Add flag --override-input to override specific lock file entries

E.g.

  $ nix flake update ~/Misc/eelco-configurations/hagbard \
    --override-input 'dwarffs/nixpkgs' ../my-nixpkgs

overrides the 'nixpkgs' input of the 'dwarffs' input of the top-level
flake.

Fixes #2837.

src/libexpr/flake/flake.cc

@@ -4,7 +4,6 @@
 #include "eval-inline.hh"
 #include "store-api.hh"
 #include "fetchers/fetchers.hh"
-#include "fetchers/regex.hh"
 
 #include <iostream>
 #include <ctime>
@@ -62,22 +61,6 @@ static void expectType(EvalState & state, ValueType type,
             showType(type), showType(value.type), pos);
 }
 
-static InputPath parseInputPath(std::string_view s, const Pos & pos)
-{
-    InputPath path;
-
-    for (auto & elem : tokenizeString<std::vector<std::string>>(s, "/")) {
-        if (!std::regex_match(elem, fetchers::flakeIdRegex))
-            throw Error("invalid flake input path element '%s' at %s", elem, pos);
-        path.push_back(elem);
-    }
-
-    if (path.empty())
-        throw Error("flake input path is empty at %s", pos);
-
-    return path;
-}
-
 static std::map<FlakeId, FlakeInput> parseFlakeInputs(
     EvalState & state, Value * value, const Pos & pos);
 
@@ -107,7 +90,11 @@ static FlakeInput parseFlakeInput(EvalState & state,
             input.overrides = parseFlakeInputs(state, attr.value, *attr.pos);
         } else if (attr.name == sFollows) {
             expectType(state, tString, *attr.value, *attr.pos);
-            input.follows = parseInputPath(attr.value->string.s, *attr.pos);
+            try {
+                input.follows = parseInputPath(attr.value->string.s);
+            } catch (Error & e) {
+                e.addPrefix("in flake attribute at '%s':\n");
+            }
         } else
             throw Error("flake input '%s' has an unsupported attribute '%s', at %s",
                 inputName, attr.name, *attr.pos);
@@ -324,7 +311,8 @@ static std::string diffLockFiles(const LockedInputs & oldLocks, const LockedInpu
 LockedFlake lockFlake(
     EvalState & state,
     const FlakeRef & topRef,
-    LockFileMode lockFileMode)
+    LockFileMode lockFileMode,
+    const LockFlags & lockFlags)
 {
     settings.requireExperimentalFeature("flakes");
 
@@ -350,6 +338,9 @@ LockedFlake lockFlake(
     // FIXME: check whether all overrides are used.
     std::map<InputPath, FlakeInput> overrides;
 
+    for (auto & i : lockFlags.inputOverrides)
+        overrides.insert_or_assign(i.first, FlakeInput { .ref = i.second });
+
     /* Compute the new lock file. This is dones as a fixpoint
        iteration: we repeat until the new lock file no longer changes
        and there are no unresolved "follows" inputs. */
@@ -664,8 +655,9 @@ void callFlake(EvalState & state,
 // This function is exposed to be used in nix files.
 static void prim_getFlake(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
+    LockFlags lockFlags;
     callFlake(state, lockFlake(state, parseFlakeRef(state.forceStringNoCtx(*args[0], pos)),
-            evalSettings.pureEval ? AllPure : UseUpdatedLockFile), v);
+            evalSettings.pureEval ? AllPure : UseUpdatedLockFile, lockFlags), v);
 }
 
 static RegisterPrimOp r2("getFlake", 1, prim_getFlake);

src/libexpr/flake/flake.hh

@@ -59,7 +59,16 @@ struct LockedFlake
     Fingerprint getFingerprint() const;
 };
 
-LockedFlake lockFlake(EvalState &, const FlakeRef &, LockFileMode);
+struct LockFlags
+{
+    std::map<InputPath, FlakeRef> inputOverrides;
+};
+
+LockedFlake lockFlake(
+    EvalState &,
+    const FlakeRef &,
+    LockFileMode,
+    const LockFlags &);
 
 void callFlake(EvalState & state,
     const Flake & flake,

src/libexpr/flake/lockfile.cc

@@ -1,5 +1,6 @@
 #include "lockfile.hh"
 #include "store-api.hh"
+#include "fetchers/regex.hh"
 
 #include <nlohmann/json.hpp>
 
@@ -104,4 +105,20 @@ void LockFile::write(const Path & path) const
     writeFile(path, fmt("%s\n", *this));
 }
 
+InputPath parseInputPath(std::string_view s)
+{
+    InputPath path;
+
+    for (auto & elem : tokenizeString<std::vector<std::string>>(s, "/")) {
+        if (!std::regex_match(elem, fetchers::flakeIdRegex))
+            throw Error("invalid flake input path element '%s'", elem);
+        path.push_back(elem);
+    }
+
+    if (path.empty())
+        throw Error("flake input path is empty");
+
+    return path;
+}
+
 }

src/libexpr/flake/lockfile.hh

@@ -81,5 +81,7 @@ struct LockFile : LockedInputs
 
 std::ostream & operator <<(std::ostream & stream, const LockFile & lockFile);
 
+InputPath parseInputPath(std::string_view s);
+
 }
 

src/nix/command.hh

@@ -4,6 +4,7 @@
 #include "args.hh"
 #include "common-eval-args.hh"
 #include "path.hh"
+#include "flake/lockfile.hh"
 
 #include <optional>
 
@@ -42,11 +43,11 @@ struct EvalCommand : virtual StoreCommand, MixEvalArgs
 struct MixFlakeOptions : virtual Args
 {
     bool recreateLockFile = false;
-
     bool saveLockFile = true;
-
     bool useRegistries = true;
 
+    flake::LockFlags lockFlags;
+
     MixFlakeOptions();
 
     flake::LockFileMode getLockFileMode();

src/nix/flake.cc

@@ -43,7 +43,7 @@ public:
 
     LockedFlake lockFlake()
     {
-        return flake::lockFlake(*getEvalState(), getFlakeRef(), getLockFileMode());
+        return flake::lockFlake(*getEvalState(), getFlakeRef(), getLockFileMode(), lockFlags);
     }
 };
 

src/nix/installables.cc

@@ -33,6 +33,17 @@ MixFlakeOptions::MixFlakeOptions()
         .longName("no-registries")
         .description("don't use flake registries")
         .set(&useRegistries, false);
+
+    mkFlag()
+        .longName("override-input")
+        .description("override a specific flake input (e.g. 'dwarffs/nixpkgs')")
+        .arity(2)
+        .labels({"input-path", "flake-url"})
+        .handler([&](std::vector<std::string> ss) {
+            lockFlags.inputOverrides.insert_or_assign(
+                flake::parseInputPath(ss[0]),
+                parseFlakeRef(ss[1], absPath(".")));
+        });
 }
 
 flake::LockFileMode MixFlakeOptions::getLockFileMode()
@@ -321,7 +332,7 @@ std::tuple<std::string, FlakeRef, flake::EvalCache::Derivation> InstallableFlake
 {
     auto state = cmd.getEvalState();
 
-    auto lockedFlake = lockFlake(*state, flakeRef, cmd.getLockFileMode());
+    auto lockedFlake = lockFlake(*state, flakeRef, cmd.getLockFileMode(), cmd.lockFlags);
 
     Value * vOutputs = nullptr;
 
@@ -375,7 +386,7 @@ std::vector<flake::EvalCache::Derivation> InstallableFlake::toDerivations()
 
 Value * InstallableFlake::toValue(EvalState & state)
 {
-    auto lockedFlake = lockFlake(state, flakeRef, cmd.getLockFileMode());
+    auto lockedFlake = lockFlake(state, flakeRef, cmd.getLockFileMode(), cmd.lockFlags);
 
     auto vOutputs = getFlakeOutputs(state, lockedFlake);
 

tests/flakes.sh

@@ -129,6 +129,12 @@ json=$(nix flake info flake1 --json | jq .)
 [[ $(echo "$json" | jq -r .description) = 'Bla bla' ]]
 [[ -d $(echo "$json" | jq -r .path) ]]
 [[ $(echo "$json" | jq -r .lastModified) = $(git -C $flake1Dir log -n1 --format=%ct) ]]
+hash1=$(echo "$json" | jq -r .revision)
+
+echo -n '# foo' >> $flake1Dir/flake.nix
+git -C $flake1Dir commit -a -m 'Foo'
+hash2=$(nix flake info flake1 --json --refresh | jq -r .revision)
+[[ $hash1 != $hash2 ]]
 
 # Test 'nix build' on a flake.
 nix build -o $TEST_ROOT/result flake1#foo
@@ -587,3 +593,14 @@ nix build -o $TEST_ROOT/result $url
 
 # Building with an incorrect SRI hash should fail.
 nix build -o $TEST_ROOT/result "file://$TEST_ROOT/flake.tar.gz?narHash=sha256-qQ2Zz4DNHViCUrp6gTS7EE4+RMqFQtUfWF2UNUtJKS0=" 2>&1 | grep 'NAR hash mismatch'
+
+# Test --override-input.
+git -C $flake3Dir reset --hard
+nix flake update $flake3Dir --override-input flake2/flake1 flake5
+[[ $(jq .inputs.flake2.inputs.flake1.url $flake3Dir/flake.lock) =~ flake5 ]]
+
+nix flake update $flake3Dir --override-input flake2/flake1 flake1
+[[ $(jq .inputs.flake2.inputs.flake1.url $flake3Dir/flake.lock) =~ flake1.*rev=$hash2 ]]
+
+nix flake update $flake3Dir --override-input flake2/flake1 flake1/master/$hash1
+[[ $(jq .inputs.flake2.inputs.flake1.url $flake3Dir/flake.lock) =~ flake1.*rev=$hash1 ]]