Factor out flake:... lookup path from evaluator

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libcmd/common-eval-args.cc

@@ -15,7 +15,20 @@
 namespace nix {
 
 EvalSettings evalSettings {
-    settings.readOnlyMode
+    settings.readOnlyMode,
+    {
+        {
+            "flake",
+            [](ref<Store> store, std::string_view rest) {
+                experimentalFeatureSettings.require(Xp::Flakes);
+                // FIXME `parseFlakeRef` should take a `std::string_view`.
+                auto flakeRef = parseFlakeRef(std::string { rest }, {}, true, false);
+                debug("fetching flake search path element '%s''", rest);
+                auto storePath = flakeRef.resolve(store).fetchTree(store).first;
+                return store->toRealPath(storePath);
+            },
+        },
+    },
 };
 
 static GlobalConfig::Register rEvalSettings(&evalSettings);

src/libexpr/eval-settings.cc

@@ -45,8 +45,9 @@ static Strings parseNixPath(const std::string & s)
     return res;
 }
 
-EvalSettings::EvalSettings(bool & readOnlyMode)
+EvalSettings::EvalSettings(bool & readOnlyMode, EvalSettings::LookupPathHooks lookupPathHooks)
     : readOnlyMode{readOnlyMode}
+    , lookupPathHooks{lookupPathHooks}
 {
     auto var = getEnv("NIX_PATH");
     if (var) nixPath = parseNixPath(*var);

src/libexpr/eval-settings.hh

@@ -5,9 +5,40 @@
 
 namespace nix {
 
+class Store;
+
 struct EvalSettings : Config
 {
-    EvalSettings(bool & readOnlyMode);
+    /**
+     * Function used to interpet look path entries of a given scheme.
+     *
+     * The argument is the non-scheme part of the lookup path entry (see
+     * `LookupPathHooks` below).
+     *
+     * The return value is (a) whether the entry was valid, and, if so,
+     * what does it map to.
+     *
+     * @todo Return (`std::optional` of) `SourceAccssor` or something
+     * more structured instead of mere `std::string`?
+     */
+    using LookupPathHook = std::optional<std::string>(ref<Store> store, std::string_view);
+
+    /**
+     * Map from "scheme" to a `LookupPathHook`.
+     *
+     * Given a lookup path value (i.e. either the whole thing, or after
+     * the `<key>=`) in the form of:
+     *
+     * ```
+     * <scheme>:<arbitrary string>
+     * ```
+     *
+     * if `<scheme>` is a key in this map, then `<arbitrary string>` is
+     * passed to the hook that is the value in this map.
+     */
+    using LookupPathHooks = std::map<std::string, std::function<LookupPathHook>>;
+
+    EvalSettings(bool & readOnlyMode, LookupPathHooks lookupPathHooks = {});
 
     bool & readOnlyMode;
 
@@ -17,6 +48,8 @@ struct EvalSettings : Config
 
     static std::string resolvePseudoUrl(std::string_view url);
 
+    LookupPathHooks lookupPathHooks;
+
     Setting<bool> enableNativeCode{this, false, "allow-unsafe-native-code-during-evaluation", R"(
         Enable built-in functions that allow executing native code.
 

src/libexpr/eval.cc

@@ -2760,14 +2760,18 @@ std::optional<std::string> EvalState::resolveLookupPathPath(const LookupPath::Pa
     auto i = lookupPathResolved.find(value);
     if (i != lookupPathResolved.end()) return i->second;
 
-    std::optional<std::string> res;
+    auto finish = [&](std::string res) {
+        debug("resolved search path element '%s' to '%s'", value, res);
+        lookupPathResolved.emplace(value, res);
+        return res;
+    };
 
     if (EvalSettings::isPseudoUrl(value)) {
         try {
             auto accessor = fetchers::downloadTarball(
                 EvalSettings::resolvePseudoUrl(value)).accessor;
             auto storePath = fetchToStore(*store, SourcePath(accessor), FetchMode::Copy);
-            res = { store->toRealPath(storePath) };
+            return finish(store->toRealPath(storePath));
         } catch (Error & e) {
             logWarning({
                 .msg = HintFmt("Nix search path entry '%1%' cannot be downloaded, ignoring", value)
@@ -2775,15 +2779,17 @@ std::optional<std::string> EvalState::resolveLookupPathPath(const LookupPath::Pa
         }
     }
 
-    else if (hasPrefix(value, "flake:")) {
-        experimentalFeatureSettings.require(Xp::Flakes);
-        auto flakeRef = parseFlakeRef(value.substr(6), {}, true, false);
-        debug("fetching flake search path element '%s''", value);
-        auto storePath = flakeRef.resolve(store).fetchTree(store).first;
-        res = { store->toRealPath(storePath) };
+    if (auto colPos = value.find(':'); colPos != value.npos) {
+        auto scheme = value.substr(0, colPos);
+        auto rest = value.substr(colPos + 1);
+        if (auto * hook = get(settings.lookupPathHooks, scheme)) {
+            auto res = (*hook)(store, rest);
+            if (res)
+                return finish(std::move(*res));
+        }
     }
 
-    else {
+    {
         auto path = absPath(value);
 
         /* Allow access to paths in the search path. */
@@ -2800,22 +2806,17 @@ std::optional<std::string> EvalState::resolveLookupPathPath(const LookupPath::Pa
         }
 
         if (pathExists(path))
-            res = { path };
+            return finish(std::move(path));
         else {
             logWarning({
                 .msg = HintFmt("Nix search path entry '%1%' does not exist, ignoring", value)
             });
-            res = std::nullopt;
         }
     }
 
-    if (res)
-        debug("resolved search path element '%s' to '%s'", value, *res);
-    else
     debug("failed to resolve search path element '%s'", value);
+    return std::nullopt;
 
-    lookupPathResolved.emplace(value, res);
-    return res;
 }