fix: follow symlinks for local assets (#8596)

- This reverts behavior that was partially unintentionally introduced in forgejo/forgejo#8143, symbolic links were no longer followed (if they escaped the asset folder) for local assets.
- Having symbolic links for user-added files is, to my understanding, a ,common usecase for NixOS and would thus have symbolic links in the asset folders. Avoiding symbolic links is not easy.
- The previous code used `http.Dir`, we cannot use that as it's not of the same type. The equivalent is `os.DirFS`.
- Unit test to prevent this regression from happening again.

Reported-by: bloxx12 (Matrix).
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8596
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

modules/assetfs/layered.go

@@ -56,14 +56,7 @@ func Local(name, base string, sub ...string) *Layer {
 		panic(fmt.Sprintf("Unable to get absolute path for %q: %v", base, err))
 	}
 	root := util.FilePathJoinAbs(base, sub...)
-	fsRoot, err := os.OpenRoot(root)
-	if err != nil {
-		if errors.Is(err, fs.ErrNotExist) {
-			return nil
-		}
-		panic(fmt.Sprintf("Unable to open layer %q", err))
-	}
-	return &Layer{name: name, fs: fsRoot.FS(), localPath: root}
+	return &Layer{name: name, fs: os.DirFS(root), localPath: root}
 }
 
 // Bindata returns a new Layer with the given name, it serves files from the given bindata asset.
@@ -80,7 +73,7 @@ type LayeredFS struct {
 
 // Layered returns a new LayeredFS with the given layers. The first layer is the top layer.
 func Layered(layers ...*Layer) *LayeredFS {
-	return &LayeredFS{layers: slices.DeleteFunc(layers, func(layer *Layer) bool { return layer == nil })}
+	return &LayeredFS{layers: layers}
 }
 
 // Open opens the named file. The caller is responsible for closing the file.

modules/assetfs/layered_test.go

@@ -1,4 +1,5 @@
 // Copyright 2023 The Gitea Authors. All rights reserved.
+// Copyright 2025 The Forgejo Authors. All rights reserved.
 // SPDX-License-Identifier: MIT
 
 package assetfs
@@ -108,3 +109,30 @@ func TestLayered(t *testing.T) {
 	assert.Equal(t, "l1", assets.GetFileLayerName("f1"))
 	assert.Equal(t, "l2", assets.GetFileLayerName("f2"))
 }
+
+// Allow layers to read symlink outside the layer root.
+func TestLayeredSymlink(t *testing.T) {
+	dir := t.TempDir()
+	dirl1 := filepath.Join(dir, "l1")
+	require.NoError(t, os.MkdirAll(dirl1, 0o755))
+
+	// Open layer in dir/l1
+	layer := Local("l1", dirl1)
+
+	// Create a file in dir/outside
+	fileContents := []byte("I am outside the layer")
+	require.NoError(t, os.WriteFile(filepath.Join(dir, "outside"), fileContents, 0o600))
+	// Symlink dir/l1/outside to dir/outside
+	require.NoError(t, os.Symlink(filepath.Join(dir, "outside"), filepath.Join(dirl1, "outside")))
+
+	// Open dir/l1/outside.
+	f, err := layer.Open("outside")
+	require.NoError(t, err)
+	defer f.Close()
+
+	// Confirm it contains the output of dir/outside
+	contents, err := io.ReadAll(f)
+	require.NoError(t, err)
+
+	assert.Equal(t, fileContents, contents)
+}