def _request_next_buffer(self):
    """Request next buffer.

    Requires self._offset and self._buffer are in consistent state.
    """
    self._buffer_future = None
    next_offset = self._offset + self._buffer.remaining()
    if next_offset != self._file_size:
      self._buffer_future = self._get_segment(next_offset,
                                              self._buffer_size)
Request next buffer.

    Requires self._offset and self._buffer are in consistent state.