def _process_ping(self):
        """
        The server will be periodically sending a PING, and if the the client
        does not reply a PONG back a number of times, it will close the connection
        sending an `-ERR 'Stale Connection'` error.
        """
        yield self.send_command(PONG_PROTO)
        if self._flush_queue.empty():
            yield self._flush_pending()