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

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

ashb commented on a change in pull request #20349:
URL: https://github.com/apache/airflow/pull/20349#discussion_r781064291



##########
File path: airflow/jobs/scheduler_job.py
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@@ -402,6 +402,14 @@ 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, continue to the next task.
+                    if not serialized_dag:
+                        self.log.error(
+                            "DAG '%s' for taskinstance %s not found in serialized_dag table",
+                            dag_id,
+                            task_instance,
+                        )
+                        continue

Review comment:
       This error/reproduction step is not quite right, but the same idea can trigger this behaviour -- if the dag is deleted at the "right" time, this bit of the scheduler will fail.
   
   I think in that case though we should fail the task instances as the DAG doesn't exist anymore, and as TP said, it can't run successfully.




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