fix heap being empty after first schedule change (#181)

* fix heap being empty after first schedule change

* add schedule equality tests

* change test name to fix line length

Author: rsichnyi

django_celery_beat/schedulers.py

@@ -194,6 +194,7 @@ class DatabaseScheduler(Scheduler):
     _schedule = None
     _last_timestamp = None
     _initial_read = True
+    _heap_invalidated = False
 
     def __init__(self, *args, **kwargs):
         """Initialize the database scheduler."""
@@ -294,6 +295,12 @@ def install_default_entries(self, data):
             )
         self.update_from_dict(entries)
 
+    def schedules_equal(self, *args, **kwargs):
+        if self._heap_invalidated:
+            self._heap_invalidated = False
+            return False
+        return super(DatabaseScheduler, self).schedules_equal(*args, **kwargs)
+
     @property
     def schedule(self):
         initial = update = False
@@ -311,6 +318,7 @@ def schedule(self):
             # the schedule changed, invalidate the heap in Scheduler.tick
             if not initial:
                 self._heap = []
+                self._heap_invalidated = True
             if logger.isEnabledFor(logging.DEBUG):
                 debug('Current schedule:\n%s', '\n'.join(
                     repr(entry) for entry in values(self._schedule)),

t/unit/test_schedulers.py

@@ -406,6 +406,22 @@ def test_update_scheduler_heap_invalidation(self, monkeypatch):
         monkeypatch.setattr(self.s, 'schedule_changed', lambda: True)
         self.s.tick()
 
+    def test_heap_size_is_constant(self, monkeypatch):
+        # heap size is constant unless the schedule changes
+        monkeypatch.setattr(self.s, 'schedule_changed', lambda: True)
+        expected_heap_size = len(self.s.schedule.values())
+        self.s.tick()
+        assert len(self.s._heap) == expected_heap_size
+        self.s.tick()
+        assert len(self.s._heap) == expected_heap_size
+
+    def test_scheduler_schedules_equality_on_change(self, monkeypatch):
+        monkeypatch.setattr(self.s, 'schedule_changed', lambda: False)
+        assert self.s.schedules_equal(self.s.schedule, self.s.schedule)
+
+        monkeypatch.setattr(self.s, 'schedule_changed', lambda: True)
+        assert not self.s.schedules_equal(self.s.schedule, self.s.schedule)
+
 
 @pytest.mark.django_db()
 class test_models(SchedulerCase):