Rework DST handling

[kiorky]

rel #32 & #23 & taichino#137

[Dunedan]

I just checked this branch to see if it fixes taichino#147. Turns out it doesn't:

from datetime import datetime

from croniter import croniter_range
from dateutil.tz import gettz

for i in croniter_range(
        datetime(2021, 3, 26, tzinfo=gettz("Europe/Berlin")),
        datetime(2021, 3, 31, tzinfo=gettz("Europe/Berlin")),
        "55 12 * * *"
):
    print(i)

2021-03-26 12:55:00+01:00
2021-03-27 12:55:00+01:00
2021-03-28 13:00:00+02:00
2021-03-29 12:55:00+02:00
2021-03-30 12:55:00+02:00

The code from this PR is also much slower than before.

I believe the best approach would be to not try to manually calculate the DST-related changes, but to let Python's datetime module handle that itself, by consequently using datetime objects internally and only convert to timestamps whenever the user wants to get timestamps.

Here is an example of Python handling that (of course without the cron-expression related logic):

from datetime import datetime, timedelta

from dateutil.tz import gettz

dt = datetime(2021, 3, 25, 12, 55, tzinfo=gettz("Europe/Berlin"))
for _ in range(0, 5):
    dt = dt + timedelta(days=1)
    print(dt)

2021-03-26 12:55:00+01:00
2021-03-27 12:55:00+01:00
2021-03-28 12:55:00+02:00
2021-03-29 12:55:00+02:00
2021-03-30 12:55:00+02:00

[kiorky]

wait for the PR to complete, as i already said, for now its just a WIP to discuss with @lowell80, and no, it's not possible to use datetime everywhere without rewriting croniter entirely.

For the speed regression, it's a tradeoff we have to put if we want to handle DST transition.

[lowell80]

I looked over the code you have so far. Unfortunately, I was trying to get my head wrapped around the core logic of it all, but it's a bit out of reach at the moment. I think I'd need to spend several consecutive hours staring at it, or more. :-)

Given that the next DST transition is Nov 1st (at least here anyway), I'm guessing there won't be anything ready for
"prod-run" by then?

I can certainly help with testing when you are ready for that phase. I have a library of user-generated CRON expressions on hand! :-) Just give a shout!

[kiorky]

Yep, and the period in France is a bit not "safe" for me right now to be as productive as i would want...

Idea is to stick with the unix strategy where that window with a blackhole exists, jobs are scheduled next hour.