return RestrictedPathError("path '%s' does not exist in Git repository '%s'", path, url);
+        };
+
+        /* Unless we're using the working tree, copy the tree into the
+           Nix store. TODO: We could have an accessor for fetching
+           files from the Git repository directly. */
+        if (input.getRef() || input.getRev() || !repoInfo.isLocal) {
+            auto storePath = fetchToStore(store, repoInfo, input);
+            auto accessor = makeStorePathAccessor(store, storePath, std::move(makeNotAllowedError));
+            accessor->setPathDisplay("«" + input.to_string() + "»");
+            return {accessor, input};
+        }
+
+        if (!repoInfo.isDirty) {
+            auto ref = getDefaultRef(repoInfo);
+            input.attrs.insert_or_assign("ref", ref);
+
+            auto rev = updateRev(input, repoInfo, ref);
+
+            input.attrs.insert_or_assign(
+                "revCount",
+                getRevCount(repoInfo, repoInfo.url, rev));
+
+            input.attrs.insert_or_assign(
+                "lastModified",
+                getLastModified(repoInfo, repoInfo.url, rev));
+        } else {
+            repoInfo.warnDirty();
+
+            // FIXME: maybe we should use the timestamp of the last
+            // modified dirty file?
+            input.attrs.insert_or_assign(
+                "lastModified",
+                getLastModified(repoInfo, repoInfo.url, "HEAD"));
+        }
+
+        return {makeFSInputAccessor(CanonPath(repoInfo.url), listFiles(repoInfo), std::move(makeNotAllowedError)), input};
+    }
+
+    bool isLocked(const Input & input) const override
+    {
+        return (bool) input.getRev();
+    }
+
+    std::optional<std::string> getFingerprint(ref<Store> store, const Input & input) const override
+    {
+        if (auto rev = input.getRev()) {
+            return fmt("%s;%s", rev->gitRev(), getSubmodulesAttr(input) ? "1" : "0");
+        } else
+            return std::nullopt;
+    }
+
 };
 
 static auto rGitInputScheme = OnStartup([] { registerInputScheme(std::make_unique<GitInputScheme>()); });

src/libfetchers/github.cc

@@ -7,6 +7,8 @@
 #include "git.hh"
 #include "fetchers.hh"
 #include "fetch-settings.hh"
+#include "input-accessor.hh"
+#include "tarball.hh"
 
 #include <optional>
 #include <nlohmann/json.hpp>
@@ -26,11 +28,11 @@ std::regex hostRegex(hostRegexS, std::regex::ECMAScript);
 
 struct GitArchiveInputScheme : InputScheme
 {
-    virtual std::string type() = 0;
+    virtual std::string type() const = 0;
 
     virtual std::optional<std::pair<std::string, std::string>> accessHeaderFromToken(const std::string & token) const = 0;
 
-    std::optional<Input> inputFromURL(const ParsedURL & url) override
+    std::optional<Input> inputFromURL(const ParsedURL & url) const override
     {
         if (url.scheme != type()) return {};
 
@@ -100,7 +102,7 @@ struct GitArchiveInputScheme : InputScheme
         return input;
     }
 
-    std::optional<Input> inputFromAttrs(const Attrs & attrs) override
+    std::optional<Input> inputFromAttrs(const Attrs & attrs) const override
     {
         if (maybeGetStrAttr(attrs, "type") != type()) return {};
 
@@ -116,7 +118,7 @@ struct GitArchiveInputScheme : InputScheme
         return input;
     }
 
-    ParsedURL toURL(const Input & input) override
+    ParsedURL toURL(const Input & input) const override
     {
         auto owner = getStrAttr(input.attrs, "owner");
         auto repo = getStrAttr(input.attrs, "repo");
@@ -132,15 +134,10 @@ struct GitArchiveInputScheme : InputScheme
         };
     }
 
-    bool hasAllInfo(const Input & input) override
-    {
-        return input.getRev() && maybeGetIntAttr(input.attrs, "lastModified");
-    }
-
     Input applyOverrides(
         const Input & _input,
         std::optional<std::string> ref,
-        std::optional<Hash> rev) override
+        std::optional<Hash> rev) const override
     {
         auto input(_input);
         if (rev && ref)
@@ -183,10 +180,8 @@ struct GitArchiveInputScheme : InputScheme
 
     virtual DownloadUrl getDownloadUrl(const Input & input) const = 0;
 
-    std::pair<StorePath, Input> fetch(ref<Store> store, const Input & _input) override
+    std::pair<StorePath, Input> downloadArchive(ref<Store> store, Input input) const
     {
-        Input input(_input);
-
         if (!maybeGetStrAttr(input.attrs, "ref")) input.attrs.insert_or_assign("ref", "HEAD");
 
         auto rev = input.getRev();
@@ -196,38 +191,53 @@ struct GitArchiveInputScheme : InputScheme
         input.attrs.insert_or_assign("rev", rev->gitRev());
 
         Attrs lockedAttrs({
-            {"type", "git-tarball"},
+            {"type", "git-zipball"},
             {"rev", rev->gitRev()},
         });
 
-        if (auto res = getCache()->lookup(store, lockedAttrs)) {
-            input.attrs.insert_or_assign("lastModified", getIntAttr(res->first, "lastModified"));
-            return {std::move(res->second), input};
-        }
+        if (auto res = getCache()->lookup(store, lockedAttrs))
+            return {std::move(res->second), std::move(input)};
 
         auto url = getDownloadUrl(input);
 
-        auto [tree, lastModified] = downloadTarball(store, url.url, input.getName(), true, url.headers);
-
-        input.attrs.insert_or_assign("lastModified", uint64_t(lastModified));
+        auto res = downloadFile(store, url.url, input.getName(), true, url.headers);
 
         getCache()->add(
             store,
             lockedAttrs,
             {
                 {"rev", rev->gitRev()},
-                {"lastModified", uint64_t(lastModified)}
             },
-            tree.storePath,
+            res.storePath,
             true);
 
-        return {std::move(tree.storePath), input};
+        return {res.storePath, std::move(input)};
+    }
+
+    std::pair<ref<InputAccessor>, Input> getAccessor(ref<Store> store, const Input & input) const override
+    {
+        auto [storePath, input2] = downloadArchive(store, input);
+
+        auto accessor = makeZipInputAccessor(CanonPath(store->toRealPath(storePath)));
+
+        auto lastModified = accessor->getLastModified();
+        assert(lastModified);
+        input2.attrs.insert_or_assign("lastModified", uint64_t(*lastModified));
+
+        accessor->setPathDisplay("«" + input2.to_string() + "»");
+
+        return {accessor, input2};
+    }
+
+    bool isLocked(const Input & input) const override
+    {
+        return (bool) input.getRev();
     }
 };
 
 struct GitHubInputScheme : GitArchiveInputScheme
 {
-    std::string type() override { return "github"; }
+    std::string type() const override { return "github"; }
 
     std::optional<std::pair<std::string, std::string>> accessHeaderFromToken(const std::string & token) const override
     {
@@ -240,14 +250,29 @@ struct GitHubInputScheme : GitArchiveInputScheme
         return std::pair<std::string, std::string>("Authorization", fmt("token %s", token));
     }
 
+    std::string getHost(const Input & input) const
+    {
+        return maybeGetStrAttr(input.attrs, "host").value_or("github.com");
+    }
+
+    std::string getOwner(const Input & input) const
+    {
+        return getStrAttr(input.attrs, "owner");
+    }
+
+    std::string getRepo(const Input & input) const
+    {
+        return getStrAttr(input.attrs, "repo");
+    }
+
     Hash getRevFromRef(nix::ref<Store> store, const Input & input) const override
     {
-        auto host = maybeGetStrAttr(input.attrs, "host").value_or("github.com");
+        auto host = getHost(input);
         auto url = fmt(
             host == "github.com"
             ? "https://api.%s/repos/%s/%s/commits/%s"
             : "https://%s/api/v3/repos/%s/%s/commits/%s",
-            host, getStrAttr(input.attrs, "owner"), getStrAttr(input.attrs, "repo"), *input.getRef());
+            host, getOwner(input), getRepo(input), *input.getRef());
 
         Headers headers = makeHeadersWithAuthTokens(host);
 
@@ -264,31 +289,42 @@ struct GitHubInputScheme : GitArchiveInputScheme
     {
         // FIXME: use regular /archive URLs instead? api.github.com
         // might have stricter rate limits.
-        auto host = maybeGetStrAttr(input.attrs, "host").value_or("github.com");
+        auto host = getHost(input);
         auto url = fmt(
             host == "github.com"
-            ? "https://api.%s/repos/%s/%s/tarball/%s"
-            : "https://%s/api/v3/repos/%s/%s/tarball/%s",
-            host, getStrAttr(input.attrs, "owner"), getStrAttr(input.attrs, "repo"),
+            ? "https://api.%s/repos/%s/%s/zipball/%s"
+            : "https://%s/api/v3/repos/%s/%s/zipball/%s",
+            host, getOwner(input), getRepo(input),
             input.getRev()->to_string(Base16, false));
 
         Headers headers = makeHeadersWithAuthTokens(host);
         return DownloadUrl { url, headers };
     }
 
-    void clone(const Input & input, const Path & destDir) override
+    void clone(const Input & input, const Path & destDir) const override
     {
-        auto host = maybeGetStrAttr(input.attrs, "host").value_or("github.com");
+        auto host = getHost(input);
         Input::fromURL(fmt("git+https://%s/%s/%s.git",
-                host, getStrAttr(input.attrs, "owner"), getStrAttr(input.attrs, "repo")))
+                host, getOwner(input), getRepo(input)))
             .applyOverrides(input.getRef(), input.getRev())
             .clone(destDir);
     }
+
+    std::pair<ref<InputAccessor>, Input> getAccessor(ref<Store> store, const Input & _input) const override
+    {
+        auto [accessor, input] = GitArchiveInputScheme::getAccessor(store, _input);
+        if (getHost(input) == "github.com")
+            accessor->setPathDisplay(fmt("https://github.com/%s/%s/blob/%s",
+                    getOwner(input),
+                    getRepo(input),
+                    input.getRev()->to_string(Base16, false)));
+        return {accessor, input};
+    }
 };
 
 struct GitLabInputScheme : GitArchiveInputScheme
 {
-    std::string type() override { return "gitlab"; }
+    std::string type() const override { return "gitlab"; }
 
     std::optional<std::pair<std::string, std::string>> accessHeaderFromToken(const std::string & token) const override
     {
@@ -335,7 +371,7 @@ struct GitLabInputScheme : GitArchiveInputScheme
         // is 10 reqs/sec/ip-addr.  See
         // https://docs.gitlab.com/ee/user/gitlab_com/index.html#gitlabcom-specific-rate-limits
         auto host = maybeGetStrAttr(input.attrs, "host").value_or("gitlab.com");
-        auto url = fmt("https://%s/api/v4/projects/%s%%2F%s/repository/archive.tar.gz?sha=%s",
+        auto url = fmt("https://%s/api/v4/projects/%s%%2F%s/repository/archive.zip?sha=%s",
             host, getStrAttr(input.attrs, "owner"), getStrAttr(input.attrs, "repo"),
             input.getRev()->to_string(Base16, false));
 
@@ -343,7 +379,7 @@ struct GitLabInputScheme : GitArchiveInputScheme
         return DownloadUrl { url, headers };
     }
 
-    void clone(const Input & input, const Path & destDir) override
+    void clone(const Input & input, const Path & destDir) const override
     {
         auto host = maybeGetStrAttr(input.attrs, "host").value_or("gitlab.com");
         // FIXME: get username somewhere
@@ -356,7 +392,7 @@ struct GitLabInputScheme : GitArchiveInputScheme
 
 struct SourceHutInputScheme : GitArchiveInputScheme
 {
-    std::string type() override { return "sourcehut"; }
+    std::string type() const override { return "sourcehut"; }
 
     std::optional<std::pair<std::string, std::string>> accessHeaderFromToken(const std::string & token) const override
     {
@@ -430,7 +466,7 @@ struct SourceHutInputScheme : GitArchiveInputScheme
         return DownloadUrl { url, headers };
     }
 
-    void clone(const Input & input, const Path & destDir) override
+    void clone(const Input & input, const Path & destDir) const override
     {
         auto host = maybeGetStrAttr(input.attrs, "host").value_or("git.sr.ht");
         Input::fromURL(fmt("git+https://%s/%s/%s",

src/libfetchers/indirect.cc

@@ -7,7 +7,7 @@ std::regex flakeRegex("[a-zA-Z][a-zA-Z0-9_-]*", std::regex::ECMAScript);
 
 struct IndirectInputScheme : InputScheme
 {
-    std::optional<Input> inputFromURL(const ParsedURL & url) override
+    std::optional<Input> inputFromURL(const ParsedURL & url) const override
     {
         if (url.scheme != "flake") return {};
 
@@ -41,7 +41,6 @@ struct IndirectInputScheme : InputScheme
         // FIXME: forbid query params?
 
         Input input;
-        input.direct = false;
         input.attrs.insert_or_assign("type", "indirect");
         input.attrs.insert_or_assign("id", id);
         if (rev) input.attrs.insert_or_assign("rev", rev->gitRev());
@@ -50,7 +49,7 @@ struct IndirectInputScheme : InputScheme
         return input;
     }
 
-    std::optional<Input> inputFromAttrs(const Attrs & attrs) override
+    std::optional<Input> inputFromAttrs(const Attrs & attrs) const override
     {
         if (maybeGetStrAttr(attrs, "type") != "indirect") return {};
 
@@ -63,12 +62,11 @@ struct IndirectInputScheme : InputScheme
             throw BadURL("'%s' is not a valid flake ID", id);
 
         Input input;
-        input.direct = false;
         input.attrs = attrs;
         return input;
     }
 
-    ParsedURL toURL(const Input & input) override
+    ParsedURL toURL(const Input & input) const override
     {
         ParsedURL url;
         url.scheme = "flake";
@@ -78,15 +76,10 @@ struct IndirectInputScheme : InputScheme
         return url;
     }
 
-    bool hasAllInfo(const Input & input) override
-    {
-        return false;
-    }
-
     Input applyOverrides(
         const Input & _input,
         std::optional<std::string> ref,
-        std::optional<Hash> rev) override
+        std::optional<Hash> rev) const override
     {
         auto input(_input);
         if (rev) input.attrs.insert_or_assign("rev", rev->gitRev());
@@ -94,10 +87,13 @@ struct IndirectInputScheme : InputScheme
         return input;
     }
 
-    std::pair<StorePath, Input> fetch(ref<Store> store, const Input & input) override
+    std::pair<ref<InputAccessor>, Input> getAccessor(ref<Store> store, const Input & input) const override
     {
         throw Error("indirect input '%s' cannot be fetched directly", input.to_string());
     }
+
+    bool isDirect(const Input & input) const override
+    { return false; }
 };
 
 static auto rIndirectInputScheme = OnStartup([] { registerInputScheme(std::make_unique<IndirectInputScheme>()); });

src/libfetchers/input-accessor.cc

@@ -0,0 +1,260 @@
+#include "input-accessor.hh"
+#include "util.hh"
+#include "store-api.hh"
+#include "cache.hh"
+
+#include <atomic>
+
+namespace nix {
+
+static std::atomic<size_t> nextNumber{0};
+
+InputAccessor::InputAccessor()
+    : number(++nextNumber)
+    , displayPrefix{"«unknown»"}
+{
+}
+
+// FIXME: merge with archive.cc.
+void InputAccessor::dumpPath(
+    const CanonPath & path,
+    Sink & sink,
+    PathFilter & filter)
+{
+    auto dumpContents = [&](const CanonPath & path)
+    {
+        // FIXME: pipe
+        auto s = readFile(path);
+        sink << "contents" << s.size();
+        sink(s);
+        writePadding(s.size(), sink);
+    };
+
+    std::function<void(const CanonPath & path)> dump;
+
+    dump = [&](const CanonPath & path) {
+        checkInterrupt();
+
+        auto st = lstat(path);
+
+        sink << "(";
+
+        if (st.type == tRegular) {
+            sink << "type" << "regular";
+            if (st.isExecutable)
+                sink << "executable" << "";
+            dumpContents(path);
+        }
+
+        else if (st.type == tDirectory) {
+            sink << "type" << "directory";
+
+            /* If we're on a case-insensitive system like macOS, undo
+               the case hack applied by restorePath(). */
+            std::map<std::string, std::string> unhacked;
+            for (auto & i : readDirectory(path))
+                if (/* archiveSettings.useCaseHack */ false) { // FIXME
+                    std::string name(i.first);
+                    size_t pos = i.first.find(caseHackSuffix);
+                    if (pos != std::string::npos) {
+                        debug("removing case hack suffix from '%s'", path + i.first);
+                        name.erase(pos);
+                    }
+                    if (!unhacked.emplace(name, i.first).second)
+                        throw Error("file name collision in between '%s' and '%s'",
+                            (path + unhacked[name]),
+                            (path + i.first));
+                } else
+                    unhacked.emplace(i.first, i.first);
+
+            for (auto & i : unhacked)
+                if (filter((path + i.first).abs())) {
+                    sink << "entry" << "(" << "name" << i.first << "node";
+                    dump(path + i.second);
+                    sink << ")";
+                }
+        }
+
+        else if (st.type == tSymlink)
+            sink << "type" << "symlink" << "target" << readLink(path);
+
+        else throw Error("file '%s' has an unsupported type", path);
+
+        sink << ")";
+    };
+
+    sink << narVersionMagic1;
+    dump(path);
+}
+
+StorePath InputAccessor::fetchToStore(
+    ref<Store> store,
+    const CanonPath & path,
+    std::string_view name,
+    PathFilter * filter,
+    RepairFlag repair)
+{
+    // FIXME: add an optimisation for the case where the accessor is
+    // an FSInputAccessor pointing to a store path.
+
+    std::optional<std::string> cacheKey;
+
+    if (!filter && fingerprint) {
+        cacheKey = *fingerprint + "|" + name + "|" + path.abs();
+        if (auto storePathS = fetchers::getCache()->queryFact(*cacheKey)) {
+            if (auto storePath = store->maybeParseStorePath(*storePathS)) {
+                if (store->isValidPath(*storePath)) {
+                    debug("store path cache hit for '%s'", showPath(path));
+                    return *storePath;
+                }
+            }
+        }
+    } else
+        debug("source path '%s' is uncacheable", showPath(path));
+
+    Activity act(*logger, lvlChatty, actUnknown, fmt("copying '%s' to the store", showPath(path)));
+
+    auto source = sinkToSource([&](Sink & sink) {
+        dumpPath(path, sink, filter ? *filter : defaultPathFilter);
+    });
+
+    auto storePath =
+        settings.readOnlyMode
+        ? store->computeStorePathFromDump(*source, name).first
+        : store->addToStoreFromDump(*source, name, FileIngestionMethod::Recursive, htSHA256, repair);
+
+    if (cacheKey)
+        fetchers::getCache()->upsertFact(*cacheKey, store->printStorePath(storePath));
+
+    return storePath;
+}
+
+std::optional<InputAccessor::Stat> InputAccessor::maybeLstat(const CanonPath & path)
+{
+    // FIXME: merge these into one operation.
+    if (!pathExists(path))
+        return {};
+    return lstat(path);
+}
+
+void InputAccessor::setPathDisplay(std::string displayPrefix, std::string displaySuffix)
+{
+    this->displayPrefix = std::move(displayPrefix);
+    this->displaySuffix = std::move(displaySuffix);
+}
+
+std::string InputAccessor::showPath(const CanonPath & path)
+{
+    return displayPrefix + path.abs() + displaySuffix;
+}
+
+SourcePath InputAccessor::root()
+{
+    return {ref(shared_from_this()), CanonPath::root};
+}
+
+std::ostream & operator << (std::ostream & str, const SourcePath & path)
+{
+    str << path.to_string();
+    return str;
+}
+
+struct MemoryInputAccessorImpl : MemoryInputAccessor
+{
+    std::map<CanonPath, std::string> files;
+
+    std::string readFile(const CanonPath & path) override
+    {
+        auto i = files.find(path);
+        if (i == files.end())
+            throw Error("file '%s' does not exist", path);
+        return i->second;
+    }
+
+    bool pathExists(const CanonPath & path) override
+    {
+        auto i = files.find(path);
+        return i != files.end();
+    }
+
+    Stat lstat(const CanonPath & path) override
+    {
+        auto i = files.find(path);
+        if (i != files.end())
+            return Stat { .type = tRegular, .isExecutable = false };
+        throw Error("file '%s' does not exist", path);
+    }
+
+    DirEntries readDirectory(const CanonPath & path) override
+    {
+        return {};
+    }
+
+    std::string readLink(const CanonPath & path) override
+    {
+        throw UnimplementedError("MemoryInputAccessor::readLink");
+    }
+
+    SourcePath addFile(CanonPath path, std::string && contents) override
+    {
+        files.emplace(path, std::move(contents));
+
+        return {ref(shared_from_this()), std::move(path)};
+    }
+};
+
+ref<MemoryInputAccessor> makeMemoryInputAccessor()
+{
+    return make_ref<MemoryInputAccessorImpl>();
+}
+
+StorePath SourcePath::fetchToStore(
+    ref<Store> store,
+    std::string_view name,
+    PathFilter * filter,
+    RepairFlag repair) const
+{
+    return accessor->fetchToStore(store, path, name, filter, repair);
+}
+
+std::string_view SourcePath::baseName() const
+{
+    return path.baseName().value_or("source");
+}
+
+SourcePath SourcePath::parent() const
+{
+    auto p = path.parent();
+    assert(p);
+    return {accessor, std::move(*p)};
+}
+
+SourcePath SourcePath::resolveSymlinks() const
+{
+    CanonPath res("/");
+
+    int linksAllowed = 1024;
+
+    for (auto & component : path) {
+        res.push(component);
+        while (true) {
+            if (auto st = accessor->maybeLstat(res)) {
+                if (!linksAllowed--)
+                    throw Error("infinite symlink recursion in path '%s'", path);
+                if (st->type != InputAccessor::tSymlink) break;
+                auto target = accessor->readLink(res);
+                if (hasPrefix(target, "/"))
+                    res = CanonPath(target);
+                else {
+                    res.pop();
+                    res.extend(CanonPath(target));
+                }
+            } else
+                break;
+        }
+    }
+
+    return {accessor, res};
+}
+
+}

src/libfetchers/input-accessor.hh

@@ -0,0 +1,184 @@
+#pragma once
+
+#include "ref.hh"
+#include "types.hh"
+#include "archive.hh"
+#include "canon-path.hh"
+#include "repair-flag.hh"
+
+namespace nix {
+
+MakeError(RestrictedPathError, Error);
+
+struct SourcePath;
+class StorePath;
+class Store;
+
+struct InputAccessor : public std::enable_shared_from_this<InputAccessor>
+{
+    const size_t number;
+
+    std::string displayPrefix, displaySuffix;
+
+    std::optional<std::string> fingerprint;
+
+    InputAccessor();
+
+    virtual ~InputAccessor()
+    { }
+
+    virtual std::string readFile(const CanonPath & path) = 0;
+
+    virtual bool pathExists(const CanonPath & path) = 0;
+
+    enum Type { tRegular, tSymlink, tDirectory, tMisc };
+
+    struct Stat
+    {
+        Type type = tMisc;
+        //uint64_t fileSize = 0; // regular files only
+        bool isExecutable = false; // regular files only
+    };
+
+    virtual Stat lstat(const CanonPath & path) = 0;
+
+    std::optional<Stat> maybeLstat(const CanonPath & path);
+
+    typedef std::optional<Type> DirEntry;
+
+    typedef std::map<std::string, DirEntry> DirEntries;
+
+    virtual DirEntries readDirectory(const CanonPath & path) = 0;
+
+    virtual std::string readLink(const CanonPath & path) = 0;
+
+    virtual void dumpPath(
+        const CanonPath & path,
+        Sink & sink,
+        PathFilter & filter = defaultPathFilter);
+
+    StorePath fetchToStore(
+        ref<Store> store,
+        const CanonPath & path,
+        std::string_view name,
+        PathFilter * filter = nullptr,
+        RepairFlag repair = NoRepair);
+
+    /* Return a corresponding path in the root filesystem, if
+       possible. This is only possible for inputs that are
+       materialized in the root filesystem. */
+    virtual std::optional<CanonPath> getPhysicalPath(const CanonPath & path)
+    { return std::nullopt; }
+
+    bool operator == (const InputAccessor & x) const
+    {
+        return number == x.number;
+    }
+
+    bool operator < (const InputAccessor & x) const
+    {
+        return number < x.number;
+    }
+
+    void setPathDisplay(std::string displayPrefix, std::string displaySuffix = "");
+
+    virtual std::string showPath(const CanonPath & path);
+
+    SourcePath root();
+
+    /* Return the maximum last-modified time of the files in this
+       tree, if available. */
+    virtual std::optional<time_t> getLastModified()
+    {
+        return std::nullopt;
+    }
+};
+
+typedef std::function<RestrictedPathError(const CanonPath & path)> MakeNotAllowedError;
+
+struct SourcePath;
+
+struct MemoryInputAccessor : InputAccessor
+{
+    virtual SourcePath addFile(CanonPath path, std::string && contents) = 0;
+};
+
+ref<MemoryInputAccessor> makeMemoryInputAccessor();
+
+ref<InputAccessor> makeZipInputAccessor(const CanonPath & path);
+
+ref<InputAccessor> makePatchingInputAccessor(
+    ref<InputAccessor> next,
+    const std::vector<std::string> & patches);
+
+struct SourcePath
+{
+    ref<InputAccessor> accessor;
+    CanonPath path;
+
+    std::string_view baseName() const;
+
+    SourcePath parent() const;
+
+    std::string readFile() const
+    { return accessor->readFile(path); }
+
+    bool pathExists() const
+    { return accessor->pathExists(path); }
+
+    InputAccessor::Stat lstat() const
+    { return accessor->lstat(path); }
+
+    std::optional<InputAccessor::Stat> maybeLstat() const
+    { return accessor->maybeLstat(path); }
+
+    InputAccessor::DirEntries readDirectory() const
+    { return accessor->readDirectory(path); }
+
+    std::string readLink() const
+    { return accessor->readLink(path); }
+
+    void dumpPath(
+        Sink & sink,
+        PathFilter & filter = defaultPathFilter) const
+    { return accessor->dumpPath(path, sink, filter); }
+
+    StorePath fetchToStore(
+        ref<Store> store,
+        std::string_view name,
+        PathFilter * filter = nullptr,
+        RepairFlag repair = NoRepair) const;
+
+    std::optional<CanonPath> getPhysicalPath() const
+    { return accessor->getPhysicalPath(path); }
+
+    std::string to_string() const
+    { return accessor->showPath(path); }
+
+    SourcePath operator + (const CanonPath & x) const
+    { return {accessor, path + x}; }
+
+    SourcePath operator + (std::string_view c) const
+    {  return {accessor, path + c}; }
+
+    bool operator == (const SourcePath & x) const
+    {
+        return std::tie(accessor, path) == std::tie(x.accessor, x.path);
+    }
+
+    bool operator != (const SourcePath & x) const
+    {
+        return std::tie(accessor, path) != std::tie(x.accessor, x.path);
+    }
+
+    bool operator < (const SourcePath & x) const
+    {
+        return std::tie(accessor, path) < std::tie(x.accessor, x.path);
+    }
+
+    SourcePath resolveSymlinks() const;
+};
+
+std::ostream & operator << (std::ostream & str, const SourcePath & path);
+
+}

src/libfetchers/local.mk

@@ -8,6 +8,6 @@ libfetchers_SOURCES := $(wildcard $(d)/*.cc)
 
 libfetchers_CXXFLAGS += -I src/libutil -I src/libstore
 
-libfetchers_LDFLAGS += -pthread
+libfetchers_LDFLAGS += -pthread -lzip
 
 libfetchers_LIBS = libutil libstore

src/libfetchers/mercurial.cc

@@ -4,7 +4,7 @@
 #include "tarfile.hh"
 #include "store-api.hh"
 #include "url-parts.hh"
-
+#include "fs-input-accessor.hh"
 #include "fetch-settings.hh"
 
 #include <sys/time.h>
@@ -43,7 +43,7 @@ static std::string runHg(const Strings & args, const std::optional<std::string>
 
 struct MercurialInputScheme : InputScheme
 {
-    std::optional<Input> inputFromURL(const ParsedURL & url) override
+    std::optional<Input> inputFromURL(const ParsedURL & url) const override
     {
         if (url.scheme != "hg+http" &&
             url.scheme != "hg+https" &&
@@ -69,7 +69,7 @@ struct MercurialInputScheme : InputScheme
         return inputFromAttrs(attrs);
     }
 
-    std::optional<Input> inputFromAttrs(const Attrs & attrs) override
+    std::optional<Input> inputFromAttrs(const Attrs & attrs) const override
     {
         if (maybeGetStrAttr(attrs, "type") != "hg") return {};
 
@@ -89,7 +89,7 @@ struct MercurialInputScheme : InputScheme
         return input;
     }
 
-    ParsedURL toURL(const Input & input) override
+    ParsedURL toURL(const Input & input) const override
     {
         auto url = parseURL(getStrAttr(input.attrs, "url"));
         url.scheme = "hg+" + url.scheme;
@@ -98,17 +98,10 @@ struct MercurialInputScheme : InputScheme
         return url;
     }
 
-    bool hasAllInfo(const Input & input) override
-    {
-        // FIXME: ugly, need to distinguish between dirty and clean
-        // default trees.
-        return input.getRef() == "default" || maybeGetIntAttr(input.attrs, "revCount");
-    }
-
     Input applyOverrides(
         const Input & input,
         std::optional<std::string> ref,
-        std::optional<Hash> rev) override
+        std::optional<Hash> rev) const override
     {
         auto res(input);
         if (rev) res.attrs.insert_or_assign("rev", rev->gitRev());
@@ -116,26 +109,29 @@ struct MercurialInputScheme : InputScheme
         return res;
     }
 
-    std::optional<Path> getSourcePath(const Input & input) override
+    void putFile(
+        const Input & input,
+        const CanonPath & path,
+        std::string_view contents,
+        std::optional<std::string> commitMsg) const
     {
-        auto url = parseURL(getStrAttr(input.attrs, "url"));
-        if (url.scheme == "file" && !input.getRef() && !input.getRev())
-            return url.path;
-        return {};
-    }
+        auto [isLocal, repoPath] = getActualUrl(input);
+        if (!isLocal)
+            throw Error("cannot commit '%s' to Mercurial repository '%s' because it's not a working tree", path, input.to_string());
 
-    void markChangedFile(const Input & input, std::string_view file, std::optional<std::string> commitMsg) override
-    {
-        auto sourcePath = getSourcePath(input);
-        assert(sourcePath);
+        auto absPath = CanonPath(repoPath) + path;
+
+        // FIXME: make sure that absPath is not a symlink that escapes
+        // the repo.
+        writeFile(absPath.abs(), contents);
 
         // FIXME: shut up if file is already tracked.
         runHg(
-            { "add", *sourcePath + "/" + std::string(file) });
+            { "add", absPath.abs() });
 
         if (commitMsg)
             runHg(
-                { "commit", *sourcePath + "/" + std::string(file), "-m", *commitMsg });
+                { "commit", absPath.abs(), "-m", *commitMsg });
     }
 
     std::pair<bool, std::string> getActualUrl(const Input & input) const
@@ -145,9 +141,9 @@ struct MercurialInputScheme : InputScheme
         return {isLocal, isLocal ? url.path : url.base};
     }
 
-    std::pair<StorePath, Input> fetch(ref<Store> store, const Input & _input) override
+    StorePath fetchToStore(ref<Store> store, Input & input) const
     {
-        Input input(_input);
+        auto origRev = input.getRev();
 
         auto name = input.getName();
 
@@ -197,7 +193,7 @@ struct MercurialInputScheme : InputScheme
 
                 auto storePath = store->addToStore(input.getName(), actualPath, FileIngestionMethod::Recursive, htSHA256, filter);
 
-                return {std::move(storePath), input};
+                return storePath;
             }
         }
 
@@ -221,13 +217,12 @@ struct MercurialInputScheme : InputScheme
             });
         };
 
-        auto makeResult = [&](const Attrs & infoAttrs, StorePath && storePath)
-            -> std::pair<StorePath, Input>
+        auto makeResult = [&](const Attrs & infoAttrs, const StorePath & storePath) -> StorePath
         {
             assert(input.getRev());
-            assert(!_input.getRev() || _input.getRev() == input.getRev());
+            assert(!origRev || origRev == input.getRev());
             input.attrs.insert_or_assign("revCount", getIntAttr(infoAttrs, "revCount"));
-            return {std::move(storePath), input};
+            return storePath;
         };
 
         if (input.getRev()) {
@@ -307,7 +302,7 @@ struct MercurialInputScheme : InputScheme
             {"revCount", (uint64_t) revCount},
         });
 
-        if (!_input.getRev())
+        if (!origRev)
             getCache()->add(
                 store,
                 unlockedAttrs,
@@ -324,6 +319,20 @@ struct MercurialInputScheme : InputScheme
 
         return makeResult(infoAttrs, std::move(storePath));
     }
+
+    std::pair<ref<InputAccessor>, Input> getAccessor(ref<Store> store, const Input & _input) const override
+    {
+        Input input(_input);
+
+        auto storePath = fetchToStore(store, input);
+
+        return {makeStorePathAccessor(store, storePath), input};
+    }
+
+    bool isLocked(const Input & input) const override
+    {
+        return (bool) input.getRev();
+    }
 };
 
 static auto rMercurialInputScheme = OnStartup([] { registerInputScheme(std::make_unique<MercurialInputScheme>()); });

src/libfetchers/patching-input-accessor.cc

@@ -0,0 +1,116 @@
+#include "input-accessor.hh"
+
+namespace nix {
+
+// TODO: handle file creation / deletion.
+struct PatchingInputAccessor : InputAccessor
+{
+    ref<InputAccessor> next;
+
+    std::map<CanonPath, std::vector<std::string>> patchesPerFile;
+
+    PatchingInputAccessor(
+        ref<InputAccessor> next,
+        const std::vector<std::string> & patches)
+        : next(next)
+    {
+        /* Extract the patches for each file. */
+        for (auto & patch : patches) {
+            std::string_view p = patch;
+            std::string_view start;
+            std::string_view fileName;
+
+            auto flush = [&]()
+            {
+                if (start.empty()) return;
+                auto contents = start.substr(0, p.data() - start.data());
+                start = "";
+                auto slash = fileName.find('/');
+                if (slash == fileName.npos) return;
+                fileName = fileName.substr(slash);
+                debug("found patch for '%s'", fileName);
+                patchesPerFile.emplace(fileName, std::vector<std::string>())
+                    .first->second.push_back(std::string(contents));
+            };
+
+            while (!p.empty()) {
+                auto [line, rest] = getLine(p);
+
+                if (hasPrefix(line, "--- ")) {
+                    flush();
+                    start = p;
+                    fileName = line.substr(4);
+                }
+
+                if (!start.empty()) {
+                    if (!(hasPrefix(line, "+++ ")
+                            || hasPrefix(line, "@@")
+                            || hasPrefix(line, "+")
+                            || hasPrefix(line, "-")
+                            || hasPrefix(line, " ")
+                            || line.empty()))
+                    {
+                        flush();
+                    }
+                }
+
+                p = rest;
+            }
+
+            flush();
+        }
+    }
+
+    std::string readFile(const CanonPath & path) override
+    {
+        auto contents = next->readFile(path);
+
+        auto i = patchesPerFile.find(path);
+        if (i != patchesPerFile.end()) {
+            for (auto & patch : i->second) {
+                auto tempDir = createTempDir();
+                AutoDelete del(tempDir);
+                auto sourceFile = tempDir + "/source";
+                auto rejFile = tempDir + "/source.rej";
+                writeFile(sourceFile, contents);
+                try {
+                    contents = runProgram("patch", true, {"--quiet", sourceFile, "--output=-", "-r", rejFile}, patch);
+                } catch (ExecError & e) {
+                    del.cancel();
+                    throw;
+                }
+            }
+        }
+
+        return contents;
+    }
+
+    bool pathExists(const CanonPath & path) override
+    {
+        return next->pathExists(path);
+    }
+
+    Stat lstat(const CanonPath & path) override
+    {
+        return next->lstat(path);
+    }
+
+    DirEntries readDirectory(const CanonPath & path) override
+    {
+        return next->readDirectory(path);
+    }
+
+    std::string readLink(const CanonPath & path) override
+    {
+        return next->readLink(path);
+    }
+};
+
+ref<InputAccessor> makePatchingInputAccessor(
+    ref<InputAccessor> next,
+    const std::vector<std::string> & patches)
+{
+    return make_ref<PatchingInputAccessor>(next, std::move(patches));
+}
+
+}

src/libfetchers/path.cc

@@ -1,12 +1,13 @@
 #include "fetchers.hh"
 #include "store-api.hh"
 #include "archive.hh"
+#include "fs-input-accessor.hh"
 
 namespace nix::fetchers {
 
 struct PathInputScheme : InputScheme
 {
-    std::optional<Input> inputFromURL(const ParsedURL & url) override
+    std::optional<Input> inputFromURL(const ParsedURL & url) const override
     {
         if (url.scheme != "path") return {};
 
@@ -26,25 +27,31 @@ struct PathInputScheme : InputScheme
                 else
                     throw Error("path URL '%s' has invalid parameter '%s'", url.to_string(), name);
             }
+            else if (name == "lock")
+                input.attrs.emplace(name, Explicit<bool> { value == "1" });
             else
                 throw Error("path URL '%s' has unsupported parameter '%s'", url.to_string(), name);
 
         return input;
     }
 
-    std::optional<Input> inputFromAttrs(const Attrs & attrs) override
+    std::optional<Input> inputFromAttrs(const Attrs & attrs) const override
     {
         if (maybeGetStrAttr(attrs, "type") != "path") return {};
 
         getStrAttr(attrs, "path");
+        maybeGetBoolAttr(attrs, "lock");
 
         for (auto & [name, value] : attrs)
             /* Allow the user to pass in "fake" tree info
                attributes. This is useful for making a pinned tree
                work the same as the repository from which is exported
-               (e.g. path:/nix/store/...-source?lastModified=1585388205&rev=b0c285...). */
-            if (name == "type" || name == "rev" || name == "revCount" || name == "lastModified" || name == "narHash" || name == "path")
-                // checked in Input::fromAttrs
+               (e.g. path:/nix/store/...-source?lastModified=1585388205&rev=b0c285...).
+               FIXME: remove this hack once we have a prepopulated
+               flake input cache mechanism.
+            */
+            if (name == "type" || name == "rev" || name == "revCount" || name == "lastModified" || name == "narHash" || name == "path" || name == "lock")
+                // checked elsewhere
                 ;
             else
                 throw Error("unsupported path input attribute '%s'", name);
@@ -54,7 +61,12 @@ struct PathInputScheme : InputScheme
         return input;
     }
 
-    ParsedURL toURL(const Input & input) override
+    bool getLockAttr(const Input & input) const
+    {
+        return maybeGetBoolAttr(input.attrs, "lock").value_or(false);
+    }
+
+    ParsedURL toURL(const Input & input) const override
     {
         auto query = attrsToQuery(input.attrs);
         query.erase("path");
@@ -66,65 +78,87 @@ struct PathInputScheme : InputScheme
         };
     }
 
-    bool hasAllInfo(const Input & input) override
+    std::optional<std::string> isRelative(const Input & input) const override
     {
-        return true;
+        auto path = getStrAttr(input.attrs, "path");
+        if (hasPrefix(path, "/"))
+            return std::nullopt;
+        else
+            return path;
     }
 
-    std::optional<Path> getSourcePath(const Input & input) override
+    bool isLocked(const Input & input) const override
     {
-        return getStrAttr(input.attrs, "path");
+        return (bool) input.getNarHash();
     }
 
-    void markChangedFile(const Input & input, std::string_view file, std::optional<std::string> commitMsg) override
+    void putFile(
+        const Input & input,
+        const CanonPath & path,
+        std::string_view contents,
+        std::optional<std::string> commitMsg) const
     {
-        // nothing to do
+        auto absPath = CanonPath(getAbsPath(input)) + path;
+
+        // FIXME: make sure that absPath is not a symlink that escapes
+        // the repo.
+        writeFile(absPath.abs(), contents);
     }
 
-    std::pair<StorePath, Input> fetch(ref<Store> store, const Input & _input) override
+    CanonPath getAbsPath(const Input & input) const
     {
-        Input input(_input);
-        std::string absPath;
         auto path = getStrAttr(input.attrs, "path");
 
-        if (path[0] != '/') {
-            if (!input.parent)
+        if (path[0] == '/')
+            return CanonPath(path);
+
         throw Error("cannot fetch input '%s' because it uses a relative path", input.to_string());
-
-            auto parent = canonPath(*input.parent);
-
-            // the path isn't relative, prefix it
-            absPath = nix::absPath(path, parent);
-
-            // for security, ensure that if the parent is a store path, it's inside it
-            if (store->isInStore(parent)) {
-                auto storePath = store->printStorePath(store->toStorePath(parent).first);
-                if (!isDirOrInDir(absPath, storePath))
-                    throw BadStorePath("relative path '%s' points outside of its parent's store path '%s'", path, storePath);
     }
+
+    std::pair<ref<InputAccessor>, Input> getAccessor(ref<Store> store, const Input & input) const override
+    {
+        auto absPath = getAbsPath(input);
+        auto input2(input);
+        input2.attrs.emplace("path", (std::string) absPath.abs());
+
+        if (getLockAttr(input2)) {
+
+            auto storePath = store->maybeParseStorePath(absPath.abs());
+
+            if (!storePath || storePath->name() != input.getName() || !store->isValidPath(*storePath)) {
+                Activity act(*logger, lvlChatty, actUnknown, fmt("copying '%s' to the store", absPath));
+                storePath = store->addToStore(input.getName(), absPath.abs());
+                auto narHash = store->queryPathInfo(*storePath)->narHash;
+                input2.attrs.insert_or_assign("narHash", narHash.to_string(SRI, true));
             } else
-            absPath = path;
+                input2.attrs.erase("narHash");
 
-        Activity act(*logger, lvlTalkative, actUnknown, fmt("copying '%s'", absPath));
+            input2.attrs.erase("lastModified");
 
-        // FIXME: check whether access to 'path' is allowed.
-        auto storePath = store->maybeParseStorePath(absPath);
+            auto makeNotAllowedError = [absPath](const CanonPath & path) -> RestrictedPathError
+            {
+                return RestrictedPathError("path '%s' does not exist'", absPath + path);
+            };
 
-        if (storePath)
-            store->addTempRoot(*storePath);
+            return {makeStorePathAccessor(store, *storePath, std::move(makeNotAllowedError)), std::move(input2)};
 
-        time_t mtime = 0;
-        if (!storePath || storePath->name() != "source" || !store->isValidPath(*storePath)) {
-            // FIXME: try to substitute storePath.
-            auto src = sinkToSource([&](Sink & sink) {
-                mtime = dumpPathAndGetMtime(absPath, sink, defaultPathFilter);
-            });
-            storePath = store->addToStoreFromDump(*src, "source");
+        } else {
+            return {makeFSInputAccessor(absPath), std::move(input2)};
         }
-        input.attrs.insert_or_assign("lastModified", uint64_t(mtime));
-
-        return {std::move(*storePath), input};
     }
+
+    std::optional<std::string> getFingerprint(ref<Store> store, const Input & input) const override
+    {
+        /* If this path is in the Nix store, we can consider it
+           locked, so just use the path as its fingerprint. Maybe we
+           should restrict this to CA paths but that's not
+           super-important. */
+        auto path = getAbsPath(input);
+        if (store->isInStore(path.abs()))
+            return path.abs();
+        return std::nullopt;
+    }
+
 };
 
 static auto rPathInputScheme = OnStartup([] { registerInputScheme(std::make_unique<PathInputScheme>()); });

src/libfetchers/registry.cc

@@ -1,5 +1,5 @@
 #include "registry.hh"
-#include "fetchers.hh"
+#include "tarball.hh"
 #include "util.hh"
 #include "globals.hh"
 #include "store-api.hh"

src/libfetchers/tarball.cc

@@ -1,3 +1,4 @@
+#include "tarball.hh"
 #include "fetchers.hh"
 #include "cache.hh"
 #include "filetransfer.hh"
@@ -7,6 +8,7 @@
 #include "tarfile.hh"
 #include "types.hh"
 #include "split.hh"
+#include "fs-input-accessor.hh"
 
 namespace nix::fetchers {
 
@@ -110,7 +112,7 @@ DownloadFileResult downloadFile(
     };
 }
 
-std::pair<Tree, time_t> downloadTarball(
+std::pair<StorePath, time_t> downloadTarball(
     ref<Store> store,
     const std::string & url,
     const std::string & name,
@@ -127,7 +129,7 @@ std::pair<Tree, time_t> downloadTarball(
 
     if (cached && !cached->expired)
         return {
-            Tree { .actualPath = store->toRealPath(cached->storePath), .storePath = std::move(cached->storePath) },
+            std::move(cached->storePath),
             getIntAttr(cached->infoAttrs, "lastModified")
         };
 
@@ -164,7 +166,7 @@ std::pair<Tree, time_t> downloadTarball(
         locked);
 
     return {
-        Tree { .actualPath = store->toRealPath(*unpackedStorePath), .storePath = std::move(*unpackedStorePath) },
+        std::move(*unpackedStorePath),
         lastModified,
     };
 }
@@ -185,7 +187,7 @@ struct CurlInputScheme : InputScheme
 
     virtual bool isValidURL(const ParsedURL & url) const = 0;
 
-    std::optional<Input> inputFromURL(const ParsedURL & url) override
+    std::optional<Input> inputFromURL(const ParsedURL & url) const override
     {
         if (!isValidURL(url))
             return std::nullopt;
@@ -203,7 +205,7 @@ struct CurlInputScheme : InputScheme
         return input;
     }
 
-    std::optional<Input> inputFromAttrs(const Attrs & attrs) override
+    std::optional<Input> inputFromAttrs(const Attrs & attrs) const override
     {
         auto type = maybeGetStrAttr(attrs, "type");
         if (type != inputType()) return {};
@@ -220,20 +222,20 @@ struct CurlInputScheme : InputScheme
         return input;
     }
 
-    ParsedURL toURL(const Input & input) override
+    ParsedURL toURL(const Input & input) const override
     {
         auto url = parseURL(getStrAttr(input.attrs, "url"));
-        // NAR hashes are preferred over file hashes since tar/zip files        // don't have a canonical representation.
+        // NAR hashes are preferred over file hashes since tar/zip
+        // files don't have a canonical representation.
         if (auto narHash = input.getNarHash())
             url.query.insert_or_assign("narHash", narHash->to_string(SRI, true));
         return url;
     }
 
-    bool hasAllInfo(const Input & input) override
+    bool isLocked(const Input & input) const override
     {
-        return true;
+        return (bool) input.getNarHash();
     }
-
 };
 
 struct FileInputScheme : CurlInputScheme
@@ -249,10 +251,17 @@ struct FileInputScheme : CurlInputScheme
                     : !hasTarballExtension(url.path));
     }
 
-    std::pair<StorePath, Input> fetch(ref<Store> store, const Input & input) override
+    std::pair<ref<InputAccessor>, Input> getAccessor(ref<Store> store, const Input & _input) const override
     {
+        auto input(_input);
+
         auto file = downloadFile(store, getStrAttr(input.attrs, "url"), input.getName(), false);
-        return {std::move(file.storePath), input};
+
+        // FIXME: remove?
+        auto narHash = store->queryPathInfo(file.storePath)->narHash;
+        input.attrs.insert_or_assign("narHash", narHash.to_string(SRI, true));
+
+        return {makeStorePathAccessor(store, file.storePath), input};
     }
 };
 
@@ -270,10 +279,17 @@ struct TarballInputScheme : CurlInputScheme
                     : hasTarballExtension(url.path));
     }
 
-    std::pair<StorePath, Input> fetch(ref<Store> store, const Input & input) override
+    std::pair<ref<InputAccessor>, Input> getAccessor(ref<Store> store, const Input & _input) const override
     {
-        auto tree = downloadTarball(store, getStrAttr(input.attrs, "url"), input.getName(), false).first;
-        return {std::move(tree.storePath), input};
+        auto input(_input);
+
+        auto storePath = downloadTarball(store, getStrAttr(input.attrs, "url"), input.getName(), false).first;
+
+        // FIXME: remove?
+        auto narHash = store->queryPathInfo(storePath)->narHash;
+        input.attrs.insert_or_assign("narHash", narHash.to_string(SRI, true));
+
+        return {makeStorePathAccessor(store, storePath), input};
     }
 };
 

src/libfetchers/tarball.hh

@@ -0,0 +1,29 @@
+#pragma once
+
+#include "types.hh"
+#include "path.hh"
+
+namespace nix::fetchers {
+
+struct DownloadFileResult
+{
+    StorePath storePath;
+    std::string etag;
+    std::string effectiveUrl;
+};
+
+DownloadFileResult downloadFile(
+    ref<Store> store,
+    const std::string & url,
+    const std::string & name,
+    bool locked,
+    const Headers & headers = {});
+
+std::pair<StorePath, time_t> downloadTarball(
+    ref<Store> store,
+    const std::string & url,
+    const std::string & name,
+    bool locked,
+    const Headers & headers = {});
+
+}

src/libfetchers/zip-input-accessor.cc

@@ -0,0 +1,196 @@
+#include "input-accessor.hh"
+
+#include <zip.h>
+#include <arpa/inet.h>
+
+namespace nix {
+
+struct cmp_str
+{
+    bool operator ()(const char * a, const char * b) const
+    {
+        return std::strcmp(a, b) < 0;
+    }
+};
+
+struct ZipMember
+{
+    struct zip_file * p = nullptr;
+    ZipMember(struct zip_file * p) : p(p) { }
+    ~ZipMember() { if (p) zip_fclose(p); }
+    operator zip_file *() { return p; }
+};
+
+struct ZipInputAccessor : InputAccessor
+{
+    CanonPath zipPath;
+    struct zip * zipFile = nullptr;
+
+    typedef std::map<const char *, struct zip_stat, cmp_str> Members;
+    Members members;
+
+    time_t lastModified = 0;
+
+    ZipInputAccessor(const CanonPath & _zipPath)
+        : zipPath(_zipPath)
+    {
+        int error;
+        zipFile = zip_open(zipPath.c_str(), ZIP_RDONLY, &error);
+        if (!zipFile) {
+            char errorMsg[1024];
+            zip_error_to_str(errorMsg, sizeof errorMsg, error, errno);
+            throw Error("couldn't open '%s': %s", zipPath, errorMsg);
+        }
+
+        /* Read the index of the zip file and put it in a map.  This
+           is unfortunately necessary because libzip's lookup
+           functions are O(n) time. */
+        struct zip_stat sb;
+        zip_uint64_t nrEntries = zip_get_num_entries(zipFile, 0);
+        for (zip_uint64_t n = 0; n < nrEntries; ++n) {
+            if (zip_stat_index(zipFile, n, 0, &sb))
+                throw Error("couldn't stat archive member #%d in '%s': %s", n, zipPath, zip_strerror(zipFile));
+
+            /* Get the timestamp of this file. */
+            #if 0
+            if (sb.valid & ZIP_STAT_MTIME)
+                lastModified = std::max(lastModified, sb.mtime);
+            #endif
+            auto nExtra = zip_file_extra_fields_count(zipFile, n, ZIP_FL_CENTRAL);
+            for (auto i = 0; i < nExtra; ++i) {
+                zip_uint16_t id, len;
+                auto extra = zip_file_extra_field_get(zipFile, i, 0, &id, &len, ZIP_FL_CENTRAL);
+                if (id == 0x5455 && len >= 5)
+                    lastModified = std::max(lastModified, (time_t) readLittleEndian<uint32_t>((unsigned char *) extra + 1));
+            }
+
+            auto slash = strchr(sb.name, '/');
+            if (!slash) continue;
+            members.emplace(slash, sb);
+        }
+    }
+
+    ~ZipInputAccessor()
+    {
+        if (zipFile) zip_close(zipFile);
+    }
+
+    std::string _readFile(const CanonPath & path)
+    {
+        auto i = members.find(((std::string) path.abs()).c_str());
+        if (i == members.end())
+            throw Error("file '%s' does not exist", showPath(path));
+
+        ZipMember member(zip_fopen_index(zipFile, i->second.index, 0));
+        if (!member)
+            throw Error("couldn't open archive member '%s': %s",
+                showPath(path), zip_strerror(zipFile));
+
+        std::string buf(i->second.size, 0);
+        if (zip_fread(member, buf.data(), i->second.size) != (zip_int64_t) i->second.size)
+            throw Error("couldn't read archive member '%s' in '%s'", path, zipPath);
+
+        return buf;
+    }
+
+    std::string readFile(const CanonPath & path) override
+    {
+        if (lstat(path).type != tRegular)
+            throw Error("file '%s' is not a regular file", path);
+
+        return _readFile(path);
+    }
+
+    bool pathExists(const CanonPath & path) override
+    {
+        return
+            members.find(path.c_str()) != members.end()
+            || members.find(((std::string) path.abs() + "/").c_str()) != members.end();
+    }
+
+    Stat lstat(const CanonPath & path) override
+    {
+        if (path.isRoot())
+            return Stat { .type = tDirectory };
+
+        Type type = tRegular;
+        bool isExecutable = false;
+
+        auto i = members.find(path.c_str());
+        if (i == members.end()) {
+            i = members.find(((std::string) path.abs() + "/").c_str());
+            type = tDirectory;
+        }
+        if (i == members.end())
+            throw Error("file '%s' does not exist", showPath(path));
+
+        // FIXME: cache this
+        zip_uint8_t opsys;
+        zip_uint32_t attributes;
+        if (zip_file_get_external_attributes(zipFile, i->second.index, ZIP_FL_UNCHANGED, &opsys, &attributes) == -1)
+            throw Error("couldn't get external attributes of '%s': %s",
+                showPath(path), zip_strerror(zipFile));
+
+        switch (opsys) {
+        case ZIP_OPSYS_UNIX:
+            auto t = (attributes >> 16) & 0770000;
+            switch (t) {
+            case 0040000: type = tDirectory; break;
+            case 0100000:
+                type = tRegular;
+                isExecutable = (attributes >> 16) & 0000100;
+                break;
+            case 0120000: type = tSymlink; break;
+            default:
+                throw Error("file '%s' has unsupported type %o", showPath(path), t);
+            }
+            break;
+        }
+
+        return Stat { .type = type, .isExecutable = isExecutable };
+    }
+
+    DirEntries readDirectory(const CanonPath & _path) override
+    {
+        std::string path(_path.abs());
+        if (path != "/") path += "/";
+
+        auto i = members.find(path.c_str());
+        if (i == members.end())
+            throw Error("directory '%s' does not exist", showPath(_path));
+
+        ++i;
+
+        DirEntries entries;
+
+        for (; i != members.end() && strncmp(i->first, path.c_str(), path.size()) == 0; ++i) {
+            auto start = i->first + path.size();
+            auto slash = strchr(start, '/');
+            if (slash && strcmp(slash, "/") != 0) continue;
+            auto name = slash ? std::string(start, slash - start) : std::string(start);
+            entries.emplace(name, std::nullopt);
+        }
+
+        return entries;
+    }
+
+    std::string readLink(const CanonPath & path) override
+    {
+        if (lstat(path).type != tSymlink)
+            throw Error("file '%s' is not a symlink", showPath(path));
+
+        return _readFile(path);
+    }
+
+    std::optional<time_t> getLastModified() override
+    {
+        return lastModified;
+    }
+};
+
+ref<InputAccessor> makeZipInputAccessor(const CanonPath & path)
+{
+    return make_ref<ZipInputAccessor>(path);
+}
+
+}

src/libstore/binary-cache-store.cc

@@ -354,7 +354,7 @@ void BinaryCacheStore::narFromPath(const StorePath & storePath, Sink & sink)
     try {
         getFile(info->url, *decompressor);
     } catch (NoSuchBinaryCacheFile & e) {
-        throw SubstituteGone(e.info());
+        throw SubstituteGone(std::move(e.info()));
     }
 
     decompressor->finish();

src/libstore/build/derivation-goal.cc

@@ -135,7 +135,7 @@ void DerivationGoal::killChild()
 void DerivationGoal::timedOut(Error && ex)
 {
     killChild();
-    done(BuildResult::TimedOut, {}, ex);
+    done(BuildResult::TimedOut, {}, std::move(ex));
 }
 
 
@@ -951,7 +951,7 @@ void DerivationGoal::buildDone()
                 BuildResult::PermanentFailure;
         }
 
-        done(st, {}, e);
+        done(st, {}, std::move(e));
         return;
     }
 }
@@ -1402,7 +1402,7 @@ void DerivationGoal::done(
         fs << worker.store.printStorePath(drvPath) << "\t" << buildResult.toString() << std::endl;
     }
 
-    amDone(buildResult.success() ? ecSuccess : ecFailed, ex);
+    amDone(buildResult.success() ? ecSuccess : ecFailed, std::move(ex));
 }
 
 

src/libstore/build/entry-points.cc

@@ -30,7 +30,7 @@ void Store::buildPaths(const std::vector<DerivedPath> & reqs, BuildMode buildMod
             if (ex)
                 logError(i->ex->info());
             else
-                ex = i->ex;
+                ex = std::move(i->ex);
         }
         if (i->exitCode != Goal::ecSuccess) {
             if (auto i2 = dynamic_cast<DerivationGoal *>(i.get())) failed.insert(i2->drvPath);
@@ -40,7 +40,7 @@ void Store::buildPaths(const std::vector<DerivedPath> & reqs, BuildMode buildMod
 
     if (failed.size() == 1 && ex) {
         ex->status = worker.exitStatus();
-        throw *ex;
+        throw std::move(*ex);
     } else if (!failed.empty()) {
         if (ex) logError(ex->info());
         throw Error(worker.exitStatus(), "build of %s failed", showPaths(failed));
@@ -109,7 +109,7 @@ void Store::ensurePath(const StorePath & path)
     if (goal->exitCode != Goal::ecSuccess) {
         if (goal->ex) {
             goal->ex->status = worker.exitStatus();
-            throw *goal->ex;
+            throw std::move(*goal->ex);
         } else
             throw Error(worker.exitStatus(), "path '%s' does not exist and cannot be created", printStorePath(path));
     }

src/libstore/build/local-derivation-goal.cc

@@ -193,7 +193,7 @@ void LocalDerivationGoal::tryLocalBuild() {
         outputLocks.unlock();
         buildUser.reset();
         worker.permanentFailure = true;
-        done(BuildResult::InputRejected, {}, e);
+        done(BuildResult::InputRejected, {}, std::move(e));
         return;
     }
 

src/libstore/derivations.cc

@@ -448,7 +448,7 @@ std::string Derivation::unparse(const Store & store, bool maskOutputs,
 
 
 // FIXME: remove
-bool isDerivation(const std::string & fileName)
+bool isDerivation(std::string_view fileName)
 {
     return hasSuffix(fileName, drvExtension);
 }

src/libstore/derivations.hh

@@ -224,7 +224,7 @@ StorePath writeDerivation(Store & store,
 Derivation parseDerivation(const Store & store, std::string && s, std::string_view name);
 
 // FIXME: remove
-bool isDerivation(const std::string & fileName);
+bool isDerivation(std::string_view fileName);
 
 /* Calculate the name that will be used for the store path for this
    output.

src/libstore/filetransfer.cc

@@ -33,14 +33,6 @@ FileTransferSettings fileTransferSettings;
 
 static GlobalConfig::Register rFileTransferSettings(&fileTransferSettings);
 
-std::string resolveUri(std::string_view uri)
-{
-    if (uri.compare(0, 8, "channel:") == 0)
-        return "https://nixos.org/channels/" + std::string(uri.substr(8)) + "/nixexprs.tar.xz";
-    else
-        return std::string(uri);
-}
-
 struct curlFileTransfer : public FileTransfer
 {
     CURLM * curlm = 0;
@@ -142,9 +134,9 @@ struct curlFileTransfer : public FileTransfer
         }
 
         template<class T>
-        void fail(const T & e)
+        void fail(T && e)
         {
-            failEx(std::make_exception_ptr(e));
+            failEx(std::make_exception_ptr(std::move(e)));
         }
 
         LambdaSink finalSink;
@@ -472,7 +464,7 @@ struct curlFileTransfer : public FileTransfer
                     fileTransfer.enqueueItem(shared_from_this());
                 }
                 else
-                    fail(exc);
+                    fail(std::move(exc));
             }
         }
     };
@@ -873,14 +865,4 @@ FileTransferError::FileTransferError(FileTransfer::Error error, std::optional<st
         err.msg = hf;
 }
 
-bool isUri(std::string_view s)
-{
-    if (s.compare(0, 8, "channel:") == 0) return true;
-    size_t pos = s.find("://");
-    if (pos == std::string::npos) return false;
-    std::string scheme(s, 0, pos);
-    return scheme == "http" || scheme == "https" || scheme == "file" || scheme == "channel" || scheme == "git" || scheme == "s3" || scheme == "ssh";
-}
-
-
 }

src/libstore/filetransfer.hh

@@ -125,9 +125,4 @@ public:
     FileTransferError(FileTransfer::Error error, std::optional<std::string> response, const Args & ... args);
 };
 
-bool isUri(std::string_view s);
-
-/* Resolve deprecated 'channel:<foo>' URLs. */
-std::string resolveUri(std::string_view uri);
-
 }

src/libstore/remote-store.cc

@@ -447,7 +447,7 @@ void RemoteStore::queryPathInfoUncached(const StorePath & path,
             } catch (Error & e) {
                 // Ugly backwards compatibility hack.
                 if (e.msg().find("is not valid") != std::string::npos)
-                    throw InvalidPath(e.info());
+                    throw InvalidPath(std::move(e.info()));
                 throw;
             }
             if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 17) {

src/libstore/store-api.cc

@@ -17,21 +17,21 @@
 namespace nix {
 
 
-bool Store::isInStore(const Path & path) const
+bool Store::isInStore(PathView path) const
 {
     return isInDir(path, storeDir);
 }
 
 
-std::pair<StorePath, Path> Store::toStorePath(const Path & path) const
+std::pair<StorePath, Path> Store::toStorePath(PathView path) const
 {
     if (!isInStore(path))
         throw Error("path '%1%' is not in the Nix store", path);
-    Path::size_type slash = path.find('/', storeDir.size() + 1);
+    auto slash = path.find('/', storeDir.size() + 1);
     if (slash == Path::npos)
         return {parseStorePath(path), ""};
     else
-        return {parseStorePath(std::string_view(path).substr(0, slash)), path.substr(slash)};
+        return {parseStorePath(path.substr(0, slash)), (Path) path.substr(slash)};
 }
 
 
@@ -220,13 +220,17 @@ StorePath Store::makeTextPath(std::string_view name, const Hash & hash,
 }
 
 
-std::pair<StorePath, Hash> Store::computeStorePathForPath(std::string_view name,
-    const Path & srcPath, FileIngestionMethod method, HashType hashAlgo, PathFilter & filter) const
+std::pair<StorePath, Hash> Store::computeStorePathFromDump(
+    Source & dump,
+    std::string_view name,
+    FileIngestionMethod method,
+    HashType hashAlgo,
+    const StorePathSet & references) const
 {
-    Hash h = method == FileIngestionMethod::Recursive
-        ? hashPath(hashAlgo, srcPath, filter).first
-        : hashFile(hashAlgo, srcPath);
-    return std::make_pair(makeFixedOutputPath(method, h, name), h);
+    HashSink sink(hashAlgo);
+    dump.drainInto(sink);
+    auto hash = sink.finish().first;
+    return {makeFixedOutputPath(method, hash, name, references), hash};
 }
 
 

src/libstore/store-api.hh

@@ -179,7 +179,7 @@ public:
 
     /* Return true if ‘path’ is in the Nix store (but not the Nix
        store itself). */
-    bool isInStore(const Path & path) const;
+    bool isInStore(PathView path) const;
 
     /* Return true if ‘path’ is a store path, i.e. a direct child of
        the Nix store. */
@@ -187,7 +187,7 @@ public:
 
     /* Split a path like /nix/store/<hash>-<name>/<bla> into
        /nix/store/<hash>-<name> and /<bla>. */
-    std::pair<StorePath, Path> toStorePath(const Path & path) const;
+    std::pair<StorePath, Path> toStorePath(PathView path) const;
 
     /* Follow symlinks until we end up with a path in the Nix store. */
     Path followLinksToStore(std::string_view path) const;
@@ -217,12 +217,14 @@ public:
         const StorePathSet & references = {},
         bool hasSelfReference = false) const;
 
-    /* This is the preparatory part of addToStore(); it computes the
-       store path to which srcPath is to be copied.  Returns the store
-       path and the cryptographic hash of the contents of srcPath. */
-    std::pair<StorePath, Hash> computeStorePathForPath(std::string_view name,
-        const Path & srcPath, FileIngestionMethod method = FileIngestionMethod::Recursive,
-        HashType hashAlgo = htSHA256, PathFilter & filter = defaultPathFilter) const;
+    /* Read-only variant of addToStoreFromDump(). It returns the store
+       path to which a NAR or flat file would be written. */
+    std::pair<StorePath, Hash> computeStorePathFromDump(
+        Source & dump,
+        std::string_view name,
+        FileIngestionMethod method = FileIngestionMethod::Recursive,
+        HashType hashAlgo = htSHA256,
+        const StorePathSet & references = {}) const;
 
     /* Preparatory part of addTextToStore().
 

src/libutil/archive.cc

@@ -35,10 +35,6 @@ static ArchiveSettings archiveSettings;
 
 static GlobalConfig::Register rArchiveSettings(&archiveSettings);
 
-const std::string narVersionMagic1 = "nix-archive-1";
-
-static std::string caseHackSuffix = "~nix~case~hack~";
-
 PathFilter defaultPathFilter = [](const Path &) { return true; };
 
 

src/libutil/archive.hh

@@ -103,7 +103,9 @@ void copyNAR(Source & source, Sink & sink);
 void copyPath(const Path & from, const Path & to);
 
 
-extern const std::string narVersionMagic1;
+inline constexpr std::string_view narVersionMagic1 = "nix-archive-1";
+
+inline constexpr std::string_view caseHackSuffix = "~nix~case~hack~";
 
 
 }

src/libutil/canon-path.cc

@@ -0,0 +1,103 @@
+#include "canon-path.hh"
+#include "util.hh"
+
+namespace nix {
+
+CanonPath CanonPath::root = CanonPath("/");
+
+CanonPath::CanonPath(std::string_view raw)
+    : path(absPath((Path) raw, "/"))
+{ }
+
+CanonPath::CanonPath(std::string_view raw, const CanonPath & root)
+    : path(absPath((Path) raw, root.abs()))
+{ }
+
+std::optional<CanonPath> CanonPath::parent() const
+{
+    if (isRoot()) return std::nullopt;
+    return CanonPath(unchecked_t(), path.substr(0, std::max((size_t) 1, path.rfind('/'))));
+}
+
+void CanonPath::pop()
+{
+    assert(!isRoot());
+    path.resize(std::max((size_t) 1, path.rfind('/')));
+}
+
+bool CanonPath::isWithin(const CanonPath & parent) const
+{
+    return !(
+        path.size() < parent.path.size()
+        || path.substr(0, parent.path.size()) != parent.path
+        || (parent.path.size() > 1 && path.size() > parent.path.size()
+            && path[parent.path.size()] != '/'));
+}
+
+CanonPath CanonPath::removePrefix(const CanonPath & prefix) const
+{
+    assert(isWithin(prefix));
+    if (prefix.isRoot()) return *this;
+    if (path.size() == prefix.path.size()) return root;
+    return CanonPath(unchecked_t(), path.substr(prefix.path.size()));
+}
+
+void CanonPath::extend(const CanonPath & x)
+{
+    if (x.isRoot()) return;
+    if (isRoot())
+        path += x.rel();
+    else
+        path += x.abs();
+}
+
+CanonPath CanonPath::operator + (const CanonPath & x) const
+{
+    auto res = *this;
+    res.extend(x);
+    return res;
+}
+
+void CanonPath::push(std::string_view c)
+{
+    assert(c.find('/') == c.npos);
+    assert(c != "." && c != "..");
+    if (!isRoot()) path += '/';
+    path += c;
+}
+
+CanonPath CanonPath::operator + (std::string_view c) const
+{
+    auto res = *this;
+    res.push(c);
+    return res;
+}
+
+bool CanonPath::isAllowed(const std::set<CanonPath> & allowed) const
+{
+    /* Check if `this` is an exact match or the parent of an
+       allowed path. */
+    auto lb = allowed.lower_bound(*this);
+    if (lb != allowed.end()) {
+        if (lb->isWithin(*this))
+            return true;
+    }
+
+    /* Check if a parent of `this` is allowed. */
+    auto path = *this;
+    while (!path.isRoot()) {
+        path.pop();
+        if (allowed.count(path))
+            return true;
+    }
+
+    return false;
+}
+
+std::ostream & operator << (std::ostream & stream, const CanonPath & path)
+{
+    stream << path.abs();
+    return stream;
+}
+
+}

src/libutil/canon-path.hh

@@ -0,0 +1,165 @@
+#pragma once
+
+#include <string>
+#include <optional>
+#include <cassert>
+#include <iostream>
+#include <set>
+
+namespace nix {
+
+/* A canonical representation of a path. It ensures the following:
+
+   - It always starts with a slash.
+
+   - It never ends with a slash, except if the path is "/".
+
+   - A slash is never followed by a slash (i.e. no empty components).
+
+   - There are no components equal to '.' or '..'.
+
+   Note that the path does not need to correspond to an actually
+   existing path, and there is no guarantee that symlinks are
+   resolved.
+*/
+class CanonPath
+{
+    std::string path;
+
+public:
+
+    /* Construct a canon path from a non-canonical path. Any '.', '..'
+       or empty components are removed. */
+    CanonPath(std::string_view raw);
+
+    explicit CanonPath(const char * raw)
+        : CanonPath(std::string_view(raw))
+    { }
+
+    struct unchecked_t { };
+
+    CanonPath(unchecked_t _, std::string path)
+        : path(std::move(path))
+    { }
+
+    static CanonPath root;
+
+    /* If `raw` starts with a slash, return
+       `CanonPath(raw)`. Otherwise return a `CanonPath` representing
+       `root + "/" + raw`. */
+    CanonPath(std::string_view raw, const CanonPath & root);
+
+    bool isRoot() const
+    { return path.size() <= 1; }
+
+    explicit operator std::string_view() const
+    { return path; }
+
+    const std::string & abs() const
+    { return path; }
+
+    const char * c_str() const
+    { return path.c_str(); }
+
+    std::string_view rel() const
+    { return ((std::string_view) path).substr(1); }
+
+    struct Iterator
+    {
+        std::string_view remaining;
+        size_t slash;
+
+        Iterator(std::string_view remaining)
+            : remaining(remaining)
+            , slash(remaining.find('/'))
+        { }
+
+        bool operator != (const Iterator & x) const
+        { return remaining.data() != x.remaining.data(); }
+
+        const std::string_view operator * () const
+        { return remaining.substr(0, slash); }
+
+        void operator ++ ()
+        {
+            if (slash == remaining.npos)
+                remaining = remaining.substr(remaining.size());
+            else {
+                remaining = remaining.substr(slash + 1);
+                slash = remaining.find('/');
+            }
+        }
+    };
+
+    Iterator begin() const { return Iterator(rel()); }
+    Iterator end() const { return Iterator(rel().substr(path.size() - 1)); }
+
+    std::optional<CanonPath> parent() const;
+
+    /* Remove the last component. Panics if this path is the root.  */
+    void pop();
+
+    std::optional<std::string_view> dirOf() const
+    {
+        if (isRoot()) return std::nullopt;
+        return path.substr(0, path.rfind('/'));
+    }
+
+    std::optional<std::string_view> baseName() const
+    {
+        if (isRoot()) return std::nullopt;
+        return ((std::string_view) path).substr(path.rfind('/') + 1);
+    }
+
+    bool operator == (const CanonPath & x) const
+    { return path == x.path; }
+
+    bool operator != (const CanonPath & x) const
+    { return path != x.path; }
+
+    /* Compare paths lexicographically except that path separators
+       are sorted before any other character. That is, in the sorted order
+       a directory is always followed directly by its children. For
+       instance, 'foo' < 'foo/bar' < 'foo!'. */
+    bool operator < (const CanonPath & x) const
+    {
+        auto i = path.begin();
+        auto j = x.path.begin();
+        for ( ; i != path.end() && j != x.path.end(); ++i, ++j) {
+            auto c_i = *i;
+            if (c_i == '/') c_i = 0;
+            auto c_j = *j;
+            if (c_j == '/') c_j = 0;
+            if (c_i < c_j) return true;
+            if (c_i > c_j) return false;
+        }
+        return i == path.end() && j != x.path.end();
+    }
+
+    /* Return true if `this` is equal to `parent` or a child of
+       `parent`. */
+    bool isWithin(const CanonPath & parent) const;
+
+    CanonPath removePrefix(const CanonPath & prefix) const;
+
+    /* Append another path to this one. */
+    void extend(const CanonPath & x);
+
+    /* Concatenate two paths. */
+    CanonPath operator + (const CanonPath & x) const;
+
+    /* Add a path component to this one. It must not contain any slashes. */
+    void push(std::string_view c);
+
+    CanonPath operator + (std::string_view c) const;
+
+    /* Check whether access to this path is allowed, which is the case
+       if 1) `this` is within any of the `allowed` paths; or 2) any of
+       the `allowed` paths are within `this`. (The latter condition
+       ensures access to the parents of allowed paths.) */
+    bool isAllowed(const std::set<CanonPath> & allowed) const;
+};
+
+std::ostream & operator << (std::ostream & stream, const CanonPath & path);
+
+}

src/libutil/error.cc

@@ -9,9 +9,9 @@ namespace nix {
 
 const std::string nativeSystem = SYSTEM;
 
-void BaseError::addTrace(std::optional<ErrPos> e, hintformat hint)
+void BaseError::addTrace(std::shared_ptr<AbstractPos> && e, hintformat hint)
 {
-    err.traces.push_front(Trace { .pos = e, .hint = hint });
+    err.traces.push_front(Trace { .pos = std::move(e), .hint = hint });
 }
 
 // c++ std::exception descendants must have a 'const char* what()' function.
@@ -35,86 +35,41 @@ std::ostream & operator<<(std::ostream & os, const hintformat & hf)
     return os << hf.str();
 }
 
-std::string showErrPos(const ErrPos & errPos)
+std::ostream & operator <<(std::ostream & str, const AbstractPos & pos)
 {
-    if (errPos.line > 0) {
-        if (errPos.column > 0) {
-            return fmt("%d:%d", errPos.line, errPos.column);
-        } else {
-            return fmt("%d", errPos.line);
-        }
-    }
-    else {
-        return "";
-    }
+    pos.print(str);
+    str << ":" << pos.line;
+    if (pos.column > 0)
+        str << ":" << pos.column;
+    return str;
 }
 
-std::optional<LinesOfCode> getCodeLines(const ErrPos & errPos)
+std::optional<LinesOfCode> AbstractPos::getCodeLines() const
 {
-    if (errPos.line <= 0)
+    if (line == 0)
         return std::nullopt;
 
-    if (errPos.origin == foFile) {
-        LinesOfCode loc;
-        try {
-            // FIXME: when running as the daemon, make sure we don't
-            // open a file to which the client doesn't have access.
-            AutoCloseFD fd = open(errPos.file.c_str(), O_RDONLY | O_CLOEXEC);
-            if (!fd) return {};
+    if (auto source = getSource()) {
 
+        std::istringstream iss(*source);
         // count the newlines.
         int count = 0;
-            std::string line;
-            int pl = errPos.line - 1;
-            do
-            {
-                line = readLine(fd.get());
-                ++count;
-                if (count < pl)
-                    ;
-                else if (count == pl)
-                    loc.prevLineOfCode = line;
-                else if (count == pl + 1)
-                    loc.errLineOfCode = line;
-                else if (count == pl + 2) {
-                    loc.nextLineOfCode = line;
-                    break;
-                }
-            } while (true);
-            return loc;
-        }
-        catch (EndOfFile & eof) {
-            if (loc.errLineOfCode.has_value())
-                return loc;
-            else
-                return std::nullopt;
-        }
-        catch (std::exception & e) {
-            return std::nullopt;
-        }
-    } else {
-        std::istringstream iss(errPos.file);
-        // count the newlines.
-        int count = 0;
-        std::string line;
-        int pl = errPos.line - 1;
+        std::string curLine;
+        int pl = line - 1;
 
         LinesOfCode loc;
 
-        do
-        {
-            std::getline(iss, line);
+        do {
+            std::getline(iss, curLine);
             ++count;
             if (count < pl)
-            {
                 ;
-            }
             else if (count == pl) {
-                loc.prevLineOfCode = line;
+                loc.prevLineOfCode = curLine;
             } else if (count == pl + 1) {
-                loc.errLineOfCode = line;
+                loc.errLineOfCode = curLine;
             } else if (count == pl + 2) {
-                loc.nextLineOfCode = line;
+                loc.nextLineOfCode = curLine;
                 break;
             }
 
@@ -124,12 +79,14 @@ std::optional<LinesOfCode> getCodeLines(const ErrPos & errPos)
 
         return loc;
     }
+
+    return std::nullopt;
 }
 
 // print lines of code to the ostream, indicating the error column.
 void printCodeLines(std::ostream & out,
     const std::string & prefix,
-    const ErrPos & errPos,
+    const AbstractPos & errPos,
     const LinesOfCode & loc)
 {
     // previous line of code.
@@ -176,28 +133,6 @@ void printCodeLines(std::ostream & out,
     }
 }
 
-void printAtPos(const ErrPos & pos, std::ostream & out)
-{
-    if (pos) {
-        switch (pos.origin) {
-            case foFile: {
-                out << fmt(ANSI_BLUE "at " ANSI_WARNING "%s:%s" ANSI_NORMAL ":", pos.file, showErrPos(pos));
-                break;
-            }
-            case foString: {
-                out << fmt(ANSI_BLUE "at " ANSI_WARNING "«string»:%s" ANSI_NORMAL ":", showErrPos(pos));
-                break;
-            }
-            case foStdin: {
-                out << fmt(ANSI_BLUE "at " ANSI_WARNING "«stdin»:%s" ANSI_NORMAL ":", showErrPos(pos));
-                break;
-            }
-            default:
-                throw Error("invalid FileOrigin in errPos");
-        }
-    }
-}
-
 static std::string indent(std::string_view indentFirst, std::string_view indentRest, std::string_view s)
 {
     std::string res;
@@ -264,18 +199,17 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
     std::ostringstream oss;
     oss << einfo.msg << "\n";
 
-    if (einfo.errPos.has_value() && *einfo.errPos) {
-        oss << "\n";
-        printAtPos(*einfo.errPos, oss);
+    auto noSource = ANSI_ITALIC " (source not available)" ANSI_NORMAL "\n";
 
-        auto loc = getCodeLines(*einfo.errPos);
+    if (einfo.errPos) {
+        oss << "\n" << ANSI_BLUE << "at " ANSI_WARNING << *einfo.errPos << ANSI_NORMAL << ":";
 
-        // lines of code.
-        if (loc.has_value()) {
+        if (auto loc = einfo.errPos->getCodeLines()) {
             oss << "\n";
             printCodeLines(oss, "", *einfo.errPos, *loc);
             oss << "\n";
-        }
+        } else
+            oss << noSource;
     }
 
     auto suggestions = einfo.suggestions.trim();
@@ -290,17 +224,15 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
         for (auto iter = einfo.traces.rbegin(); iter != einfo.traces.rend(); ++iter) {
             oss << "\n" << "… " << iter->hint.str() << "\n";
 
-            if (iter->pos.has_value() && (*iter->pos)) {
-                auto pos = iter->pos.value();
-                oss << "\n";
-                printAtPos(pos, oss);
+            if (iter->pos) {
+                oss << "\n" << ANSI_BLUE << "at " ANSI_WARNING << *iter->pos << ANSI_NORMAL << ":";
 
-                auto loc = getCodeLines(pos);
-                if (loc.has_value()) {
+                if (auto loc = iter->pos->getCodeLines()) {
                     oss << "\n";
-                    printCodeLines(oss, "", pos, *loc);
+                    printCodeLines(oss, "", *iter->pos, *loc);
                     oss << "\n";
-                }
+                } else
+                    oss << noSource;
             }
         }
     }

src/libutil/error.hh

@@ -54,13 +54,6 @@ typedef enum {
     lvlVomit
 } Verbosity;
 
-/* adjust Pos::origin bit width when adding stuff here */
-typedef enum {
-    foFile,
-    foStdin,
-    foString
-} FileOrigin;
-
 // the lines of code surrounding an error.
 struct LinesOfCode {
     std::optional<std::string> prevLineOfCode;
@@ -68,54 +61,37 @@ struct LinesOfCode {
     std::optional<std::string> nextLineOfCode;
 };
 
-// ErrPos indicates the location of an error in a nix file.
-struct ErrPos {
-    int line = 0;
-    int column = 0;
-    std::string file;
-    FileOrigin origin;
-
-    operator bool() const
+/* An abstract type that represents a location in a source file. */
+struct AbstractPos
 {
-        return line != 0;
-    }
+    uint32_t line = 0;
+    uint32_t column = 0;
 
-    // convert from the Pos struct, found in libexpr.
-    template <class P>
-    ErrPos & operator=(const P & pos)
-    {
-        origin = pos.origin;
-        line = pos.line;
-        column = pos.column;
-        file = pos.file;
-        return *this;
-    }
+    /* Return the contents of the source file. */
+    virtual std::optional<std::string> getSource() const
+    { return std::nullopt; };
 
-    template <class P>
-    ErrPos(const P & p)
-    {
-        *this = p;
-    }
+    virtual void print(std::ostream & out) const = 0;
+
+    std::optional<LinesOfCode> getCodeLines() const;
 };
 
-std::optional<LinesOfCode> getCodeLines(const ErrPos & errPos);
+std::ostream & operator << (std::ostream & str, const AbstractPos & pos);
 
 void printCodeLines(std::ostream & out,
     const std::string & prefix,
-    const ErrPos & errPos,
+    const AbstractPos & errPos,
     const LinesOfCode & loc);
 
-void printAtPos(const ErrPos & pos, std::ostream & out);
-
 struct Trace {
-    std::optional<ErrPos> pos;
+    std::shared_ptr<AbstractPos> pos;
     hintformat hint;
 };
 
 struct ErrorInfo {
     Verbosity level;
     hintformat msg;
-    std::optional<ErrPos> errPos;
+    std::shared_ptr<AbstractPos> errPos;
     std::list<Trace> traces;
 
     Suggestions suggestions;
@@ -177,12 +153,12 @@ public:
     const ErrorInfo & info() const { calcWhat(); return err; }
 
     template<typename... Args>
-    void addTrace(std::optional<ErrPos> e, const std::string & fs, const Args & ... args)
+    void addTrace(std::shared_ptr<AbstractPos> && e, const std::string & fs, const Args & ... args)
     {
-        addTrace(e, hintfmt(fs, args...));
+        addTrace(std::move(e), hintfmt(fs, args...));
     }
 
-    void addTrace(std::optional<ErrPos> e, hintformat hint);
+    void addTrace(std::shared_ptr<AbstractPos> && e, hintformat hint);
 
     bool hasTrace() const { return !err.traces.empty(); }
 };

src/libutil/fmt.hh

@@ -148,7 +148,7 @@ inline hintformat hintfmt(const std::string & fs, const Args & ... args)
     return f;
 }
 
-inline hintformat hintfmt(std::string plain_string)
+inline hintformat hintfmt(const std::string & plain_string)
 {
     // we won't be receiving any args in this case, so just print the original string
     return hintfmt("%s", normaltxt(plain_string));

src/libutil/logging.cc

@@ -105,14 +105,6 @@ public:
 
 Verbosity verbosity = lvlInfo;
 
-void warnOnce(bool & haveWarned, const FormatOrString & fs)
-{
-    if (!haveWarned) {
-        warn(fs.s);
-        haveWarned = true;
-    }
-}
-
 void writeToStderr(std::string_view s)
 {
     try {
@@ -130,11 +122,11 @@ Logger * makeSimpleLogger(bool printBuildLogs)
     return new SimpleLogger(printBuildLogs);
 }
 
-std::atomic<uint64_t> nextId{(uint64_t) getpid() << 32};
+std::atomic<uint64_t> nextId{0};
 
 Activity::Activity(Logger & logger, Verbosity lvl, ActivityType type,
     const std::string & s, const Logger::Fields & fields, ActivityId parent)
-    : logger(logger), id(nextId++)
+    : logger(logger), id(nextId++ + (((uint64_t) getpid()) << 32))
 {
     logger.startActivity(id, lvl, type, s, fields, parent);
 }
@@ -186,10 +178,11 @@ struct JSONLogger : Logger {
         json["msg"] = oss.str();
         json["raw_msg"] = ei.msg.str();
 
-        if (ei.errPos.has_value() && (*ei.errPos)) {
+        if (ei.errPos) {
             json["line"] = ei.errPos->line;
             json["column"] = ei.errPos->column;
-            json["file"] = ei.errPos->file;
+            //json["file"] = ei.errPos->file;
+            json["file"] = nullptr;
         } else {
             json["line"] = nullptr;
             json["column"] = nullptr;
@@ -201,10 +194,11 @@ struct JSONLogger : Logger {
             for (auto iter = ei.traces.rbegin(); iter != ei.traces.rend(); ++iter) {
                 nlohmann::json stackFrame;
                 stackFrame["raw_msg"] = iter->hint.str();
-                if (iter->pos.has_value() && (*iter->pos)) {
+                if (iter->pos) {
                     stackFrame["line"] = iter->pos->line;
                     stackFrame["column"] = iter->pos->column;
-                    stackFrame["file"] = iter->pos->file;
+                    //stackFrame["file"] = iter->pos->file;
+                    stackFrame["file"] = nullptr;
                 }
                 traces.push_back(stackFrame);
             }

src/libutil/logging.hh

@@ -225,7 +225,11 @@ inline void warn(const std::string & fs, const Args & ... args)
     logger->warn(f.str());
 }
 
-void warnOnce(bool & haveWarned, const FormatOrString & fs);
+#define warnOnce(haveWarned, args...) \
+    if (!haveWarned) {                \
+        haveWarned = true;            \
+        warn(args);                   \
+    }
 
 void writeToStderr(std::string_view s);
 

src/libutil/ref.hh

@@ -83,6 +83,11 @@ public:
         return p != other.p;
     }
 
+    bool operator < (const ref<T> & other) const
+    {
+        return p < other.p;
+    }
+
 private:
 
     template<typename T2, typename... Args>

src/libutil/serialise.cc

@@ -338,7 +338,7 @@ Sink & operator << (Sink & sink, const StringSet & s)
 
 Sink & operator << (Sink & sink, const Error & ex)
 {
-    auto info = ex.info();
+    auto & info = ex.info();
     sink
         << "Error"
         << info.level

src/libutil/serialise.hh

@@ -331,15 +331,7 @@ T readNum(Source & source)
     unsigned char buf[8];
     source((char *) buf, sizeof(buf));
 
-    uint64_t n =
-        ((uint64_t) buf[0]) |
-        ((uint64_t) buf[1] << 8) |
-        ((uint64_t) buf[2] << 16) |
-        ((uint64_t) buf[3] << 24) |
-        ((uint64_t) buf[4] << 32) |
-        ((uint64_t) buf[5] << 40) |
-        ((uint64_t) buf[6] << 48) |
-        ((uint64_t) buf[7] << 56);
+    auto n = readLittleEndian<uint64_t>(buf);
 
     if (n > (uint64_t) std::numeric_limits<T>::max())
         throw SerialisationError("serialised integer %d is too large for type '%s'", n, typeid(T).name());

src/libutil/tests/canon-path.cc

@@ -0,0 +1,155 @@
+#include "canon-path.hh"
+
+#include <gtest/gtest.h>
+
+namespace nix {
+
+    TEST(CanonPath, basic) {
+        {
+            CanonPath p("/");
+            ASSERT_EQ(p.abs(), "/");
+            ASSERT_EQ(p.rel(), "");
+            ASSERT_EQ(p.baseName(), std::nullopt);
+            ASSERT_EQ(p.dirOf(), std::nullopt);
+            ASSERT_FALSE(p.parent());
+        }
+
+        {
+            CanonPath p("/foo//");
+            ASSERT_EQ(p.abs(), "/foo");
+            ASSERT_EQ(p.rel(), "foo");
+            ASSERT_EQ(*p.baseName(), "foo");
+            ASSERT_EQ(*p.dirOf(), ""); // FIXME: do we want this?
+            ASSERT_EQ(p.parent()->abs(), "/");
+        }
+
+        {
+            CanonPath p("foo/bar");
+            ASSERT_EQ(p.abs(), "/foo/bar");
+            ASSERT_EQ(p.rel(), "foo/bar");
+            ASSERT_EQ(*p.baseName(), "bar");
+            ASSERT_EQ(*p.dirOf(), "/foo");
+            ASSERT_EQ(p.parent()->abs(), "/foo");
+        }
+
+        {
+            CanonPath p("foo//bar/");
+            ASSERT_EQ(p.abs(), "/foo/bar");
+            ASSERT_EQ(p.rel(), "foo/bar");
+            ASSERT_EQ(*p.baseName(), "bar");
+            ASSERT_EQ(*p.dirOf(), "/foo");
+        }
+    }
+
+    TEST(CanonPath, pop) {
+        CanonPath p("foo/bar/x");
+        ASSERT_EQ(p.abs(), "/foo/bar/x");
+        p.pop();
+        ASSERT_EQ(p.abs(), "/foo/bar");
+        p.pop();
+        ASSERT_EQ(p.abs(), "/foo");
+        p.pop();
+        ASSERT_EQ(p.abs(), "/");
+    }
+
+    TEST(CanonPath, removePrefix) {
+        CanonPath p1("foo/bar");
+        CanonPath p2("foo/bar/a/b/c");
+        ASSERT_EQ(p2.removePrefix(p1).abs(), "/a/b/c");
+        ASSERT_EQ(p1.removePrefix(p1).abs(), "/");
+        ASSERT_EQ(p1.removePrefix(CanonPath("/")).abs(), "/foo/bar");
+    }
+
+    TEST(CanonPath, iter) {
+        {
+            CanonPath p("a//foo/bar//");
+            std::vector<std::string_view> ss;
+            for (auto & c : p) ss.push_back(c);
+            ASSERT_EQ(ss, std::vector<std::string_view>({"a", "foo", "bar"}));
+        }
+
+        {
+            CanonPath p("/");
+            std::vector<std::string_view> ss;
+            for (auto & c : p) ss.push_back(c);
+            ASSERT_EQ(ss, std::vector<std::string_view>());
+        }
+    }
+
+    TEST(CanonPath, concat) {
+        {
+            CanonPath p1("a//foo/bar//");
+            CanonPath p2("xyzzy/bla");
+            ASSERT_EQ((p1 + p2).abs(), "/a/foo/bar/xyzzy/bla");
+        }
+
+        {
+            CanonPath p1("/");
+            CanonPath p2("/a/b");
+            ASSERT_EQ((p1 + p2).abs(), "/a/b");
+        }
+
+        {
+            CanonPath p1("/a/b");
+            CanonPath p2("/");
+            ASSERT_EQ((p1 + p2).abs(), "/a/b");
+        }
+
+        {
+            CanonPath p("/foo/bar");
+            ASSERT_EQ((p + "x").abs(), "/foo/bar/x");
+        }
+
+        {
+            CanonPath p("/");
+            ASSERT_EQ((p + "foo" + "bar").abs(), "/foo/bar");
+        }
+    }
+
+    TEST(CanonPath, within) {
+        {
+            ASSERT_TRUE(CanonPath("foo").isWithin(CanonPath("foo")));
+            ASSERT_FALSE(CanonPath("foo").isWithin(CanonPath("bar")));
+            ASSERT_FALSE(CanonPath("foo").isWithin(CanonPath("fo")));
+            ASSERT_TRUE(CanonPath("foo/bar").isWithin(CanonPath("foo")));
+            ASSERT_FALSE(CanonPath("foo").isWithin(CanonPath("foo/bar")));
+            ASSERT_TRUE(CanonPath("/foo/bar/default.nix").isWithin(CanonPath("/")));
+            ASSERT_TRUE(CanonPath("/").isWithin(CanonPath("/")));
+        }
+    }
+
+    TEST(CanonPath, sort) {
+        ASSERT_FALSE(CanonPath("foo") < CanonPath("foo"));
+        ASSERT_TRUE (CanonPath("foo") < CanonPath("foo/bar"));
+        ASSERT_TRUE (CanonPath("foo/bar") < CanonPath("foo!"));
+        ASSERT_FALSE(CanonPath("foo!") < CanonPath("foo"));
+        ASSERT_TRUE (CanonPath("foo") < CanonPath("foo!"));
+    }
+
+    TEST(CanonPath, allowed) {
+        {
+            std::set<CanonPath> allowed {
+                CanonPath("foo/bar"),
+                CanonPath("foo!"),
+                CanonPath("xyzzy"),
+                CanonPath("a/b/c"),
+            };
+
+            ASSERT_TRUE (CanonPath("foo/bar").isAllowed(allowed));
+            ASSERT_TRUE (CanonPath("foo/bar/bla").isAllowed(allowed));
+            ASSERT_TRUE (CanonPath("foo").isAllowed(allowed));
+            ASSERT_FALSE(CanonPath("bar").isAllowed(allowed));
+            ASSERT_FALSE(CanonPath("bar/a").isAllowed(allowed));
+            ASSERT_TRUE (CanonPath("a").isAllowed(allowed));
+            ASSERT_TRUE (CanonPath("a/b").isAllowed(allowed));
+            ASSERT_TRUE (CanonPath("a/b/c").isAllowed(allowed));
+            ASSERT_TRUE (CanonPath("a/b/c/d").isAllowed(allowed));
+            ASSERT_TRUE (CanonPath("a/b/c/d/e").isAllowed(allowed));
+            ASSERT_FALSE(CanonPath("a/b/a").isAllowed(allowed));
+            ASSERT_FALSE(CanonPath("a/b/d").isAllowed(allowed));
+            ASSERT_FALSE(CanonPath("aaa").isAllowed(allowed));
+            ASSERT_FALSE(CanonPath("zzz").isAllowed(allowed));
+            ASSERT_TRUE (CanonPath("/").isAllowed(allowed));
+        }
+    }
+}

src/libutil/tests/tests.cc

@@ -311,6 +311,42 @@ namespace nix {
         ASSERT_THROW(base64Decode("cXVvZCBlcm_0IGRlbW9uc3RyYW5kdW0="), Error);
     }
 
+    /* ----------------------------------------------------------------------------
+     * getLine
+     * --------------------------------------------------------------------------*/
+
+    TEST(getLine, all) {
+        {
+            auto [line, rest] = getLine("foo\nbar\nxyzzy");
+            ASSERT_EQ(line, "foo");
+            ASSERT_EQ(rest, "bar\nxyzzy");
+        }
+
+        {
+            auto [line, rest] = getLine("foo\r\nbar\r\nxyzzy");
+            ASSERT_EQ(line, "foo");
+            ASSERT_EQ(rest, "bar\r\nxyzzy");
+        }
+
+        {
+            auto [line, rest] = getLine("foo\n");
+            ASSERT_EQ(line, "foo");
+            ASSERT_EQ(rest, "");
+        }
+
+        {
+            auto [line, rest] = getLine("foo");
+            ASSERT_EQ(line, "foo");
+            ASSERT_EQ(rest, "");
+        }
+
+        {
+            auto [line, rest] = getLine("");
+            ASSERT_EQ(line, "");
+            ASSERT_EQ(rest, "");
+        }
+    }
+
     /* ----------------------------------------------------------------------------
      * toLower
      * --------------------------------------------------------------------------*/

src/libutil/util.cc

@@ -1617,6 +1617,21 @@ std::string stripIndentation(std::string_view s)
 }
 
 
+std::pair<std::string_view, std::string_view> getLine(std::string_view s)
+{
+    auto newline = s.find('\n');
+
+    if (newline == s.npos) {
+        return {s, ""};
+    } else {
+        auto line = s.substr(0, newline);
+        if (!line.empty() && line[line.size() - 1] == '\r')
+            line = line.substr(0, line.size() - 1);
+        return {line, s.substr(newline + 1)};
+    }
+}
+
+
 //////////////////////////////////////////////////////////////////////
 
 static Sync<std::pair<unsigned short, unsigned short>> windowSize{{0, 0}};

src/libutil/util.hh

@@ -510,6 +510,17 @@ std::optional<N> string2Float(const std::string_view s)
 }
 
 
+/* Convert a little-endian integer to host order. */
+template<typename T>
+T readLittleEndian(unsigned char * p)
+{
+    T x = 0;
+    for (size_t i = 0; i < sizeof(x); ++i)
+        x |= ((T) *p++) << (i * 8);
+    return x;
+}
+
+
 /* Return true iff `s' starts with `prefix'. */
 bool hasPrefix(std::string_view s, std::string_view prefix);
 
@@ -563,6 +574,12 @@ std::string base64Decode(std::string_view s);
 std::string stripIndentation(std::string_view s);
 
 
+/* Get the prefix of 's' up to and excluding the next line break (LF
+   optionally preceded by CR), and the remainder following the line
+   break. */
+std::pair<std::string_view, std::string_view> getLine(std::string_view s);
+
+
 /* Get a value for the specified key from an associate container. */
 template <class T>
 const typename T::mapped_type * get(const T & map, const typename T::key_type & key)
@@ -737,4 +754,11 @@ inline std::string operator + (std::string && s, std::string_view s2)
     return std::move(s);
 }
 
+inline std::string operator + (std::string_view s1, const char * s2)
+{
+    std::string s(s1);
+    s.append(s2);
+    return s;
+}
+
 }

src/nix-build/nix-build.cc

@@ -289,7 +289,7 @@ static void main_nix_build(int argc, char * * argv)
     else
         for (auto i : left) {
             if (fromArgs)
-                exprs.push_back(state->parseExprFromString(std::move(i), absPath(".")));
+                exprs.push_back(state->parseExprFromString(std::move(i), state->rootPath(absPath("."))));
             else {
                 auto absolute = i;
                 try {
@@ -301,8 +301,11 @@ static void main_nix_build(int argc, char * * argv)
                 else
                     /* If we're in a #! script, interpret filenames
                        relative to the script. */
-                    exprs.push_back(state->parseExprFromFile(resolveExprPath(state->checkSourcePath(lookupFileArg(*state,
-                                        inShebang && !packages ? absPath(i, absPath(dirOf(script))) : i)))));
+                    exprs.push_back(
+                        state->parseExprFromFile(
+                            resolveExprPath(
+                                lookupFileArg(*state,
+                                    inShebang && !packages ? absPath(i, absPath(dirOf(script))) : i))));
             }
         }
 
@@ -385,7 +388,9 @@ static void main_nix_build(int argc, char * * argv)
         if (!shell) {
 
             try {
-                auto expr = state->parseExprFromString("(import <nixpkgs> {}).bashInteractive", absPath("."));
+                auto expr = state->parseExprFromString(
+                    "(import <nixpkgs> {}).bashInteractive",
+                    state->rootPath(absPath(".")));
 
                 Value v;
                 state->eval(expr, v);

src/nix-channel/nix-channel.cc

@@ -3,7 +3,7 @@
 #include "filetransfer.hh"
 #include "store-api.hh"
 #include "legacy.hh"
-#include "fetchers.hh"
+#include "tarball.hh"
 
 #include <fcntl.h>
 #include <regex>

src/nix-env/nix-env.cc

@@ -44,7 +44,7 @@ typedef enum {
 struct InstallSourceInfo
 {
     InstallSourceType type;
-    Path nixExprPath; /* for srcNixExprDrvs, srcNixExprs */
+    std::shared_ptr<SourcePath> nixExprPath; /* for srcNixExprDrvs, srcNixExprs */
     Path profile; /* for srcProfile */
     std::string systemFilter; /* for srcNixExprDrvs */
     Bindings * autoArgs;
@@ -92,9 +92,11 @@ static bool parseInstallSourceOptions(Globals & globals,
 }
 
 
-static bool isNixExpr(const Path & path, struct stat & st)
+static bool isNixExpr(const SourcePath & path, struct InputAccessor::Stat & st)
 {
-    return S_ISREG(st.st_mode) || (S_ISDIR(st.st_mode) && pathExists(path + "/default.nix"));
+    return
+        st.type == InputAccessor::tRegular
+        || (st.type == InputAccessor::tDirectory && (path + "default.nix").pathExists());
 }
 
 
@@ -102,10 +104,10 @@ static constexpr size_t maxAttrs = 1024;
 
 
 static void getAllExprs(EvalState & state,
-    const Path & path, StringSet & seen, BindingsBuilder & attrs)
+    const SourcePath & path, StringSet & seen, BindingsBuilder & attrs)
 {
     StringSet namesSorted;
-    for (auto & i : readDirectory(path)) namesSorted.insert(i.name);
+    for (auto & [name, _] : path.readDirectory()) namesSorted.insert(name);
 
     for (auto & i : namesSorted) {
         /* Ignore the manifest.nix used by profiles.  This is
@@ -113,13 +115,16 @@ static void getAllExprs(EvalState & state,
            are implemented using profiles). */
         if (i == "manifest.nix") continue;
 
-        Path path2 = path + "/" + i;
+        SourcePath path2 = path + i;
 
-        struct stat st;
-        if (stat(path2.c_str(), &st) == -1)
+        InputAccessor::Stat st;
+        try {
+            st = path2.resolveSymlinks().lstat();
+        } catch (Error &) {
             continue; // ignore dangling symlinks in ~/.nix-defexpr
+        }
 
-        if (isNixExpr(path2, st) && (!S_ISREG(st.st_mode) || hasSuffix(path2, ".nix"))) {
+        if (isNixExpr(path2, st) && (st.type != InputAccessor::tRegular || hasSuffix(path2.baseName(), ".nix"))) {
             /* Strip off the `.nix' filename suffix (if applicable),
                otherwise the attribute cannot be selected with the
                `-A' option.  Useful if you want to stick a Nix
@@ -129,21 +134,20 @@ static void getAllExprs(EvalState & state,
                 attrName = std::string(attrName, 0, attrName.size() - 4);
             if (!seen.insert(attrName).second) {
                 std::string suggestionMessage = "";
-                if (path2.find("channels") != std::string::npos && path.find("channels") != std::string::npos) {
+                if (path2.path.abs().find("channels") != std::string::npos && path.path.abs().find("channels") != std::string::npos)
                     suggestionMessage = fmt("\nsuggestion: remove '%s' from either the root channels or the user channels", attrName);
-                }
                 printError("warning: name collision in input Nix expressions, skipping '%1%'"
                             "%2%", path2, suggestionMessage);
                 continue;
             }
             /* Load the expression on demand. */
             auto vArg = state.allocValue();
-            vArg->mkString(path2);
+            vArg->mkPath(path2);
             if (seen.size() == maxAttrs)
                 throw Error("too many Nix expressions in directory '%1%'", path);
             attrs.alloc(attrName).mkApp(&state.getBuiltin("import"), vArg);
         }
-        else if (S_ISDIR(st.st_mode))
+        else if (st.type == InputAccessor::tDirectory)
             /* `path2' is a directory (with no default.nix in it);
                recurse into it. */
             getAllExprs(state, path2, seen, attrs);
@@ -152,11 +156,9 @@ static void getAllExprs(EvalState & state,
 
 
 
-static void loadSourceExpr(EvalState & state, const Path & path, Value & v)
+static void loadSourceExpr(EvalState & state, const SourcePath & path, Value & v)
 {
-    struct stat st;
-    if (stat(path.c_str(), &st) == -1)
-        throw SysError("getting information about '%1%'", path);
+    auto st = path.resolveSymlinks().lstat();
 
     if (isNixExpr(path, st))
         state.evalFile(path, v);
@@ -167,7 +169,7 @@ static void loadSourceExpr(EvalState & state, const Path & path, Value & v)
        set flat, not nested, to make it easier for a user to have a
        ~/.nix-defexpr directory that includes some system-wide
        directory). */
-    else if (S_ISDIR(st.st_mode)) {
+    else if (st.type == InputAccessor::tDirectory) {
         auto attrs = state.buildBindings(maxAttrs);
         attrs.alloc("_combineChannels").mkList(0);
         StringSet seen;
@@ -179,7 +181,7 @@ static void loadSourceExpr(EvalState & state, const Path & path, Value & v)
 }
 
 
-static void loadDerivations(EvalState & state, Path nixExprPath,
+static void loadDerivations(EvalState & state, const SourcePath & nixExprPath,
     std::string systemFilter, Bindings & autoArgs,
     const std::string & pathPrefix, DrvInfos & elems)
 {
@@ -390,7 +392,7 @@ static void queryInstSources(EvalState & state,
             /* Load the derivations from the (default or specified)
                Nix expression. */
             DrvInfos allElems;
-            loadDerivations(state, instSource.nixExprPath,
+            loadDerivations(state, *instSource.nixExprPath,
                 instSource.systemFilter, *instSource.autoArgs, "", allElems);
 
             elems = filterBySelector(state, allElems, args, newestOnly);
@@ -407,10 +409,10 @@ static void queryInstSources(EvalState & state,
         case srcNixExprs: {
 
             Value vArg;
-            loadSourceExpr(state, instSource.nixExprPath, vArg);
+            loadSourceExpr(state, *instSource.nixExprPath, vArg);
 
             for (auto & i : args) {
-                Expr * eFun = state.parseExprFromString(i, absPath("."));
+                Expr * eFun = state.parseExprFromString(i, state.rootPath(absPath(".")));
                 Value vFun, vTmp;
                 state.eval(eFun, vFun);
                 vTmp.mkApp(&vFun, &vArg);
@@ -462,7 +464,7 @@ static void queryInstSources(EvalState & state,
 
         case srcAttrPath: {
             Value vRoot;
-            loadSourceExpr(state, instSource.nixExprPath, vRoot);
+            loadSourceExpr(state, *instSource.nixExprPath, vRoot);
             for (auto & i : args) {
                 Value & v(*findAlongAttrPath(state, i, *instSource.autoArgs, vRoot).first);
                 getDerivations(state, v, "", *instSource.autoArgs, elems, true);
@@ -647,7 +649,7 @@ static void upgradeDerivations(Globals & globals,
                 } else newElems.push_back(i);
 
             } catch (Error & e) {
-                e.addTrace(std::nullopt, "while trying to find an upgrade for '%s'", i.queryName());
+                e.addTrace(nullptr, "while trying to find an upgrade for '%s'", i.queryName());
                 throw;
             }
         }
@@ -954,7 +956,7 @@ static void queryJSON(Globals & globals, std::vector<DrvInfo> & elems, bool prin
         } catch (AssertionError & e) {
             printMsg(lvlTalkative, "skipping derivation named '%1%' which gives an assertion failure", i.queryName());
         } catch (Error & e) {
-            e.addTrace(std::nullopt, "while querying the derivation named '%1%'", i.queryName());
+            e.addTrace(nullptr, "while querying the derivation named '%1%'", i.queryName());
             throw;
         }
     }
@@ -1015,7 +1017,7 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
         installedElems = queryInstalled(*globals.state, globals.profile);
 
     if (source == sAvailable || compareVersions)
-        loadDerivations(*globals.state, globals.instSource.nixExprPath,
+        loadDerivations(*globals.state, *globals.instSource.nixExprPath,
             globals.instSource.systemFilter, *globals.instSource.autoArgs,
             attrPath, availElems);
 
@@ -1257,7 +1259,7 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
         } catch (AssertionError & e) {
             printMsg(lvlTalkative, "skipping derivation named '%1%' which gives an assertion failure", i.queryName());
         } catch (Error & e) {
-            e.addTrace(std::nullopt, "while querying the derivation named '%1%'", i.queryName());
+            e.addTrace(nullptr, "while querying the derivation named '%1%'", i.queryName());
             throw;
         }
     }
@@ -1374,23 +1376,24 @@ static int main_nix_env(int argc, char * * argv)
         Operation op = 0;
         RepairFlag repair = NoRepair;
         std::string file;
+        Path nixExprPath;
 
         Globals globals;
 
         globals.instSource.type = srcUnknown;
-        globals.instSource.nixExprPath = getHome() + "/.nix-defexpr";
+        nixExprPath = getHome() + "/.nix-defexpr";
         globals.instSource.systemFilter = "*";
 
-        if (!pathExists(globals.instSource.nixExprPath)) {
+        if (!pathExists(nixExprPath)) {
             try {
-                createDirs(globals.instSource.nixExprPath);
+                createDirs(nixExprPath);
                 replaceSymlink(
                     fmt("%s/profiles/per-user/%s/channels", settings.nixStateDir, getUserName()),
-                    globals.instSource.nixExprPath + "/channels");
+                    nixExprPath + "/channels");
                 if (getuid() != 0)
                     replaceSymlink(
                         fmt("%s/profiles/per-user/root/channels", settings.nixStateDir),
-                        globals.instSource.nixExprPath + "/channels_root");
+                        nixExprPath + "/channels_root");
             } catch (Error &) { }
         }
 
@@ -1474,8 +1477,10 @@ static int main_nix_env(int argc, char * * argv)
         globals.state = std::shared_ptr<EvalState>(new EvalState(myArgs.searchPath, store));
         globals.state->repair = repair;
 
-        if (file != "")
-            globals.instSource.nixExprPath = lookupFileArg(*globals.state, file);
+        globals.instSource.nixExprPath = std::make_shared<SourcePath>(
+            file != ""
+            ? lookupFileArg(*globals.state, file)
+            : globals.state->rootPath(nixExprPath));
 
         globals.instSource.autoArgs = myArgs.getAutoArgs(*globals.state);
 

src/nix-env/user-env.cc

@@ -22,7 +22,7 @@ DrvInfos queryInstalled(EvalState & state, const Path & userEnv)
     Path manifestFile = userEnv + "/manifest.nix";
     if (pathExists(manifestFile)) {
         Value v;
-        state.evalFile(manifestFile, v);
+        state.evalFile(state.rootPath(manifestFile), v);
         Bindings & bindings(*state.allocBindings(0));
         getDerivations(state, v, "", bindings, elems, false);
     }
@@ -114,7 +114,7 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
     Value envBuilder;
     state.eval(state.parseExprFromString(
         #include "buildenv.nix.gen.hh"
-            , "/"), envBuilder);
+            , state.rootPath("/")), envBuilder);
 
     /* Construct a Nix expression that calls the user environment
        builder with the manifest as argument. */

src/nix-instantiate/nix-instantiate.cc

@@ -168,9 +168,11 @@ static int main_nix_instantiate(int argc, char * * argv)
 
         if (findFile) {
             for (auto & i : files) {
-                Path p = state->findFile(i);
-                if (p == "") throw Error("unable to find '%1%'", i);
-                std::cout << p << std::endl;
+                auto p = state->findFile(i);
+                if (auto fn = p.getPhysicalPath())
+                    std::cout << fn->abs() << std::endl;
+                else
+                    throw Error("'%s' has no physical path", p);
             }
             return 0;
         }
@@ -184,8 +186,8 @@ static int main_nix_instantiate(int argc, char * * argv)
 
         for (auto & i : files) {
             Expr * e = fromArgs
-                ? state->parseExprFromString(i, absPath("."))
-                : state->parseExprFromFile(resolveExprPath(state->checkSourcePath(lookupFileArg(*state, i))));
+                ? state->parseExprFromString(i, state->rootPath(absPath(".")))
+                : state->parseExprFromFile(resolveExprPath(lookupFileArg(*state, i)));
             processExpr(*state, attrPaths, parseOnly, strict, autoArgs,
                 evalOnly, outputKind, xmlOutputSourceLocation, e);
         }

src/nix-store/nix-store.cc

@@ -926,7 +926,6 @@ static void opServe(Strings opFlags, Strings opArgs)
                     worker_proto::write(*store, out, status.builtOutputs);
                 }
 
-
                 break;
             }
 

src/nix/daemon.cc

@@ -257,7 +257,7 @@ static void daemonLoop()
         } catch (Interrupted & e) {
             return;
         } catch (Error & error) {
-            ErrorInfo ei = error.info();
+            auto ei = error.info();
             // FIXME: add to trace?
             ei.msg = hintfmt("error processing connection: %1%", ei.msg.str());
             logError(ei);

src/nix/develop.cc

@@ -559,7 +559,9 @@ struct CmdDevelop : Common, MixEnvironment
             // chdir if installable is a flake of type git+file or path
             auto installableFlake = std::dynamic_pointer_cast<InstallableFlake>(installable);
             if (installableFlake) {
-                auto sourcePath = installableFlake->getLockedFlake()->flake.resolvedRef.input.getSourcePath();
+                auto sourcePath = installableFlake->getLockedFlake()
+                    ->flake.resolvedRef.input.getAccessor(store).first
+                    ->root().getPhysicalPath();
                 if (sourcePath) {
                     if (chdir(sourcePath->c_str()) == -1) {
                         throw SysError("chdir to '%s' failed", *sourcePath);

src/nix/eval.cc

@@ -65,7 +65,7 @@ struct CmdEval : MixJSON, InstallableCommand
 
         if (apply) {
             auto vApply = state->allocValue();
-            state->eval(state->parseExprFromString(*apply, absPath(".")), *vApply);
+            state->eval(state->parseExprFromString(*apply, state->rootPath(absPath("."))), *vApply);
             auto vRes = state->allocValue();
             state->callFunction(*vApply, *v, *vRes, noPos);
             v = vRes;

src/nix/flake-archive.md

@@ -15,11 +15,10 @@ R""(
   # nix flake archive dwarffs
   ```
 
-* Print the store paths of the flake sources of NixOps without
-  fetching them:
+* Copy and print the store paths of the flake sources of NixOps:
 
   ```console
-  # nix flake archive --json --dry-run nixops
+  # nix flake archive --json nixops
   ```
 
 # Description

src/nix/flake-prefetch.md

@@ -2,21 +2,18 @@ R""(
 
 # Examples
 
-* Download a tarball and unpack it:
+* Download a tarball:
 
   ```console
   # nix flake prefetch https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.10.5.tar.xz
-  Downloaded 'https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.10.5.tar.xz?narHash=sha256-3XYHZANT6AFBV0BqegkAZHbba6oeDkIUCDwbATLMhAY='
-  to '/nix/store/sl5vvk8mb4ma1sjyy03kwpvkz50hd22d-source' (hash
-  'sha256-3XYHZANT6AFBV0BqegkAZHbba6oeDkIUCDwbATLMhAY=').
+  Fetched 'https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.10.5.tar.xz?narHash=sha256-3XYHZANT6AFBV0BqegkAZHbba6oeDkIUCDwbATLMhAY='.
   ```
 
 * Download the `dwarffs` flake (looked up in the flake registry):
 
   ```console
   # nix flake prefetch dwarffs --json
-  {"hash":"sha256-VHg3MYVgQ12LeRSU2PSoDeKlSPD8PYYEFxxwkVVDRd0="
-  ,"storePath":"/nix/store/hang3792qwdmm2n0d9nsrs5n6bsws6kv-source"}
+  {}
   ```
 
 # Description

src/nix/flake-update.md

@@ -16,7 +16,7 @@ R""(
 # Description
 
 This command recreates the lock file of a flake (`flake.lock`), thus
-updating the lock for every mutable input (like `nixpkgs`) to its
+updating the lock for every unlocked input (like `nixpkgs`) to its
 current version. This is equivalent to passing `--recreate-lock-file`
 to any command that operates on a flake. That is,
 

src/nix/flake.cc

@@ -182,8 +182,7 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
                 j["revCount"] = *revCount;
             if (auto lastModified = flake.lockedRef.input.getLastModified())
                 j["lastModified"] = *lastModified;
-            j["path"] = store->printStorePath(flake.sourceInfo->storePath);
-            j["locks"] = lockedFlake.lockFile.toJSON();
+            j["locks"] = lockedFlake.lockFile.toJSON().first;
             logger->cout("%s", j.dump());
         } else {
             logger->cout(
@@ -196,9 +195,6 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
                 logger->cout(
                     ANSI_BOLD "Description:" ANSI_NORMAL "   %s",
                     *flake.description);
-            logger->cout(
-                ANSI_BOLD "Path:" ANSI_NORMAL "          %s",
-                store->printStorePath(flake.sourceInfo->storePath));
             if (auto rev = flake.lockedRef.input.getRev())
                 logger->cout(
                     ANSI_BOLD "Revision:" ANSI_NORMAL "      %s",
@@ -215,7 +211,7 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
             if (!lockedFlake.lockFile.root->inputs.empty())
                 logger->cout(ANSI_BOLD "Inputs:" ANSI_NORMAL);
 
-            std::unordered_set<std::shared_ptr<Node>> visited;
+            std::set<ref<Node>> visited;
 
             std::function<void(const Node & node, const std::string & prefix)> recurse;
 
@@ -227,7 +223,7 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
                     if (auto lockedNode = std::get_if<0>(&input.second)) {
                         logger->cout("%s" ANSI_BOLD "%s" ANSI_NORMAL ": %s",
                             prefix + (last ? treeLast : treeConn), input.first,
-                            *lockedNode ? (*lockedNode)->lockedRef : flake.lockedRef);
+                            (*lockedNode)->lockedRef);
 
                         bool firstVisit = visited.insert(*lockedNode).second;
 
@@ -456,8 +452,8 @@ struct CmdFlakeCheck : FlakeCommand
                     if (attr->name == state->symbols.create("path")) {
                         PathSet context;
                         auto path = state->coerceToPath(attr->pos, *attr->value, context);
-                        if (!store->isInStore(path))
-                            throw Error("template '%s' has a bad 'path' attribute");
+                        if (!path.pathExists())
+                            throw Error("template '%s' refers to a non-existent path '%s'", attrPath, path);
                         // TODO: recursively check the flake in 'path'.
                     }
                 } else
@@ -730,33 +726,28 @@ struct CmdFlakeInitCommon : virtual Args, EvalCommand
 
         auto cursor = installable.getCursor(*evalState);
 
-        auto templateDirAttr = cursor->getAttr("path");
-        auto templateDir = templateDirAttr->getString();
+        auto templateDirAttr = cursor->getAttr("path")->forceValue();
+        PathSet context;
+        auto templateDir = evalState->coerceToPath(noPos, templateDirAttr, context);
 
-        if (!store->isInStore(templateDir))
-            throw TypeError(
-                "'%s' was not found in the Nix store\n"
-                "If you've set '%s' to a string, try using a path instead.",
-                templateDir, templateDirAttr->getAttrPathStr());
+        std::vector<CanonPath> changedFiles;
+        std::vector<CanonPath> conflictedFiles;
 
-        std::vector<Path> changedFiles;
-        std::vector<Path> conflictedFiles;
-
-        std::function<void(const Path & from, const Path & to)> copyDir;
-        copyDir = [&](const Path & from, const Path & to)
+        std::function<void(const SourcePath & from, const CanonPath & to)> copyDir;
+        copyDir = [&](const SourcePath & from, const CanonPath & to)
         {
-            createDirs(to);
+            createDirs(to.abs());
 
-            for (auto & entry : readDirectory(from)) {
-                auto from2 = from + "/" + entry.name;
-                auto to2 = to + "/" + entry.name;
-                auto st = lstat(from2);
-                if (S_ISDIR(st.st_mode))
+            for (auto & [name, entry] : from.readDirectory()) {
+                auto from2 = from + name;
+                auto to2 = to + name;
+                auto st = from2.lstat();
+                if (st.type == InputAccessor::tDirectory)
                     copyDir(from2, to2);
-                else if (S_ISREG(st.st_mode)) {
-                    auto contents = readFile(from2);
-                    if (pathExists(to2)) {
-                        auto contents2 = readFile(to2);
+                else if (st.type == InputAccessor::tRegular) {
+                    auto contents = from2.readFile();
+                    if (pathExists(to2.abs())) {
+                        auto contents2 = readFile(to2.abs());
                         if (contents != contents2) {
                             printError("refusing to overwrite existing file '%s'\nplease merge it manually with '%s'", to2, from2);
                             conflictedFiles.push_back(to2);
@@ -765,12 +756,12 @@ struct CmdFlakeInitCommon : virtual Args, EvalCommand
                         }
                         continue;
                     } else
-                        writeFile(to2, contents);
+                        writeFile(to2.abs(), contents);
                 }
-                else if (S_ISLNK(st.st_mode)) {
-                    auto target = readLink(from2);
-                    if (pathExists(to2)) {
-                        if (readLink(to2) != target) {
+                else if (st.type == InputAccessor::tSymlink) {
+                    auto target = from2.readLink();
+                    if (pathExists(to2.abs())) {
+                        if (readLink(to2.abs()) != target) {
                             printError("refusing to overwrite existing file '%s'\n please merge it manually with '%s'", to2, from2);
                             conflictedFiles.push_back(to2);
                         } else {
@@ -778,7 +769,7 @@ struct CmdFlakeInitCommon : virtual Args, EvalCommand
                         }
                         continue;
                     } else
-                          createSymlink(target, to2);
+                        createSymlink(target, to2.abs());
                 }
                 else
                     throw Error("file '%s' has unsupported type", from2);
@@ -787,21 +778,21 @@ struct CmdFlakeInitCommon : virtual Args, EvalCommand
             }
         };
 
-        copyDir(templateDir, flakeDir);
+        copyDir(templateDir, CanonPath(flakeDir));
 
         if (!changedFiles.empty() && pathExists(flakeDir + "/.git")) {
             Strings args = { "-C", flakeDir, "add", "--intent-to-add", "--force", "--" };
-            for (auto & s : changedFiles) args.push_back(s);
+            for (auto & s : changedFiles) args.push_back(s.abs());
             runProgram("git", true, args);
         }
-        auto welcomeText = cursor->maybeGetAttr("welcomeText");
-        if (welcomeText) {
+
+        if (auto welcomeText = cursor->maybeGetAttr("welcomeText")) {
             notice("\n");
             notice(renderMarkdownToTerminal(welcomeText->getString()));
         }
 
         if (!conflictedFiles.empty())
-            throw Error("Encountered %d conflicts - see above", conflictedFiles.size());
+            throw Error("encountered %d conflicts - see above", conflictedFiles.size());
     }
 };
 
@@ -885,7 +876,7 @@ struct CmdFlakeClone : FlakeCommand
     }
 };
 
-struct CmdFlakeArchive : FlakeCommand, MixJSON, MixDryRun
+struct CmdFlakeArchive : FlakeCommand, MixJSON
 {
     std::string dstUri;
 
@@ -913,42 +904,42 @@ struct CmdFlakeArchive : FlakeCommand, MixJSON, MixDryRun
 
     void run(nix::ref<nix::Store> store) override
     {
+        auto dstStore = store;
+        if (!dstUri.empty())
+            dstStore = openStore(dstUri);
+
         auto flake = lockFlake();
 
         auto jsonRoot = json ? std::optional<JSONObject>(std::cout) : std::nullopt;
 
-        StorePathSet sources;
-
-        sources.insert(flake.flake.sourceInfo->storePath);
+        {
+            Activity act(*logger, lvlChatty, actUnknown, fmt("archiving root"));
+            auto storePath = flake.flake.lockedRef.input.fetchToStore(dstStore).first;
             if (jsonRoot)
-            jsonRoot->attr("path", store->printStorePath(flake.flake.sourceInfo->storePath));
+                jsonRoot->attr("path", store->printStorePath(storePath));
+        }
 
         // FIXME: use graph output, handle cycles.
-        std::function<void(const Node & node, std::optional<JSONObject> & jsonObj)> traverse;
-        traverse = [&](const Node & node, std::optional<JSONObject> & jsonObj)
+        std::function<void(const Node & node, const InputPath & parentPath, std::optional<JSONObject> & jsonObj)> traverse;
+        traverse = [&](const Node & node, const InputPath & parentPath, std::optional<JSONObject> & jsonObj)
         {
             auto jsonObj2 = jsonObj ? jsonObj->object("inputs") : std::optional<JSONObject>();
             for (auto & [inputName, input] : node.inputs) {
                 if (auto inputNode = std::get_if<0>(&input)) {
+                    auto inputPath = parentPath;
+                    inputPath.push_back(inputName);
+                    Activity act(*logger, lvlChatty, actUnknown,
+                        fmt("archiving input '%s'", printInputPath(inputPath)));
                     auto jsonObj3 = jsonObj2 ? jsonObj2->object(inputName) : std::optional<JSONObject>();
-                    auto storePath =
-                        dryRun
-                        ? (*inputNode)->lockedRef.input.computeStorePath(*store)
-                        : (*inputNode)->lockedRef.input.fetch(store).first.storePath;
+                    auto storePath = (*inputNode)->lockedRef.input.fetchToStore(dstStore).first;
                     if (jsonObj3)
                         jsonObj3->attr("path", store->printStorePath(storePath));
-                    sources.insert(std::move(storePath));
-                    traverse(**inputNode, jsonObj3);
+                    traverse(**inputNode, inputPath, jsonObj3);
                 }
             }
         };
 
-        traverse(*flake.lockFile.root, jsonRoot);
-
-        if (!dryRun && !dstUri.empty()) {
-            ref<Store> dstStore = dstUri.empty() ? openStore() : openStore(dstUri);
-            copyPaths(*store, *dstStore, sources);
-        }
+        traverse(*flake.lockFile.root, {}, jsonRoot);
     }
 };
 
@@ -1184,7 +1175,7 @@ struct CmdFlakePrefetch : FlakeCommand, MixJSON
 
     std::string description() override
     {
-        return "download the source tree denoted by a flake reference into the Nix store";
+        return "fetch the source tree denoted by a flake reference";
     }
 
     std::string doc() override
@@ -1198,19 +1189,13 @@ struct CmdFlakePrefetch : FlakeCommand, MixJSON
     {
         auto originalRef = getFlakeRef();
         auto resolvedRef = originalRef.resolve(store);
-        auto [tree, lockedRef] = resolvedRef.fetchTree(store);
-        auto hash = store->queryPathInfo(tree.storePath)->narHash;
+        auto [accessor, lockedRef] = resolvedRef.lazyFetch(store);
 
         if (json) {
             auto res = nlohmann::json::object();
-            res["storePath"] = store->printStorePath(tree.storePath);
-            res["hash"] = hash.to_string(SRI, true);
             logger->cout(res.dump());
         } else {
-            notice("Downloaded '%s' to '%s' (hash '%s').",
-                lockedRef.to_string(),
-                store->printStorePath(tree.storePath),
-                hash.to_string(SRI, true));
+            notice("Fetched '%s'.", lockedRef.to_string());
         }
     }
 };