Trigger activity for the caller by writting a NUL to the self-pipe.

def _poll_trigger(self):
        """
        Trigger activity for the caller by writting a NUL to the self-pipe.
    """
        try:
            os.write(self._poll_send, '\0'.encode('utf-8'))
        except Exception as e:
            log = self._getparam('log', self._discard)
            log.debug("Ignoring self-pipe write error -- %s", e)