Re: git-2.13.0: log --date=format:%z not working

[Jeff King <peff@peff.net>]

On Sat, May 27, 2017 at 06:57:08PM +0200, Ævar Arnfjörð Bjarmason wrote:

> There's another test which breaks if we just s/gmtime/localtime/g. As
> far as I can tell to make the non-local case work we'd need to do a
> whole dance where we set the TZ variable to e.g. UTC$offset, then call
> strftime(), then call it again. Maybe there's some way to just specify
> the tz offset, but I didn't find any in a quick skimming of time.h.

There isn't. At least on _some_ platforms, the zone information is
embedded in "struct tm" and stored by gmtime() and localtime(), but the
fields aren't publicly accessible. Which is why your patch worked for
format-local (it swaps out gmtime() for localtime() which sets those
fields behind the scenes). But:

  - I'm not sure that's guaranteed by the standard; strftime() might get
    its zone information elsewhere (if it needs to reliably distinguish
    between gmtime() and localtime() results it has to at least set a
    bit in the "struct tm", but that bit may not be the full zone info).

  - Even if it does work, you're stuck with only the local timezone. In
    theory you could temporarily tweak the process's timezone, call
    localtime(), and then tweak it back. I was never able to get that to
    work (links below).

On glibc, at least, you can access the zone fields in "struct tm" by
compiling with _DEFAULT_SOURCE.

So I think the best we could do is probably to have a feature macro like
TM_HAS_GMTOFF, and set tm->tm_gmtoff and tm->tm_zone on platforms that
support it. I'm not sure what we'd put in the latter, though; we don't
actually have the timezone name at all (we just have "+0200" or whatever
we parsed from the git object, but that would be better than nothing).

That leaves other platforms still broken, but like I said, I don't think
there's a portable solution.

Here are some links to past explorations:

  http://public-inbox.org/git/20160208152858.GA17226@sigill.intra.peff.net/

  http://public-inbox.org/git/87vb2d37ea.fsf@web.de/

-Peff