code: def _rewrite_tmpfile(self, mainfile, tmpfile, writer_spec):
      
      if mainfile.closed:
        # can happen when finalize fails
        return
  
      account_id = self._get_tmp_account_id(writer_spec)
      f = cloudstorage_api.open(tmpfile, _account_id=account_id)
      # both reads and writes are buffered - the number here doesn't matter
      data = f.read(self._REWRITE_BLOCK_SIZE)
      while data:
        mainfile.write(data)
        data = f.read(self._REWRITE_BLOCK_SIZE)
      f.close()
      mainfile.flush()
docs: Copies contents of tmpfile (name) to mainfile (buffer).