jedcunningham opened a new pull request, #66132:
URL: https://github.com/apache/airflow/pull/66132
## What's the bug
`_DataIntervalTimetable.next_dagrun_info` aligns the next run's `start` onto
`data_interval.end` of the previous run. When that previous interval is
zero-length (`start == end`), `start` lands on the *same* point as the existing
run's `logical_date`, so the scheduler tries to create a run that collides with
the existing one and logs:
```
run already exists; skipping dagrun creation
```
It then calls
`calculate_dagrun_date_fields(last_automated_run=existing_run)` to advance —
which feeds the same zero-length interval back in and recomputes the same
value. The scheduler loops on the skip warning until the next cron boundary
nudges `_skip_to_latest` to a different result, at which point the gate may
have already rolled past the slot — so the intended run is silently skipped.
## When does this happen?
Zero-length `data_interval`s are the normal output of `CronTriggerTimetable`
(point-in-time) but never of `CronDataIntervalTimetable` (windowed, validated
`> 0`). They appear on a `CronDataIntervalTimetable` DAG when its scheduling
history was produced by `CronTriggerTimetable` and the DAG later switches to
`CronDataIntervalTimetable` — for example, when `[scheduler]
create_cron_data_intervals` is flipped from `False` (Airflow 3 default) to
`True` after some runs have already been created.
## The fix
In `_DataIntervalTimetable.next_dagrun_info`, after computing `start`, if
the previous interval was zero-length and `start` falls on it, advance `start`
by one period so the new run gets a fresh `logical_date`.
The guard only fires for `_DataIntervalTimetable` subclasses
(`CronDataIntervalTimetable`, `DeltaDataIntervalTimetable`).
`CronTriggerTimetable` / `DeltaTriggerTimetable` extend a separate
`_TriggerTimetable` base with its own `next_dagrun_info` and never go through
this code path, so their normal zero-length output is unaffected.
## Tests
Added `test_zero_length_last_interval_does_not_re_emit_logical_date`
parametrized on `catchup ∈ {True, False}` — both branches converge on the same
expected interval, demonstrating the guard absorbs the difference between the
two paths into the previous-run handling.
related: #59618
##### Was generative AI tooling used to co-author this PR?
- [X] Yes — Claude Code (Anthropic)
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