reader: add method to get Primary Volume label

image_reader.go

@@ -58,6 +58,16 @@ func (i *Image) RootDir() (*File, error) {
 	return nil, os.ErrNotExist
 }
 
+// RootDir returns the label of the first Primary Volume
+func (i *Image) Label() (string, error) {
+	for _, vd := range i.volumeDescriptors {
+		if vd.Type() == volumeTypePrimary {
+			return string(vd.Primary.VolumeIdentifier), nil
+		}
+	}
+	return "", os.ErrNotExist
+}
+
 // File is a os.FileInfo-compatible wrapper around an ISO9660 directory entry
 type File struct {
 	ra       io.ReaderAt

image_reader_test.go

@@ -30,6 +30,10 @@ func TestImageReader(t *testing.T) {
 		assert.Equal(t, volumeTypeTerminator, image.volumeDescriptors[1].Header.Type)
 	}
 
+	label, err := image.Label()
+	assert.NoError(t, err)
+	assert.Equal(t, "my-vol-id", label)
+
 	rootDir, err := image.RootDir()
 	assert.NoError(t, err)
 	assert.True(t, rootDir.IsDir())