Re: [Outreachy][PATCH 2/2] Port helper/test-date.c to unit-tests/t-date.c

[Patrick Steinhardt <ps@pks.im>]

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On Thu, Mar 28, 2024 at 09:35:39AM -0700, Junio C Hamano wrote:
> Ghanshyam Thakkar <shyamthakkar001@gmail.com> writes:
> 
> > I believe the issue might not be related to the setenv function, but rather
> > with tzset(). As you can see here[1], when we set TZ before we call the
> > unit-testing binaries, the tests which were failing (EST5 ones that I
> > separated with t-datetest) pass on 'win test (0)', and the ones which
> > were passing (UTC ones, t-date) fail. (Although some tests on linux are also
> > failing, but that can be explained by the fact that t-date runs first
> > and sets the TZ to UTC, afterwhich t-datetest runs and fails, although
> > this is not conclusive). Therefore, I am almost certain that the issue
> > is with changing the timezone during runtime on windows and not with setting
> > TZ variable with setenv(). CC'ing Johannes to see if he has any insights
> > on this.
> 
> Interesting.  Sometime before I started working on Git, I learned
> that no program did tzset() after it started running to switch
> multiple timezones and worked correctly on many different variants
> of UNIXes (there were many of them back then), and because I never
> got interested in writing a world-clock program, I didn't know, and
> kind of surprised to learn that it works on some platforms (like
> Linux and macOS) to switch zones with tzset() these days ;-).
> 
> So, if Windows runtime is unhappy with the program calling tzset()
> more than once, I wouldn't be too surprised.
> 
> Thanks.

As I was debugging other Windows-specific issues in a VM already, Chris
asked me to also have a look at this issue. And indeed, most of the
tests fail deterministically. I also found a fix:

    diff --git a/t/unit-tests/t-date.c b/t/unit-tests/t-date.c
    index dd5dbbb2e0..2d7b1f085a 100644
    --- a/t/unit-tests/t-date.c
    +++ b/t/unit-tests/t-date.c
    @@ -31,7 +31,7 @@ static int check_prereqs(unsigned int prereqs) {
     }
     
     static void set_TZ_env(const char *zone) {
    -	setenv("TZ", zone, 1);
    +	_putenv_s("TZ", zone);
        tzset();
     }

I have no idea why that works though, and the fix is of course not
portable. But with this change, the timezones do get picked up by
`tzset()` and related date functions as expected.

I'm quite dumb when it comes to the Windows API, so I don't have much of
a clue why this works. The documentation also didn't point out anything
obvious. Dscho, do you happen to have an explanation for this?

Patrick

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