styndall opened a new issue, #76926:
URL: https://github.com/apache/airflow/issues/76926
### Under which category would you file this issue?
Airflow Core
### Apache Airflow version
3.3.2 (also present on `main` as of e42b5a92ae)
### What happened and how to reproduce it?
With the MySQL metadata DB and **mysqlclient 2.3.1** (released 2026-11-10),
the scheduler crash-loops with:
```
AssertionError: No such polymorphic_identity 'CallbackType.TRIGGER' is
defined
```
raised from the deadline query in `SchedulerJobRunner._run_scheduler_loop`
(`selectinload(Deadline.callback)`).
mysqlclient 2.3.1 changed `Connection.literal()` from `isinstance(o, str)`
to `type(o) is str` ("built-in handling only applies to exact built-in types;
subclasses use the connection's converter mapping"). A `str`-subclass Enum
without `__str__` now falls through and is escaped via `str()`, i.e.
`CallbackType.TRIGGERER` is bound as `'CallbackType.TRIGGERER'`:
```python
>>> conn = engine.raw_connection()
>>> conn.literal(CallbackType.TRIGGERER) # mysqlclient 2.3.1
b"'CallbackType.TRIGGERER'"
>>> conn.literal(CallbackType.TRIGGERER) # mysqlclient 2.3.0
b"'triggerer'"
```
`callback.type` is `String(20)`, so with a non-strict `sql_mode` MySQL
truncates it to `'CallbackType.TRIGGER'`, which matches no polymorphic
identity. `callback.fetch_method` is stored as `'CallbackFetchMethod.'` the
same way. Every DAG run with a deadline creates such a row, so the scheduler
keeps crashing until the rows are repaired. (With a strict `sql_mode` the
insert fails instead.)
To reproduce: MySQL backend, `mysqlclient==2.3.1`, a DAG with a
`DeadlineAlert`. Trigger a run and inspect `select type, fetch_method from
callback`.
We hit this in production. Repair after pinning the driver back to 2.3.0:
```sql
UPDATE callback SET type='triggerer', fetch_method='import_path' WHERE
type='CallbackType.TRIGGER';
UPDATE callback SET type='dag_processor', fetch_method='dag_attribute' WHERE
type='CallbackType.DAG_PRO';
```
### What you think should happen instead?
Enum members bound to DB columns should be stored as their `.value`,
regardless of driver. `CallbackState`, `TaskInstanceState`, `DagRunState` etc.
already define `__str__` returning `self.value`, which is why their columns
were unaffected.
The same gap exists for these `str`-Enums that are written to DB columns
without `.value`:
- `CallbackType` (`callback.type`, via `polymorphic_identity`)
- `CallbackFetchMethod` (`callback.fetch_method`)
- `ReprocessBehavior` (`backfill.reprocess_behavior`)
- `BackfillDagRunExceptionReason` (`backfill_dag_run.exception_reason`)
(`DagWarningType` is safe: `get_warning_type_value` stores `.value`.)
Postgres is unaffected: psycopg adapts `str` subclasses by their string
value.
### Operating System
Debian (official-image based)
### Deployment
Other 3rd-party Helm chart
### Anything else?
Workaround: pin `mysqlclient<2.3.1` (Airflow's official constraints pin
2.2.8, so constraint-based installs are not affected).
### Are you willing to submit PR?
- [X] Yes I am willing to submit a PR!
### Code of Conduct
- [X] I agree to follow this project's Code of Conduct
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