Invalid crontab is created, but breaks database scheduler

[giolucasd]

Summary:

There is validation while creating CrontabSchedule objects. But such validation allows crontabs that never runs, such as * * 1 31 2, to be created and used within PeriodicTask objects. Having such crontabs while using beat as separate process celery -A [project-name] beat -l info -S django lead to RuntimeError.

• Django Version: 5.0.5
• Celery Version: 5.4.0
• Celery-Beat Version: 2.6.0

Exact steps to reproduce the issue:

1. Create a CrontabSchedule:

problematic_crontab = {
    "minute": "*",
    "hour": "*",
    "day_of_week": "1",
    "day_of_month": "31",
    "month_of_year": "2",
}
schedule, _ = CrontabSchedule.objects.create(**problematic_crontab)

2. Use the created schedule within a PeriodicTask:

PeriodicTask.objects.create(
    task="your_task",
    name="Problematic Task",
    crontab=schedule,
)

3. Run beat scheduler:

celery -A [project-name] beat -l info -S django

Detailed information

I'm running django with celery and have a user newsletter that may be sent monthly, weekly or never. For that purpose, I decided to use 3 crontabs to do the job and the "never" crontab I chose was 31 of february instead of the classical year 3099. Project decisions apart, I believe that allowing the crontab creation while throwing a RuntimeError on that crontab existence is a bug and it led me to waisting way more time than I would have if the create validation already stated the problem.

The system runs on docker containers following cookiecutter-django patterns.

Logs:

[2024-05-07 12:10:10,954: CRITICAL/SpawnProcess-1] beat raised exception <class 'RuntimeError'>: RuntimeError('unable to rollover, time specification is probably invalid')
Traceback (most recent call last):
  File "/usr/local/lib/python3.11/site-packages/celery/apps/beat.py", line 113, in start_scheduler
    service.start()
  File "/usr/local/lib/python3.11/site-packages/celery/beat.py", line 643, in start
    interval = self.scheduler.tick()
               ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/celery/beat.py", line 339, in tick
    self.populate_heap()
  File "/usr/local/lib/python3.11/site-packages/celery/beat.py", line 313, in populate_heap
    is_due, next_call_delay = entry.is_due()
                              ^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/django_celery_beat/schedulers.py", line 136, in is_due
    return self.schedule.is_due(last_run_at_in_tz)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/django_celery_beat/tzcrontab.py", line 41, in is_due
    rem_delta = self.remaining_estimate(last_run_at)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/celery/schedules.py", line 617, in remaining_estimate
    return remaining(*self.remaining_delta(last_run_at, ffwd=ffwd))
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/celery/schedules.py", line 604, in remaining_delta
    delta = self._delta_to_next(last_run_at,
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/celery/schedules.py", line 516, in _delta_to_next
    roll_over()
  File "/usr/local/lib/python3.11/site-packages/celery/schedules.py", line 505, in roll_over
    raise RuntimeError('unable to rollover, '
RuntimeError: unable to rollover, time specification is probably invalid