[chinmayasameeru]

Fix #73432 — the dag processor can get stuck in an infinite loop after a Dag-level callback runs.

Here's what happens: when a Dag file gets processed in "callback-only" mode (meaning it was triggered by a Dag callback, not a regular scan), process_parse_results creates a new DagFileStat with last_finish_time=None. That stat overwrites the existing one in _file_stats, so on the next scan the file looks like it's never been parsed. The scheduler re-queues it, processes it again, creates another stat with last_finish_time=None, and the cycle repeats forever.

The fix is to preserve the existing last_finish_time when doing callback-only processing. I added a last_finish_time parameter to process_parse_results — when it's passed (from the existing stat), the is_callback_only branch uses it instead of the current finish_time. I updated both call sites:

• handle_parsing_result: passes self._file_stats[file].last_finish_time — this is the main fix for the reported issue
• process_unparsed_files: passes db_stat.last_finish_time to DagFileStat.from_db — this handles the case where a file is already in the DB

I couldn't run the full airflow test suite here (the test environment needs a database and a bunch of infra), but I did validate the Python syntax with py_compile and the logic is pretty straightforward — it's just passing a timestamp through instead of dropping it.

[boring-cyborg]

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[Eason09053360]

This duplicates #73433 (opened earlier by the issue reporter).
Also, ruff format fails on the function signature — please run prek install so the static checks run before pushing. :)

[henry3260]

Did you reproduce the bug yourself and confirm this fix resolves it? Note that this duplicates #73433, and the lint checks are currently failing, I may close this PR unless those are addressed.