code: func (s *Store) TmpFile() (*os.File, error) {
  	dir, err := s.TmpDir()
  	if err != nil {
  		return nil, err
  	}
  	return ioutil.TempFile(dir, "")
  }
docs: // TODO(sgotti), unexport this and provide other functions for external users
  // TmpFile returns an *os.File local to the same filesystem as the Store, or
  // any error encountered