Celery Cleanup Task Excluded In Schedule

Summary

When upgrading to 2.8.1 we've had a unit test fail for our custom Scheduler class, due to celery.backend_cleanup missing from the scheduler's schedule.

Detailed Information

It appears this is intended to be included, as it is explicitly added to the hours_to_include:

django-celery-beat/django_celery_beat/schedulers.py

Lines 311 to 315 in 2a20e5a

	# Window of +/- 2 hours around the current hour in server tz.
	hours_to_include = [
	(server_hour + offset) % 24 for offset in range(-2, 3)
	]
	hours_to_include += [4] # celery's default cleanup task

Yet the default schedule in DatabaseScheduler.install_default_entries() does not specify a timezone for celery.backend_cleanup, so the crontab will default to CELERY_TIMEZONE:

django-celery-beat/django_celery_beat/models.py

Lines 64 to 76 in 2a20e5a

	def crontab_schedule_celery_timezone():
	"""Return timezone string from Django settings ``CELERY_TIMEZONE`` variable.
	If is not defined or is not a valid timezone, return ``"UTC"`` instead.
	"""
	try:
	CELERY_TIMEZONE = getattr(
	settings, '%s_TIMEZONE' % current_app.namespace)
	except AttributeError:
	return 'UTC'
	if CELERY_TIMEZONE in available_timezones():
	return CELERY_TIMEZONE
	return 'UTC'

This then may be excluded, as the server_hour for the cleanup task may not be within the hours_to_include:

django-celery-beat/django_celery_beat/schedulers.py

Lines 331 to 347 in 2a20e5a

	# Calculate server-hour based on timezone offset
	server_hour=Case(
	# Handle each timezone specifically
	*[
	When(
	timezone=timezone_name,
	then=(
	F('hour_int')
	+ self._get_timezone_offset(timezone_name)
	+ 24
	) % 24
	)
	for timezone_name in self._get_unique_timezone_names()
	],
	# Default case - use hour as is
	default=F('hour_int')
	)

e.g.
Expected hour: 4
sever_hour in Australia/Melbourne: 17

All tasks appear to still be running fine, but just wanted to point this out as it seems unintentional.

[auvipy]

@alirafiei75 up for another one?