Unlock the task, set its status to `S_READY` so that the scheduler can submit it.
        source_node is the :class:`Node` that removed the lock
        Call task.check_status if check_status is True.

def unlock(self, source_node, check_status=True):
        """
        Unlock the task, set its status to `S_READY` so that the scheduler can submit it.
        source_node is the :class:`Node` that removed the lock
        Call task.check_status if check_status is True.
        """
        if self.status != self.S_LOCKED:
            raise RuntimeError("Trying to unlock a task with status %s" % self.status)

        self._status = self.S_READY
        if check_status: self.check_status()
        self.history.info("Unlocked by %s", source_node)