Set the result or exception from the job if it is complete.

def _set_future_result(self):
        """Set the result or exception from the job if it is complete."""
        # This must be done in a lock to prevent the polling thread
        # and main thread from both executing the completion logic
        # at the same time.
        with self._completion_lock:
            # If the operation isn't complete or if the result has already been
            # set, do not call set_result/set_exception again.
            # Note: self._result_set is set to True in set_result and
            # set_exception, in case those methods are invoked directly.
            if self.state != _DONE_STATE or self._result_set:
                return

            if self.error_result is not None:
                exception = _error_result_to_exception(self.error_result)
                self.set_exception(exception)
            else:
                self.set_result(self)