Conrad Lee created AIRFLOW-1428:
-----------------------------------
Summary: DagRun deadlocks when all tasks' dependencies have
skipped state
Key: AIRFLOW-1428
URL: https://issues.apache.org/jira/browse/AIRFLOW-1428
Project: Apache Airflow
Issue Type: Bug
Components: DagRun, dependencies
Affects Versions: 1.8.2
Environment: LocalExecutor with postgres
Reporter: Conrad Lee
Assignee: Bolke de Bruin
Priority: Critical
In controlling which tasks are executed in a DagRun, it's common for tasks to
skip themselves, e.g., by raising an AirflowSkipException. One controls how
skips propagate using trigger rules. It is currently unclear to me how to
propagate skipped states without causing the DagRun to deadlock.
Consider the following simple example
{code}
def raise_skip():
raise AirflowSkipException
skip_op = PythonOperator(
task_id='skip_op',
python_callable=raise_skip,
dag=dag)
skipped_child_op = DummyOperator(task_id='skipped_child_op', dag=dag)
skip_op.set_downstream(skipped_child_op)
{code}
When I run the code above, the DagRun deadlocks. I have dug into why:
* The deadlock is detected by DagRun.update_state
[here](https://github.com/apache/incubator-airflow/blob/master/airflow/models.py#L4290-L4293).
* That's raised because `no_dependencies_met` is `True`, when it should be
`False`
* no_dependencies_met is True because when you call
`skipped_child_op.get_failed_dep_statuses()`, it returns `
[TIDepStatus(dep_name='Trigger Rule', passed=False, reason="Task's trigger rule
'all_success' requires all upstream tasks to have succeeded, but found 1
non-success(es). upstream_tasks_state={'successes': 0, 'skipped': 1, 'failed':
0, 'upstream_failed': 0, 'done': 1},
upstream_task_ids=['metas_update_skipper']")]`
So basically, because `skipped_child_op`'s parent is skipped, it is considered
failed. I have looked for a trigger rule that would cause skipped parents to
be not failed, but that doesn't seem to exist. I don't want to use `ALL_DONE`
because I want `skipped_child_op` to be skipped. Thus, there seems to be no
way for me to properly implement this very simply DAG.
It seems that the airflow community has gone back and forth on how to handle
skipped tasks--see
[AIRFLOW-983](https://issues.apache.org/jira/browse/AIRFLOW-983),
[AIRFLOW-992](https://issues.apache.org/jira/browse/AIRFLOW-992), and
[AIRFLOW-719](https://issues.apache.org/jira/browse/AIRFLOW-719). There seems
to be support for adding a trigger rule called `ALL_SUCCESS_OR_SKIPPED`, but
nobody has implemented it.
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