Fix a daylight savings time issue in CronTrigger

[hlobit]

Changes

Fixes #980.

[agronholm]

How did you get the test to pass for you locally? Are the new tests sensitive to the local time zone?

[agronholm]

Either way, I'll take a look at this tomorrow.

[agronholm]

Oh, and I'll need a new changelog entry for this too (don't delete the pull request template, as it contains a check list of things for the PR to be accepted).

[coveralls]

coverage: 91.996% (-0.01%) from 92.006%
when pulling 7455b52 on hlobit:cron-trigger-dst-fold-fix
into 8660590 on agronholm:master.

[agronholm]

Oops, it's been 2 weeks already! I got distracted by several other projects I'm working on. Anyways, the workings of the cron trigger are not fresh in my memory, so would you mind explaining these changes? They look fine, but I'd like to understand why these specific changes were necessary.

[hlobit]

No problem !
I think what is key in this change is the replacement of a statement like

self._last_fire_time + timedelta(seconds=1)

by

datetime.fromtimestamp(
    self._last_fire_time.timestamp() + 1, self.timezone
)

For any reason, the first one does not take DST into account.
https://docs.python.org/3/library/datetime.html#datetime.datetime.fold

Note that no time zone adjustments are done even if the input is an aware object.

The second one does the trick because it is doing operations using the timestamp(), which handles the fold attribute.
https://docs.python.org/3/library/datetime.html#datetime.datetime.timestamp

The timestamp() method uses the fold attribute to disambiguate the times during a repeated interval.

The other change is to make sure the value of fold attribute is not lost while returning a new aware datetime object from _set_field_value().

This made my use cases pass, see the tests.

And thanks for the great apscheduler ❤️

[agronholm]

Nice work, but I'd like a few tweaks before I merge.

[agronholm]

This would make me feel better about the correctness.

Suggested change

	assert next_date == correct_next_date.replace(tzinfo=timezone)
	assert next_date.fold == fold
	assert next_date == correct_next_date.replace(tzinfo=timezone, fold=fold)
	assert str(next_date) == str(correct_next_date)

[agronholm]

Why bother with from_crontab()? This could just be the minute field.

[agronholm]

Suggested change

	return datetime(**values, tzinfo=self.timezone).replace(fold=dateval.fold)
	return datetime(**values, tzinfo=self.timezone, fold=dateval.fold)