retrieve root path from FS instances

Signed-off-by: CrazyMax <[email protected]>

fs.go

@@ -44,6 +44,16 @@ type fs struct {
 	root string
 }
 
+// GetRoot returns root dir for FS implementations created by NewFS.
+// Returns an empty string and false if the FS implementation does not have a
+// root path.
+func GetRoot(fsys FS) (string, bool) {
+	if fsi, ok := fsys.(*fs); ok {
+		return fsi.root, true
+	}
+	return "", false
+}
+
 func (fs *fs) Walk(ctx context.Context, target string, fn gofs.WalkDirFunc) error {
 	seenFiles := make(map[uint64]string)
 	return filepath.WalkDir(filepath.Join(fs.root, target), func(path string, dirEntry gofs.DirEntry, walkErr error) (retErr error) {

fs_test.go

@@ -12,6 +12,50 @@ import (
 	"github.com/tonistiigi/fsutil/types"
 )
 
+func TestGetRoot(t *testing.T) {
+	testCases := []struct {
+		name     string
+		setupFS  func() FS
+		wantRoot string
+		wantOk   bool
+	}{
+		{
+			name: "Valid FS",
+			setupFS: func() FS {
+				fsys, err := NewFS("/tmp")
+				require.NoError(t, err)
+				return fsys
+			},
+			wantRoot: filepath.FromSlash("/tmp"),
+			wantOk:   true,
+		},
+		{
+			name: "Invalid FS",
+			setupFS: func() FS {
+				return nil
+			},
+			wantRoot: "",
+			wantOk:   false,
+		},
+	}
+	for _, tt := range testCases {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			fsys := tt.setupFS()
+			if fsys == nil && tt.wantOk {
+				t.Fatal("FS setup returned nil, but test case expects a valid FS")
+			}
+			gotRoot, gotOk := GetRoot(fsys)
+			if gotOk != tt.wantOk {
+				t.Errorf("GetRoot() gotOk = %v, want %v", gotOk, tt.wantOk)
+			}
+			if gotRoot != tt.wantRoot {
+				t.Errorf("GetRoot() gotRoot = %q, want %q", gotRoot, tt.wantRoot)
+			}
+		})
+	}
+}
+
 func TestWalk(t *testing.T) {
 	tmpDir := t.TempDir()