Added fix for get_next_fire_time not advancing through fold with unfolded previous_fire_time (#1094)

Author: SoulofAkuma

docs/versionhistory.rst

@@ -4,6 +4,13 @@ Version history
 To find out how to migrate your application from a previous version of
 APScheduler, see the :doc:`migration section <migration>`.
 
+**UNRELEASED**
+
+- Fixed an issue where a job using a ``CronTrigger`` scheduled in a repeated time
+  interval during DST transitions could cause the scheduler to get stuck in an infinite
+  loop
+  (#1021 <https://github.com/agronholm/apscheduler/issues/1021>_; PR by @soulofakuma)
+
 **3.11.1**
 
 - Fixed ``scheduler.shutdown()`` not raising ``SchedulerNotRunning`` (or raising the

pyproject.toml

@@ -51,6 +51,7 @@ zookeeper = ["kazoo"]
 test = [
     "APScheduler[mongodb,redis,rethinkdb,sqlalchemy,tornado,zookeeper,etcd]",
     "pytest",
+    "pytest-timeout",
     "anyio >= 4.5.2",
     "PySide6; python_implementation == 'CPython' and python_version < '3.14'",
     "gevent; python_version < '3.14'",

src/apscheduler/job.py

@@ -1,4 +1,5 @@
 from collections.abc import Iterable, Mapping
+from datetime import timezone
 from inspect import isclass, ismethod
 from uuid import uuid4
 
@@ -12,6 +13,8 @@
     ref_to_obj,
 )
 
+UTC = timezone.utc
+
 
 class Job:
     """
@@ -145,7 +148,7 @@ def _get_run_times(self, now):
         """
         run_times = []
         next_run_time = self.next_run_time
-        while next_run_time and next_run_time <= now:
+        while next_run_time and next_run_time.astimezone(UTC) <= now.astimezone(UTC):
             run_times.append(next_run_time)
             next_run_time = self.trigger.get_next_fire_time(next_run_time, now)
 

src/apscheduler/triggers/cron/__init__.py

@@ -1,4 +1,4 @@
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, timezone
 
 from tzlocal import get_localzone
 
@@ -16,8 +16,11 @@
     convert_to_datetime,
     datetime_ceil,
     datetime_repr,
+    datetime_utc_add,
 )
 
+UTC = timezone.utc
+
 
 class CronTrigger(BaseTrigger):
     """
@@ -183,9 +186,7 @@ def _increment_field_value(self, dateval, fieldnum):
                     i += 1
 
         difference = datetime(**values) - dateval.replace(tzinfo=None)
-        dateval = datetime.fromtimestamp(
-            dateval.timestamp() + difference.total_seconds(), self.timezone
-        )
+        dateval = datetime_utc_add(dateval, difference)
         return dateval, fieldnum
 
     def _set_field_value(self, dateval, fieldnum, new_value):
@@ -202,19 +203,23 @@ def _set_field_value(self, dateval, fieldnum, new_value):
         return datetime(**values, tzinfo=self.timezone, fold=dateval.fold)
 
     def get_next_fire_time(self, previous_fire_time, now):
-        # If datetime is folded, cast in ISO format to ensure they advance correctly
-        if previous_fire_time and previous_fire_time.fold == 1:
-            previous_fire_time = datetime.fromisoformat(previous_fire_time.isoformat())
-
-        if now.fold == 1:
-            now = datetime.fromisoformat(now.isoformat()) + timedelta(microseconds=1)
-
         if previous_fire_time:
-            start_date = min(now, previous_fire_time + timedelta(microseconds=1))
+            start_date = min(
+                now.astimezone(UTC),
+                datetime_utc_add(
+                    previous_fire_time, timedelta(microseconds=1)
+                ).astimezone(UTC),
+            ).astimezone(self.timezone)
             if start_date == previous_fire_time:
-                start_date += timedelta(microseconds=1)
+                start_date = datetime_utc_add(start_date, timedelta(microseconds=1))
         else:
-            start_date = max(now, self.start_date) if self.start_date else now
+            start_date = (
+                max(now.astimezone(UTC), self.start_date.astimezone(UTC)).astimezone(
+                    self.timezone
+                )
+                if self.start_date
+                else now
+            )
 
         fieldnum = 0
         next_date = datetime_ceil(start_date).astimezone(self.timezone)

src/apscheduler/util.py

@@ -35,6 +35,8 @@
 else:
     from zoneinfo import ZoneInfo
 
+UTC = timezone.utc
+
 
 class _Undefined:
     def __nonzero__(self):
@@ -236,10 +238,32 @@ def datetime_ceil(dateval):
 
     """
     if dateval.microsecond > 0:
-        return dateval + timedelta(seconds=1, microseconds=-dateval.microsecond)
+        return datetime_utc_add(
+            dateval, timedelta(seconds=1, microseconds=-dateval.microsecond)
+        )
+
     return dateval
 
 
+def datetime_utc_add(dateval: datetime, tdelta: timedelta) -> datetime:
+    """
+    Adds an timedelta to a datetime in UTC for correct datetime arithmetic across
+    Daylight Saving Time changes
+
+    :param dateval: The date to add to
+    :type dateval: datetime
+    :param operand: The timedelta to add to the datetime
+    :type operand: timedelta
+    :return: The sum of the datetime and the timedelta
+    :rtype: datetime
+    """
+    original_tz = dateval.tzinfo
+    if original_tz is None:
+        return dateval + tdelta
+
+    return (dateval.astimezone(UTC) + tdelta).astimezone(original_tz)
+
+
 def datetime_repr(dateval):
     return dateval.strftime("%Y-%m-%d %H:%M:%S %Z") if dateval else "None"
 

tests/test_job.py

@@ -1,4 +1,5 @@
 import gc
+import sys
 import weakref
 from datetime import datetime, timedelta
 from functools import partial
@@ -11,6 +12,11 @@
 from apscheduler.triggers.date import DateTrigger
 from apscheduler.util import localize
 
+if sys.version_info < (3, 9):
+    from backports.zoneinfo import ZoneInfo
+else:
+    from zoneinfo import ZoneInfo
+
 
 def dummyfunc():
     pass
@@ -103,6 +109,32 @@ def test_get_run_times(create_job, timezone):
     assert run_times == expected_times
 
 
+@pytest.mark.timeout(5)
+def test_get_run_times_dst_transition(create_job):
+    """Tests that Job._get_run_times does not run into an endless loop due to datetime comparison"""
+    timezone = ZoneInfo("US/Eastern")
+    next_run_time = datetime(2025, 11, 2, 1, 0, 10, tzinfo=timezone)
+    now = datetime(2025, 11, 2, 1, 0, 10, 10, tzinfo=timezone)
+    next_next_run_time = datetime(2025, 11, 2, 1, 0, 10, fold=1, tzinfo=timezone)
+    job = create_job(
+        trigger="cron",
+        trigger_args={"timezone": timezone, "hour": 1, "minute": 0, "second": 10},
+        next_run_time=next_next_run_time,
+        func=dummyfunc,
+    )
+    job.next_run_time = next_run_time
+
+    run_times = job._get_run_times(now)
+    assert len(run_times) == 1
+    assert str(run_times[0]) == str(next_run_time)
+
+    run_times = job._get_run_times(now.replace(fold=1))
+    assert len(run_times) == 2
+    assert list(map(str, run_times)) == list(
+        map(str, [next_run_time, next_next_run_time])
+    )
+
+
 def test_private_modify_bad_id(job):
     """Tests that only strings are accepted for job IDs."""
     del job.id

tests/triggers/test_cron.py

@@ -265,31 +265,59 @@ def test_different_tz(timezone):
 
 
 @pytest.mark.parametrize(
-    "trigger_args, start_date, start_date_fold, correct_next_date",
+    "trigger_args, start_date, start_date_fold, correct_next_date, correct_next_date_fold",
     [
-        ({"hour": 8}, datetime(2013, 3, 9, 12), False, datetime(2013, 3, 10, 8)),
-        ({"hour": 8}, datetime(2013, 11, 2, 12), True, datetime(2013, 11, 3, 8)),
+        ({"hour": 8}, datetime(2013, 3, 9, 12), 0, datetime(2013, 3, 10, 8), 0),
+        ({"hour": 8}, datetime(2013, 11, 2, 12), 1, datetime(2013, 11, 3, 8), 0),
+        (
+            {"hour": 1, "minute": 30},
+            datetime(2013, 11, 3, 0, 30),
+            0,
+            datetime(2013, 11, 3, 1, 30),
+            0,
+        ),
+        (
+            {"hour": 1, "minute": 30},
+            datetime(2013, 11, 3, 1, 30, 5),
+            0,
+            datetime(2013, 11, 3, 1, 30),
+            1,
+        ),
+        (
+            {"hour": 1, "minute": 30},
+            datetime(2013, 11, 3, 1, 30, 5),
+            1,
+            datetime(2013, 11, 4, 1, 30),
+            0,
+        ),
         (
             {"minute": "*/30"},
             datetime(2013, 3, 10, 1, 35),
             1,
             datetime(2013, 3, 10, 3),
+            0,
         ),
         (
             {"minute": "*/30"},
             datetime(2013, 11, 3, 1, 35),
             0,
             datetime(2013, 11, 3, 1),
+            1,
         ),
     ],
     ids=[
         "absolute_spring",
         "absolute_autumn",
+        "absolute_autumn_from_before_into_repeated_interval",
+        "absolute_autumn_from_repeated_into_repeated_interval",
+        "absolute_autumn_from_repeated_interval_to_after",
         "interval_spring",
         "interval_autumn",
     ],
 )
-def test_dst_change(trigger_args, start_date, start_date_fold, correct_next_date):
+def test_dst_change(
+    trigger_args, start_date, start_date_fold, correct_next_date, correct_next_date_fold
+):
     """
     Making sure that CronTrigger works correctly when crossing the DST switch threshold.
     Note that you should explicitly compare datetimes as strings to avoid the internal datetime
@@ -299,8 +327,16 @@ def test_dst_change(trigger_args, start_date, start_date_fold, correct_next_date
     timezone = ZoneInfo("US/Eastern")
     trigger = CronTrigger(timezone=timezone, **trigger_args)
     start_date = start_date.replace(tzinfo=timezone, fold=start_date_fold)
-    correct_next_date = correct_next_date.replace(tzinfo=timezone, fold=1)
+    correct_next_date = correct_next_date.replace(
+        tzinfo=timezone, fold=correct_next_date_fold
+    )
     assert str(trigger.get_next_fire_time(None, start_date)) == str(correct_next_date)
+    assert str(trigger.get_next_fire_time(start_date, start_date)) == str(
+        correct_next_date
+    )
+    assert str(trigger.get_next_fire_time(start_date, correct_next_date)) == str(
+        correct_next_date
+    )
 
 
 def test_dst_change_2(timezone):
@@ -310,7 +346,7 @@ def test_dst_change_2(timezone):
     """
     timezone = ZoneInfo("Europe/Helsinki")
     trigger = CronTrigger(minute=30, timezone=timezone)
-    start_date = datetime(2017, 10, 29, 3, 30, tzinfo=timezone, fold=1)
+    start_date = datetime(2017, 10, 29, 3, 30, 0, 5, tzinfo=timezone, fold=1)
     correct_next_date = datetime(2017, 10, 29, 4, 30, tzinfo=timezone, fold=0)
     assert str(trigger.get_next_fire_time(None, start_date)) == str(correct_next_date)
     assert str(trigger.get_next_fire_time(start_date, start_date)) == str(