Move MountedSourceAccessor to libutil

src/libutil/mounted-source-accessor.cc

@@ -0,0 +1,79 @@
+#include "mounted-source-accessor.hh"
+
+namespace nix {
+
+struct MountedSourceAccessor : SourceAccessor
+{
+    std::map<CanonPath, ref<SourceAccessor>> mounts;
+
+    MountedSourceAccessor(std::map<CanonPath, ref<SourceAccessor>> _mounts)
+        : mounts(std::move(_mounts))
+    {
+        displayPrefix.clear();
+
+        // Currently we require a root filesystem. This could be relaxed.
+        assert(mounts.contains(CanonPath::root));
+
+        // FIXME: return dummy parent directories automatically?
+    }
+
+    std::string readFile(const CanonPath & path) override
+    {
+        auto [accessor, subpath] = resolve(path);
+        return accessor->readFile(subpath);
+    }
+
+    bool pathExists(const CanonPath & path) override
+    {
+        auto [accessor, subpath] = resolve(path);
+        return accessor->pathExists(subpath);
+    }
+
+    std::optional<Stat> maybeLstat(const CanonPath & path) override
+    {
+        auto [accessor, subpath] = resolve(path);
+        return accessor->maybeLstat(subpath);
+    }
+
+    DirEntries readDirectory(const CanonPath & path) override
+    {
+        auto [accessor, subpath] = resolve(path);
+        return accessor->readDirectory(subpath);
+    }
+
+    std::string readLink(const CanonPath & path) override
+    {
+        auto [accessor, subpath] = resolve(path);
+        return accessor->readLink(subpath);
+    }
+
+    std::string showPath(const CanonPath & path) override
+    {
+        auto [accessor, subpath] = resolve(path);
+        return displayPrefix + accessor->showPath(subpath) + displaySuffix;
+    }
+
+    std::pair<ref<SourceAccessor>, CanonPath> resolve(CanonPath path)
+    {
+        // Find the nearest parent of `path` that is a mount point.
+        std::vector<std::string> subpath;
+        while (true) {
+            auto i = mounts.find(path);
+            if (i != mounts.end()) {
+                std::reverse(subpath.begin(), subpath.end());
+                return {i->second, CanonPath(subpath)};
+            }
+
+            assert(!path.isRoot());
+            subpath.push_back(std::string(*path.baseName()));
+            path.pop();
+        }
+    }
+};
+
+ref<SourceAccessor> makeMountedSourceAccessor(std::map<CanonPath, ref<SourceAccessor>> mounts)
+{
+    return make_ref<MountedSourceAccessor>(std::move(mounts));
+}
+
+}