CrontabSchedule task with start_time but last_run_at is None will run before scheduled time

[WeJie]

Summary:

CrontabSchedule task, which last_run_at is None, with start_time(e.g. at 14:00) that scheduled run at fixed time(e.g. at 15:00) will run immediately at celery beat start(or reload), if start_time < right now(e.g. at 14:02) < schedule time.

Exact steps to reproduce the issue:

let's say right now is 14:00 today

1. Stop celery beat.
2. Create PeriodicTask with start_time is now - 2mins with CrontabSchedule 0 15 * * *
3. Start celery beat.

Detailed information

When last_run_at is None and start_time is not None, current logic only consider interval typed of task, but does not cater tasks run at every fixed time.

class ModelEntry(ScheduleEntry):
    def __init__(self, model, app=None):
        ....
        if not model.last_run_at:
            model.last_run_at = model.date_changed or self._default_now()
            # if last_run_at is not set and
            # model.start_time last_run_at should be in way past.
            # This will trigger the job to run at start_time
            # and avoid the heap block.
            if self.model.start_time:
                # issue: because the last_run_at is way too past, fixed time task will 
                # run immediately like it already pass the last schedule time, which is actually not.
                model.last_run_at = model.last_run_at \
                    - datetime.timedelta(days=365 * 30)

• Celery Version: 5.5.3
• Celery-Beat Version: 2.8.1