code: def _get_offset_from_gcs(self):
      
      headers = {'content-range': 'bytes */*'}
      status, response_headers, content = self._api.put_object(
          self._path_with_token, headers=headers)
      errors.check_status(status, [308], self._path, headers,
                          response_headers, content,
                          {'upload_path': self._path_with_token})
      val = response_headers.get('range')
      if val is None:
        return -1
      _, offset = val.rsplit('-', 1)
      return int(offset)
docs: Get the last offset that has been written to GCS.
  
      This is a utility method that does not modify self.
  
      Returns:
        an int of the last offset written to GCS by this upload, inclusive.
        -1 means nothing has been written.