[GitHub] [airflow] ephraimbuddy commented on a change in pull request #20349: Fix Scheduler crash when executing task instances of missing DAG

[GitBox <gi...@apache.org>]

ephraimbuddy commented on a change in pull request #20349:
URL: https://github.com/apache/airflow/pull/20349#discussion_r783670984



##########
File path: airflow/jobs/scheduler_job.py
##########
@@ -403,6 +403,15 @@ def _executable_task_instances_to_queued(self, max_tis: int, session: Session =
                     # Many dags don't have a task_concurrency, so where we can avoid loading the full
                     # serialized DAG the better.
                     serialized_dag = self.dagbag.get_dag(dag_id, session=session)
+                    # If the dag is missing, fail the task and continue to the next task.
+                    if not serialized_dag:
+                        self.log.error(
+                            "DAG '%s' for task instance %s not found in serialized_dag table",
+                            dag_id,
+                            task_instance,
+                        )
+                        task_instance.set_state(State.FAILED, session=session)

Review comment:
       I will opt for setting all scheduled tasks to None. I doubt that failing the DAG here will really fail it when there're tasks being executed in executor. There's a bug that when you mark a DAG as failed, it comes up again as running https://github.com/apache/airflow/issues/16078
   
   So I propose to set all scheduled tasks to None. The scheduler will no longer move the task instances to scheduled when the dag can no longer be found and it makes sense to set it to None instead of failing it since the task instances won't have logs




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