Re: timezone related bug of git

[Jeff King <peff@peff.net>]

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++;