Send a signal to all task processes.

def signal(self, details):
        """
        Send a signal to all task processes.
    """
        log = self._params.get('log', self._discard)
        if '_signal' not in dir(self._parent) or not callable(getattr(self._parent, '_signal')):
            log.error("Event parent '%s' has no '_signal' method", self._name)
            return
        sig = utils.signum(self._handler_arg)
        if sig is None:
            log.error("Invalid signal '%s' for task '%s'", self._handler_arg, sig._name)
            return
        log.info("sending %s to all '%s' processes", utils.signame(sig), self._name)
        self._parent._signal(sig)