bug#45818: 28.0.50; Test solar-sunrise-sunset fails

[Basil L. Contovounesios]

Mattias Engdegård <mattiase@acm.org> writes:

> 12 jan. 2021 kl. 21.06 skrev Basil L. Contovounesios <contovob@tcd.ie>:
>
>> So I'm guessing dst-adjust-time adjusts for DST in my locale (based on
>> some cache), but not yours?  The following change lets the test succeed
>> for me:
>
> Thank you, and I agree that it's probably the pragmatic choice. Much as I 
> would
> have liked to track down and explain the exact reason for the observed
> phenomenon, I gave up and pushed your suggested patch.
>
> If I would hazard a guess, it's related to the fact that Ireland uses daylight
> saving in winter (with opposite adjustment) rather than in summer; this may
> trigger a daylight-saving adjustment for the dates in question (late
> December). The fact that IST is used for both Irish and Indian time may be
> significant or a mere coincidence; it certainly didn't help matters.
>
> Date and time calculations can be both infuriatingly and delightfully messy, 
> but
> in this case the code didn't really help either, with way too many implicit
> input parameters -- global variables, caches, system settings, time-zone 
> files,
> and so on. Not something to make a test writer happy!

Agreed.  If this is the sort of stuff that gets you (or Stefan, CCed)
out of bed in the morning, then you might also be interested in looking
at lunar-test-phase-list.  I tried applying the same
calendar-daylight-savings-starts trick in with-lunar-test, but that just
brought down the difference in actual vs expected times from 2hrs to
1hr.

My guess as to why this happens is because calendar-dst-find-data (or
similar) calls current-time-zone without an explicit time zone argument,
meaning tests can't truly specify a time zone other than the system's in
general.

Is this new wishlist bug report material, or are we happy to leave
lunar-test-phase-list marked as unstable and forget about this?

> Thanks again for the report, and for persevering!

Thanks for writing the test and making it more robust!  That's two of my
'make check' failures fixed in one day, only one to go :).

-- 
Basil