Subject: Re: Regression tests fail with tzdata 2024b

From: Sven Klemm <[email-masked]>
Date: Mon, 16 Sep 2024 10:33:56 +0200

On Mon, Sep 16, 2024 at 7:09 AM Tom Lane <[email-masked]> wrote:

> Wolfgang Walther <[email-masked]> writes:
> > Tom Lane:
> >> Also, as a real place to a greater extent
> >> than "PST8PDT" is, it's more subject to historical revisionism when
> >> somebody turns up evidence of local law having been different than
> >> TZDB currently thinks.
>
> > I now tried all versions of tzdata which we had in tree back to 2018g,
> > they all work fine with the same regression test output. 2018g was an
> > arbitrary cutoff, I just didn't try any further.
>
> Yeah, my belly-aching above is just about hypothetical future
> instability.  In reality, I'm sure America/Los_Angeles is pretty
> well researched and so it's unlikely that there will be unexpected
> changes in its zone history.
>
> > In the end, we don't need a default timezone that will never change.
>
> We do, really.  For example, there's a nonzero chance the USA will
> cancel DST altogether at some future time.  (This would be driven by
> politicians who don't remember the last time, but there's no shortage
> of those.)  That'd likely break some of our test results, and even if
> it happened not to, it'd still be bad because we'd probably lose some
> coverage of the DST-transition-related code paths in src/timezone/.
> So I'd really be way happier with a test timezone that never changes
> but does include DST behavior.  I thought PST8PDT fit those
> requirements pretty well, and I'm annoyed at Eggert for having tossed
> it overboard for no benefit whatever.  But I don't run tzdb, so
> here we are.
>
> > We just need one that didn't change in a reasonable number of
> > releases going backwards.
>
> We've had this sort of fire-drill before, e.g. commit 8d7af8fbe.
> It's no fun, and the potential surface area for unexpected changes
> is now much greater than the few tests affected by that change.
>
> But here we are, so I pushed your patch with a couple of other
> cosmetic bits.  There are still a couple of references to PST8PDT
> in the tree, but they don't appear to care what the actual meaning
> of that zone is, so I left them be.
>

This is an unfortunate change as this will break extensions tests using
pg_regress for testing. We run our tests against multiple minor versions
and this getting backported means our tests will fail with the next minor
pg release. Is there a workaround available to make the timezone for
pg_regress configurable without going into every test?

Regards, Sven Klemm

---

From: Tom Lane <[email-masked]>
Date: Mon, 16 Sep 2024 11:19:42 -0400

> pg_regress configurable without going into every test?

Configurable to what?  If your test cases are dependent on the
historical behavior of PST8PDT, you're out of luck, because that
simply isn't available anymore (or won't be once 2024b reaches
your platform, anyway).

			regards, tom lane

---

From: Sven Klemm <[email-masked]>
Date: Tue, 17 Sep 2024 07:38:15 +0200

> simply isn't available anymore (or won't be once 2024b reaches

I was wondering whether the timezone used by pg_regress could be made
configurable.

-- 
Regards, Sven Klemm

---

From: Tom Lane <[email-masked]>
Date: Tue, 17 Sep 2024 01:42:35 -0400

> I was wondering whether the timezone used by pg_regress could be made

Yes, I understood that you were suggesting that.  My point is that
it wouldn't do you any good: you will still have to change any
regression test cases that depend on behavior PST8PDT has/had that
is different from America/Los_Angeles.  That being the case,
I don't see much value in making it configurable.

			regards, tom lane

---

From: Wolfgang Walther <[email-masked]>
Date: Tue, 17 Sep 2024 08:39:53 +0200

> I don't see much value in making it configurable.

Just changing it back to PST8PDT wouldn't really help as Tom pointed 
out. You'd still get different results depending on which tzdata version 
you are running with.

The core regression tests need to be run with a timezone that tests 
special cases in the timezone handling code. But that might not be true 
for extensions - all they want could be a stable output across major and 
minor versions of postgres and versions of tzdata. It could be helpful 
to set pg_regress' timezone to UTC, for example?

Best,

Wolfgang

---

From: Tom Lane <[email-masked]>
Date: Tue, 17 Sep 2024 10:15:25 -0400

> to set pg_regress' timezone to UTC, for example?

I would not recommend that choice.  It would mask simple errors such
as failing to apply the correct conversion between timestamptz and
timestamp values.  Also, if you have test cases that are affected by
this issue at all, you probably have a need/desire to test things like
DST transitions.

			regards, tom lane