code: func (s *Store) TmpDir() (string, error) {
  	dir := filepath.Join(s.dir, "tmp")
  	if err := os.MkdirAll(dir, defaultPathPerm); err != nil {
  		return "", err
  	}
  	return dir, nil
  }
docs: // TODO(sgotti), unexport this and provide other functions for external users
  // TmpDir creates and returns dir local to the same filesystem as the Store,
  // or any error encountered