Fix CrontabSchedule task run before schedule time

Author: WeJie

django_celery_beat/schedulers.py

@@ -94,13 +94,19 @@ 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:
-                model.last_run_at = model.last_run_at \
-                    - datetime.timedelta(days=365 * 30)
+                if isinstance(model.schedule, schedules.schedule):
+                    # 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.
+                    model.last_run_at = model.last_run_at \
+                        - datetime.timedelta(days=365 * 30)
+                else:
+                    # last_run_at should be the time the task started.
+                    model.last_run_at = model.start_time
+
 
         self.last_run_at = model.last_run_at
 

t/unit/test_schedulers.py

@@ -1035,6 +1035,24 @@ def test_crontab_with_start_time_after_crontab(self, app):
         assert not is_due
         assert next_check == pytest.approx(expected_delay, abs=60)
 
+    def test_crontab_with_start_time_before_crontab(self, app):
+        now = app.now()
+        delay_minutes = 2
+        test_start_time = now - timedelta(minutes=delay_minutes)
+        crontab_time = now + timedelta(minutes=delay_minutes)
+
+        # start_time(now - 2min) < now < crontab_time(now + 2min)
+        task = self.create_model_crontab(
+            crontab(minute=f'{crontab_time.minute}'),
+            start_time=test_start_time)
+
+        entry = EntryTrackSave(task, app=app)
+        is_due, next_check = entry.is_due()
+
+        expected_delay = delay_minutes * 60
+        assert not is_due
+        assert next_check < expected_delay
+
     def test_crontab_with_start_time_different_time_zone(self, app):
         now = app.now()