Re: timezone related bug of git

[Dongsheng Song <dongsheng.song@gmail.com>]

Hi Jeff,

Your patch looks good to me.  Here is my test result:

$ patch -p1 < x.patch
patching file cache.h
patching file date.c
patching file strbuf.c

$ make

$ ~/git-testing/bin/git --version

$ ~/git-testing/bin/git log 11990eba -1 --date=format:%s
commit 11990eba0be50d1ad0655ede4062b7130326c41f (HEAD -> trunk,
origin/trunk, origin/HEAD)
Author: rillig <rillig@NetBSD.org>
Date:   1635633678

    indent: move debugging functions to a separate section


$ ~/git-testing/bin/git cat-file -p 11990eba
tree 5d62150f5e2bafd3db76641450ca5d902302a039
parent 892557a74bd49983fac28366b772b53c9216ca73
author rillig <rillig@NetBSD.org> 1635633678 +0000
committer rillig <rillig@NetBSD.org> 1635633678 +0000

indent: move debugging functions to a separate section


PS:
Thanks Junio for clarification, Jeff's explanation is very clear,
I'm just frustrated with the limitations of the system, I greatly
appreciate Jeff's work.

Best regards,
Dongsheng Song

On Mon, Nov 1, 2021 at 12:03 PM Jeff King <peff@peff.net> wrote:
>
> On Sun, Oct 31, 2021 at 11:46:40AM -0700, Junio C Hamano wrote:
>
> > Dongsheng Song <dongsheng.song@gmail.com> writes:
> >
> > > Thank you for the clarification, it's really a disappointing answer.
> >
> > The situation may be disappointing, but I found the answer eminently
> > clear and helpful.
>
> The most disappointing thing IMHO is the lousy state of system-level
> date routines. ;)
>
> I have some patches working towards allowing timestamps before 1970, and
> the system routines are quite unreliable (both in giving insufficient
> portable interfaces, but also just doing weird things with negative
> values).
>
> > > Perhaps the manual needs to be clearer about this limitation.
> >
> > Sounds like we have a volunteer ;-)?
>
> Yeah, I'd be happy if somebody wanted to note this in the manual. But if
> anybody wants to pursue manually intercepting %s, I think the patch
> below might point them in the right direction.
>
> I won't be at all surprised if it has funny corner cases. Our
> tm_to_time_t() is pretty basic and hacky. We can't use mktime() because
> it only handles the current system timezone. OTOH, I think the tz_offset
> we're undoing here originally came from comparing mktime() versus
> tm_to_time_t() via local_time_tzoffset(), so it could be cancelling out
> any bugs exactly. :)
>
> So maybe the code below is sufficient, but we'd probably at least want
> some tests on top. Maybe something somebody interested would like to
> pick up and run with?
>
> ---
> diff --git a/cache.h b/cache.h
> index eba12487b9..aa6f380d10 100644
> --- a/cache.h
> +++ b/cache.h
> @@ -1588,6 +1588,7 @@ timestamp_t approxidate_careful(const char *, int *);
>  timestamp_t approxidate_relative(const char *date);
>  void parse_date_format(const char *format, struct date_mode *mode);
>  int date_overflows(timestamp_t date);
> +time_t tm_to_time_t(const struct tm *tm);
>
>  #define IDENT_STRICT          1
>  #define IDENT_NO_DATE         2
> diff --git a/date.c b/date.c
> index c55ea47e96..84bb4451c1 100644
> --- a/date.c
> +++ b/date.c
> @@ -9,7 +9,7 @@
>  /*
>   * This is like mktime, but without normalization of tm_wday and tm_yday.
>   */
> -static time_t tm_to_time_t(const struct tm *tm)
> +time_t tm_to_time_t(const struct tm *tm)
>  {
>         static const int mdays[] = {
>             0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334
> diff --git a/strbuf.c b/strbuf.c
> index b22e981655..8b8b1900bc 100644
> --- a/strbuf.c
> +++ b/strbuf.c
> @@ -1019,6 +1019,13 @@ void strbuf_addftime(struct strbuf *sb, const char *fmt, const struct tm *tm,
>                         strbuf_addstr(&munged_fmt, "%%");
>                         fmt++;
>                         break;
> +               case 's':
> +                       strbuf_addf(&munged_fmt, "%"PRItime,
> +                                   tm_to_time_t(tm) -
> +                                   3600 * (tz_offset / 100) -
> +                                   60 * (tz_offset % 100));
> +                       fmt++;
> +                       break;
>                 case 'z':
>                         strbuf_addf(&munged_fmt, "%+05d", tz_offset);
>                         fmt++;