dstandish commented on code in PR #35887:
URL: https://github.com/apache/airflow/pull/35887#discussion_r1416630188
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airflow/timetables/_cron.py:
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@@ -32,21 +31,32 @@
from pendulum import DateTime
-def _is_schedule_fixed(expression: str) -> bool:
- """Figures out if the schedule has a fixed time (e.g. 3 AM every day).
+def _covers_every_hour(cron: croniter) -> bool:
+ """Check whether the given cron runs at least once an hour.
- :return: True if the schedule has a fixed time, False if not.
+ This indicates whether we need to implement a workaround for (what I call)
+ the "fold hour problem". Folding happens when a region switches time
+ backwards, usually as a part of ending a DST period, causing a block of
time
+ to occur twice in the wall clock. This is indicated by the ``fold`` flag on
+ datetime.
- Detection is done by "peeking" the next two cron trigger time; if the
- two times have the same minute and hour value, the schedule is fixed,
- and we *don't* need to perform the DST fix.
+ As an example, Switzerland in 2023 ended DST on 3am (wall clock time,
UTC+2)
+ by dialing back the clock to 2am (UTC+1). So for (say) ``30 * * * *``, if
+ the last run was 2:30am (UTC+2), the next needs to be 2:30am (UTC+1,
folded)
+ instead of 3:30am.
- This assumes DST happens on whole minute changes (e.g. 12:59 -> 12:00).
+ While this technically happens for all runs (in such a timezone), we only
+ really care about runs that happen at least once an hour, and can
+ provide a somewhat reasonable rationale to skip the fold hour for things
+ such as ``*/2`` (every two hour). So we try to *minially* peak into
croniter
+ internals to work around the issue.
Review Comment:
```suggestion
While this technically happens for all cron schedules (in such a
timezone), we only
really care about schedules that run at least once an hour; for
schedules that run
less frequently, such as ``*/2`` (every two hours), it seems reasonable
to skip the
fold hour. So we try to *minially* peak into croniter internals to work
around the issue.
```
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