run_after not respected when last DagRun is scheduled but not executed

[awenest]

Description

When using a custom Timetable that specifies run_after later than the data interval end (e.g. scheduling at 04:00 UTC instead of midnight), Airflow’s scheduler may issue the next interval prematurely if the previous run hasn’t yet started. This leads to: • Skipped intervals • Misaligned scheduling

Steps to Reproduce 1. Setup a Timetable that returns:

 DagRunInfo(
    data_interval=DataInterval(start, end),
    run_after=DateTime.combine(end.date(), schedule_at).replace(tzinfo=UTC),
)

where schedule_at = Time(4, 0)

2.	On day X, at 00:00 UTC, the scheduler calls next_dagrun_info(), sees no prior run, and emits interval A (X–X+1 day) with run_after = X+1 04:00 UTC
3.	Since A hasn’t triggered yet, last_automated_data_interval is still None. The scheduler calls next_dagrun_info() again (before 04:00 UTC), again thinking “no prior run,” and emits interval B (X+1–X+2 day)
4.	As a result, interval A is skipped forever

Expected Behavior

Airflow should: • Not emit a new interval before the prior interval’s run_after has passed, even if the prior run hasn’t executed yet

Or expose to the timetable logic: • The timestamp of the last scheduled but not necessarily executed DagRun, so custom logic can guard against premature scheduling

Impact

This bug causes custom daily schedules using non-midnight run times to lose a day of data and/or misalign downstream processes, requiring developers to implement brittle workarounds.

Proposed Fixes

Hold off calling next_dagrun_info() again until the last scheduled run’s run_after has passed. Even if last_automated_data_interval is still None, recognize that an interval has been scheduled. 2. Timetable interface improvement Expose an argument like last_scheduled_run_after to next_dagrun_info(), so custom timetables can make informed decisions. 3. Documentation update Make clear in Timetable guide that: If run_after differs from data_interval.end, timetable authors must guard against re-emission of intervals before the previous run_after has passed.

References • Timetable documentation: https://airflow.apache.org/docs/apache-airflow/stable/timetables.html

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