docs: def _flush(self):
          """
          Flush the write buffers of the stream.
          """
          # Initialize multipart upload
          if self._upload_id is None:
              with _handle_oss_error():
                  self._upload_id = self._bucket.init_multipart_upload(
                      self._key).upload_id
  
          # Upload part with workers
          response = self._workers.submit(
              self._bucket.upload_part, key=self._key, upload_id=self._upload_id,
              part_number=self._seek, data=self._get_buffer().tobytes())
  
          # Save part information
          self._write_futures.append(
              dict(response=response, part_number=self._seek))