TaskChunkSeries rule: do not schedule instances before start date

Otherwise, an infinite loop scheduling new instances on the same day
could occur.

task/models.py

@@ -287,8 +287,8 @@ def apply_rule(self, last: Optional[date] = None) -> Optional[date]:
         """
         if last and last < self.start:
             # if the start date is modified after chunks are scheduled
-            # already, prevent from scheduling any more chunks in the
-            # past
+            # already, prevent from scheduling any more chunks before the
+            # start date
             last = None
 
         apply = getattr(self, '_apply_{}'.format(self.rule), None)
@@ -315,7 +315,10 @@ def _apply_monthly(self, last: Optional[date], day: Optional[int] = None) -> Opt
             day = self.monthly_day
 
         if not last:
-            return self._replace_day(self.start, day)
+            next = self._replace_day(self.start, day)
+            if next < self.start:
+                next = self._add_months(next, 1)
+            return next
 
         next = self._add_months(last, self.monthly_months)
         return self._replace_day(next, day)

task/tests.py

@@ -3085,6 +3085,33 @@ def test_apply_rule_monthlyweekday_last(self):
             series.apply_rule(date(2010, 5, 16)),
             date(2010, 6, 26))
 
+    @freeze_time('2010-02-24')
+    def test_apply_rule_monthlyweekday_start_before_first_potential_schedule(self):
+        """
+        Test applying the monthlyweekday rule if the instance of
+        the start month would be before the actual start date.
+        """
+        series = TaskChunkSeries.objects.create(
+            task=self.task,
+            start=date(2010, 1, 28),
+            rule='monthlyweekday',
+            monthly_months=1,
+            monthlyweekday_weekday=1,
+            monthlyweekday_nth=1)
+
+        # interval without previous occurrence schedules for month of start
+        self.assertEqual(
+            series.apply_rule(),
+            date(2010, 2, 2))
+
+        self.assertEqual(
+            series.apply_rule(series.start),
+            date(2010, 2, 2))
+
+        self.assertEqual(
+            series.apply_rule(date(2010, 2, 5)),
+            date(2010, 3, 2))
+
     def test_schedule_limit_count(self):
         series = TaskChunkSeries.objects.create(
             task=self.task,