def _invoke_callbacks(self):
        """Record the task_end_time & task_cost_time, set result for self._callback_result."""
        self.task_end_time = time.time()
        self.task_cost_time = self.task_end_time - self.task_start_time
        with self._condition:
            for callback in self._done_callbacks:
                try:
                    result = callback(self)
                    if callback in self._user_callbacks:
                        self._callback_result = result
                except Exception as e:
                    Config.main_logger.error("exception calling callback for %s" % e)
            self._condition.notify_all()
Record the task_end_time & task_cost_time, set result for self._callback_result.