Add SourceAccessor::getFingerprint()

This returns the fingerprint for a specific subpath. This is intended
for "composite" accessors like MountedSourceAccessor, where different
subdirectories can have different fingerprints.

src/libfetchers/filtering-source-accessor.cc

@@ -60,6 +60,13 @@ std::string FilteringSourceAccessor::showPath(const CanonPath & path)
     return displayPrefix + next->showPath(prefix / path) + displaySuffix;
 }
 
+std::pair<CanonPath, std::optional<std::string>> FilteringSourceAccessor::getFingerprint(const CanonPath & path)
+{
+    if (fingerprint)
+        return {path, fingerprint};
+    return next->getFingerprint(prefix / path);
+}
+
 void FilteringSourceAccessor::checkAccess(const CanonPath & path)
 {
     if (!isAllowed(path))

src/libfetchers/include/nix/fetchers/filtering-source-accessor.hh

@@ -50,6 +50,8 @@ struct FilteringSourceAccessor : SourceAccessor
 
     std::string showPath(const CanonPath & path) override;
 
+    std::pair<CanonPath, std::optional<std::string>> getFingerprint(const CanonPath & path) override;
+
     /**
      * Call `makeNotAllowedError` to throw a `RestrictedPathError`
      * exception if `isAllowed()` returns `false` for `path`.

src/libutil/include/nix/util/source-accessor.hh

@@ -180,6 +180,27 @@ struct SourceAccessor : std::enable_shared_from_this<SourceAccessor>
      */
     std::optional<std::string> fingerprint;
 
+    /**
+     * Return the fingerprint for `path`. This is usually the
+     * fingerprint of the current accessor, but for composite
+     * accessors (like `MountedSourceAccessor`), we want to return the
+     * fingerprint of the "inner" accessor if the current one lacks a
+     * fingerprint.
+     *
+     * So this method is intended to return the most-outer accessor
+     * that has a fingerprint for `path`. It also returns the path that `path`
+     * corresponds to in that accessor.
+     *
+     * For example: in a `MountedSourceAccessor` that has
+     * `/nix/store/foo` mounted,
+     * `getFingerprint("/nix/store/foo/bar")` will return the path
+     * `/bar` and the fingerprint of the `/nix/store/foo` accessor.
+     */
+    virtual std::pair<CanonPath, std::optional<std::string>> getFingerprint(const CanonPath & path)
+    {
+        return {path, fingerprint};
+    }
+
     /**
      * Return the maximum last-modified time of the files in this
      * tree, if available.

src/libutil/mounted-source-accessor.cc

@@ -91,6 +91,14 @@ struct MountedSourceAccessorImpl : MountedSourceAccessor
         else
             return nullptr;
     }
+
+    std::pair<CanonPath, std::optional<std::string>> getFingerprint(const CanonPath & path) override
+    {
+        if (fingerprint)
+            return {path, fingerprint};
+        auto [accessor, subpath] = resolve(path);
+        return accessor->getFingerprint(subpath);
+    }
 };
 
 ref<MountedSourceAccessor> makeMountedSourceAccessor(std::map<CanonPath, ref<SourceAccessor>> mounts)

src/libutil/union-source-accessor.cc

@@ -72,6 +72,18 @@ struct UnionSourceAccessor : SourceAccessor
         }
         return std::nullopt;
     }
+
+    std::pair<CanonPath, std::optional<std::string>> getFingerprint(const CanonPath & path) override
+    {
+        if (fingerprint)
+            return {path, fingerprint};
+        for (auto & accessor : accessors) {
+            auto [subpath, fingerprint] = accessor->getFingerprint(path);
+            if (fingerprint)
+                return {subpath, fingerprint};
+        }
+        return {path, std::nullopt};
+    }
 };
 
 ref<SourceAccessor> makeUnionSourceAccessor(std::vector<ref<SourceAccessor>> && accessors)