Fixed a daylight savings time issue in CronTrigger (#981)

* Removed the `timedelta` operations - which are not timezone aware
* Made sure the "fold" attribute remains when incrementing

docs/versionhistory.rst

@@ -62,6 +62,8 @@ APScheduler, see the :doc:`migration section <migration>`.
   acquire the same schedules at once
 - Changed ``SQLAlchemyDataStore`` to automatically create the explicitly specified
   schema if it's missing (PR by @zhu0629)
+- Fixed an issue with ``CronTrigger`` infinitely looping to get next date when DST ends
+  (`#980 <https://github.com/agronholm/apscheduler/issues/980>`_; PR by @hlobit)
 
 **4.0.0a5**
 

src/apscheduler/triggers/cron/__init__.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 from collections.abc import Sequence
-from datetime import datetime, timedelta, tzinfo
+from datetime import datetime, tzinfo
 from typing import Any, ClassVar
 
 import attrs
@@ -207,16 +207,17 @@ def _set_field_value(
                 else:
                     values[field.name] = new_value
 
-        return datetime(**values, tzinfo=self.timezone)
+        return datetime(**values, tzinfo=self.timezone, fold=dateval.fold)
 
     def next(self) -> datetime | None:
         if self._last_fire_time:
-            start_time = self._last_fire_time + timedelta(microseconds=1)
+            next_time = datetime.fromtimestamp(
+                self._last_fire_time.timestamp() + 1, self.timezone
+            )
         else:
-            start_time = self.start_time
+            next_time = self.start_time
 
         fieldnum = 0
-        next_time = datetime_ceil(start_time).astimezone(self.timezone)
         while 0 <= fieldnum < len(self._fields):
             field = self._fields[fieldnum]
             curr_value = field.get_value(next_time)
@@ -276,11 +277,3 @@ def __repr__(self) -> str:
 
         fields.append(f"timezone={timezone_repr(self.timezone)!r}")
         return f"CronTrigger({', '.join(fields)})"
-
-
-def datetime_ceil(dateval: datetime) -> datetime:
-    """Round the given datetime object upwards."""
-    if dateval.microsecond > 0:
-        return dateval + timedelta(seconds=1, microseconds=-dateval.microsecond)
-
-    return dateval

tests/triggers/test_cron.py

@@ -400,6 +400,45 @@ def test_dst_change(
     )
 
 
+@pytest.mark.parametrize(
+    "minute, start_time, correct_next_dates",
+    [
+        (
+            0,
+            datetime(2024, 10, 27, 2, 0, 0, 0),
+            [
+                (datetime(2024, 10, 27, 2, 0, 0, 0), 0),
+                (datetime(2024, 10, 27, 2, 0, 0, 0), 1),
+                (datetime(2024, 10, 27, 3, 0, 0, 0), 0),
+            ],
+        ),
+        (
+            1,
+            datetime(2024, 10, 27, 2, 1, 0, 0),
+            [
+                (datetime(2024, 10, 27, 2, 1, 0, 0), 0),
+                (datetime(2024, 10, 27, 2, 1, 0, 0), 1),
+                (datetime(2024, 10, 27, 3, 1, 0, 0), 0),
+            ],
+        ),
+    ],
+    ids=["dst_change_0", "dst_change_1"],
+)
+def test_dst_change2(
+    minute,
+    start_time,
+    correct_next_dates,
+    timezone,
+):
+    trigger = CronTrigger(minute=minute, timezone=timezone)
+    trigger.start_time = start_time.replace(tzinfo=timezone)
+    for correct_next_date, fold in correct_next_dates:
+        correct_next_date = correct_next_date.replace(tzinfo=timezone, fold=fold)
+        next_date = trigger.next()
+        assert next_date == correct_next_date
+        assert str(next_date) == str(correct_next_date)
+
+
 def test_zero_value(timezone):
     start_time = datetime(2020, 1, 1, tzinfo=timezone)
     trigger = CronTrigger(