TSKVenkat opened a new pull request, #72250:
URL: https://github.com/apache/airflow/pull/72250
Changing a Dag from an hourly to a daily cron stops it being scheduled
again. The scheduler keeps logging `run already exists; skipping dagrun
creation` for a `logical_date` that is already taken and never advances past
it, so the Dag silently stops producing runs until rows are deleted by hand.
`_align_to_prev` maps the previous run's interval end back onto the new
schedule. When the new cron is coarser than the one that produced that run, the
result is the boundary the run already occupies, so `next_dagrun_info` proposes
a `logical_date` identical to the existing run's.
Reproduced on `main` (3.4.0) with the reporter's scenario — an hourly run at
`2026-05-04 00:00` with interval `[00:00, 01:00)`, then a switch to `0 0 * * *`:
```
previous run logical_date : 2026-05-04 00:00:00+00:00
proposed next logical_date: 2026-05-04 00:00:00+00:00 <- collides,
scheduler stalls
```
and after the change:
```
proposed next logical_date: 2026-05-05 00:00:00+00:00 <- advances
```
A guard for this already existed, added in #66132, but it only covered the
zero-length intervals `CronTriggerTimetable` leaves behind (`start == end`).
The collision is not specific to those: it happens whenever the previous run
came from any finer schedule. Comparing the proposed `logical_date` against the
previous run's expresses the invariant the scheduler actually depends on — a
proposed run must be strictly later than the last one — and subsumes the
zero-length case, so the existing test for it still passes unchanged.
Tests: `test_coarser_schedule_does_not_re_emit_logical_date` covers the
hourly-to-daily switch for both catchup settings. It fails on `main` (2 failed)
and passes with this change, while
`test_zero_length_last_interval_does_not_re_emit_logical_date` passes on both
sides. Full `airflow-core/tests/unit/timetables/` run: 253 passed, 1
pre-existing failure in `test_workday_timetable.py` caused by `pandas` missing
from my environment (it fails identically on unmodified `main`).
closes: #66754
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