func (s *snapshotter) Commit(ctx context.Context, name, key string, opts ...snapshots.Opt) error {
        return update(ctx, s.db, func(tx *bolt.Tx) error {
		bkt := getSnapshotterBucket(tx, ns, s.name)
		if bkt == nil {
			return errors.Wrapf(errdefs.ErrNotFound, "snapshot %v does not exist", key)
		}
                ......
                obkt := bkt.Bucket([]byte(key))
		if obkt == nil {
			return errors.Wrapf(errdefs.ErrNotFound, "snapshot %v does not exist", key)
		}