amoghrajesh opened a new pull request, #65688:
URL: https://github.com/apache/airflow/pull/65688
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### What?
When downgrading from Airflow 3.2.0 to 3.1.x, the scheduler enters a crash
loop due to `UniqueViolation` errors on the `dag_run` table, causing all DAG
scheduling to stop.
### Current behaviour
Airflow 3.2.0 (related PR: https://github.com/apache/airflow/pull/59115)
changed how run_id is generated for scheduled DAG runs:
- 3.1.x: `run_id = scheduled__<logical_date>`
- 3.2.0: `run_id = scheduled__<run_after>`
For a daily DAG, these timestamps differ by one interval. After downgrading
to 3.1.x, `DagModel.next_dagrun` still holds a value set by 3.2.0. The 3.1.x
scheduler uses that value to generate a run_id that already exists in dag_run
(created by 3.2.0 with the new format) leading to DB insertion error.
This is made worse by the known session handling issue in
_create_dag_runs_timetable (issue:
https://github.com/apache/airflow/issues/59120): one UniqueViolation messed the
entire SQLAlchemy session, causing errors to cascade to every other DAG in the
same scheduler batch. The result is a full scheduler crash loop affecting all
DAGs, not just the one with the collision.
### Proposed change
Add a data migration to the downgrade path of existing migration: 0107, so
that on downgrade, we null out `next_dagrun`, `next_dagrun_create_after`,
`next_dagrun_data_interval_start`, and `next_dagrun_data_interval_end` for all
DAGs.
Nulling is intentional so that the scheduler as it already handles NULL can
start recalculating these fields from the last completed run on its next cycle.
After downgrade, the 3.1.x scheduler recalculates using 3.1.x semantics,
generating run_ids that will not collide with existing runs.
### What does this mean for 3.2 -> 3.2.1? (assuming we make this into 3.2.2)
For 3.2 and 3.2.1:
- This fix is not in it
- Downgrade path is broken
- Anyone who upgraded to 3.2.0 and rolled back to 3.1.x will hit the crash
loop
Anyone on these versions who needs to roll back to 3.1.x should manually run
the sql workaround (null out the 4 fields) before starting the 3.1.x scheduler.
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