potiuk opened a new pull request, #25266:
URL: https://github.com/apache/airflow/pull/25266
This PR attempts to fix the deadlock that occurs when task instance
is being run in parallel to running _do_scheduling operation
executing get_next_dagruns_to_examine.
The whole scheduling is based on actually locking DagRuns scheduler
operats on - and it basically means that state of ANY task instances
for that DagRun should not change during the scheduling.
However there are some cases where task instance is locked
FOR UPDATE without prior locking of the DagRun table - this
happens for example when local task job executes the task
and runs "check_and_change_state_before_execution" method on the
task instance it runs. There is no earlier DagRun locking
happening and the "refresh_from_db" run with lock_for_update
will get the lock on both TaskInstance row as well as on the
DagRun row. The problem is this locking happens in reverse sequence
in this case:
1) get_next_dagruns_to_examine - locks DagRun first and THEN
tries to locks some task instances for that DagRun
2) "check_and_change_state_before_execution" runs effectively the
query: select ... from task_instance join dag_run ... for update
which FIRST locks TaskInstance and then DagRun table.
This reverse sequence of locking is what causes the deadlock.
The fix is to force locking the DagRun before running the task instance
query that joins dag_run to task_instance.
Fixes: #23361
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