fix: allow access to '..' in mapfs

pkg/mapfs/fs.go

@@ -123,7 +123,7 @@ func (m *FS) CopyFilesUnder(dir string) error {
 
 // Stat returns a FileInfo describing the file.
 func (m *FS) Stat(name string) (fs.FileInfo, error) {
-	if strings.HasPrefix(name, "../") && m.underlyingRoot != "" {
+	if m.isPathAboveRoot(name) {
 		return os.Stat(filepath.Join(m.underlyingRoot, name))
 	}
 
@@ -145,15 +145,15 @@ func (m *FS) Stat(name string) (fs.FileInfo, error) {
 // ReadDir reads the named directory
 // and returns a list of directory entries sorted by filename.
 func (m *FS) ReadDir(name string) ([]fs.DirEntry, error) {
-	if strings.HasPrefix(name, "../") && m.underlyingRoot != "" {
+	if m.isPathAboveRoot(name) {
 		return os.ReadDir(filepath.Join(m.underlyingRoot, name))
 	}
 	return m.root.ReadDir(cleanPath(name))
 }
 
 // Open opens the named file for reading.
 func (m *FS) Open(name string) (fs.File, error) {
-	if strings.HasPrefix(name, "../") && m.underlyingRoot != "" {
+	if m.isPathAboveRoot(name) {
 		return os.Open(filepath.Join(m.underlyingRoot, name))
 	}
 	return m.root.Open(cleanPath(name))
@@ -188,7 +188,7 @@ func (m *FS) MkdirAll(path string, perm fs.FileMode) error {
 // The caller is permitted to modify the returned byte slice.
 // This method should return a copy of the underlying data.
 func (m *FS) ReadFile(name string) ([]byte, error) {
-	if strings.HasPrefix(name, "../") && m.underlyingRoot != "" {
+	if m.isPathAboveRoot(name) {
 		return os.ReadFile(filepath.Join(m.underlyingRoot, name))
 	}
 
@@ -245,3 +245,7 @@ func cleanPath(path string) string {
 	path = strings.TrimLeft(path, "/") // Remove the leading slash
 	return path
 }
+
+func (m *FS) isPathAboveRoot(name string) bool {
+	return strings.HasPrefix(name, "..") && m.underlyingRoot != ""
+}

pkg/mapfs/fs_test.go

@@ -478,3 +478,21 @@ func TestFS_RemoveAll(t *testing.T) {
 		require.ErrorIs(t, err, fs.ErrNotExist)
 	})
 }
+
+func TestFS_WithUnderlyingRoot(t *testing.T) {
+	root := "testdata/subdir"
+	fsys := mapfs.New(mapfs.WithUnderlyingRoot(root))
+	fsys.WriteFile("foo.txt", root+"/foo.txt")
+
+	fi, err := fsys.Stat("..")
+	require.NoError(t, err)
+	assert.True(t, fi.IsDir())
+
+	fi, err = fsys.Stat("../hello.txt")
+	require.NoError(t, err)
+	assert.False(t, fi.IsDir())
+
+	fi, err = fsys.Stat("foo.txt")
+	require.NoError(t, err)
+	assert.False(t, fi.IsDir())
+}