[PATCH v2 0/3] Phase out non-reentrant time functions

gmtime/localtime is considered unsafe in multithread environment.

git was started as single-thread application, but we have some multi-thread code, right now.

replace all usage of gmtime/localtime by their respective reentrant ones.

On most POSIX systems, we won't notice any differences in performance, since they implemented gmtime and gmtime_r mostly the same.

On Windows, we may be taxed, since gmtime_r and localtime_r is our compat functions, because we memcpy from returned data of gmtime/localtime.

To address that, I made patch #3 and included it together with this series. I'm not sure how much portable it is.

Doan Tran Cong Danh (3):
  date.c: switch to reentrant {gm,local}time_r
  archive-zip.c: switch to reentrant localtime_r
  mingw: use {gm,local}time_s as backend for {gm,local}time_r

 archive-zip.c  | 10 +++++-----
 compat/mingw.c | 12 ++++++------
 date.c         | 18 ++++++++++--------
 3 files changed, 21 insertions(+), 19 deletions(-)

Range-diff against v1:
1:  1e7e08b3c7 < -:  ---------- date.c::datestamp: switch to reentrant localtime_r
2:  130eba77db ! 1:  71de738864 date.c::time_to_tm_local: use reentrant localtime_r(3)
    @@ Metadata
     Author: Doan Tran Cong Danh <congdanhqx@gmail.com>
     
      ## Commit message ##
    -    date.c::time_to_tm_local: use reentrant localtime_r(3)
    +    date.c: switch to reentrant {gm,local}time_r
     
    -    Phase out `localtime(3)' in favour of reentrant `localtime_r(3)'
    +    Originally, git was intended to be single-thread executable.
    +    `gmtime(3)' and `localtime(3)' can be used in such codebase
    +    for cleaner code.
    +
    +    Overtime, we're employing multithread in our code base.
    +
    +    Let's phase out `gmtime(3)' and `localtime(3)' in favour of
    +    `gmtime_r(3)' and `localtime_r(3)'.
     
         Signed-off-by: Doan Tran Cong Danh <congdanhqx@gmail.com>
     
      ## date.c ##
    -@@ date.c: static struct tm *time_to_tm(timestamp_t time, int tz)
    - 	return gmtime(&t);
    +@@ date.c: static time_t gm_time_t(timestamp_t time, int tz)
    +  * thing, which means that tz -0100 is passed in as the integer -100,
    +  * even though it means "sixty minutes off"
    +  */
    +-static struct tm *time_to_tm(timestamp_t time, int tz)
    ++static struct tm *time_to_tm(timestamp_t time, int tz, struct tm *tm)
    + {
    + 	time_t t = gm_time_t(time, tz);
    +-	return gmtime(&t);
    ++	return gmtime_r(&t, tm);
      }
      
     -static struct tm *time_to_tm_local(timestamp_t time)
    @@ date.c: const char *show_date(timestamp_t time, int tz, const struct date_mode *
     -		tm = time_to_tm_local(time);
     +		tm = time_to_tm_local(time, &tmbuf);
      	else
    - 		tm = time_to_tm(time, tz);
    +-		tm = time_to_tm(time, tz);
    ++		tm = time_to_tm(time, tz, &tmbuf);
      	if (!tm) {
    +-		tm = time_to_tm(0, 0);
    ++		tm = time_to_tm(0, 0, &tmbuf);
    + 		tz = 0;
    + 	}
    + 
    +@@ date.c: void datestamp(struct strbuf *out)
    + {
    + 	time_t now;
    + 	int offset;
    ++	struct tm tm = { 0 };
    + 
    + 	time(&now);
    + 
    +-	offset = tm_to_time_t(localtime(&now)) - now;
    ++	offset = tm_to_time_t(localtime_r(&now, &tm)) - now;
    + 	offset /= 60;
    + 
    + 	date_string(now, offset, out);
3:  ce7fe7bcf3 < -:  ---------- date.c::time_to_tm: use reentrant gmtime_r(3)
4:  f6fd89dfe1 ! 2:  77798f1773 archive-zip: use reentrant localtime_r(3)
    @@ Metadata
     Author: Doan Tran Cong Danh <congdanhqx@gmail.com>
     
      ## Commit message ##
    -    archive-zip: use reentrant localtime_r(3)
    +    archive-zip.c: switch to reentrant localtime_r
    +
    +    Originally, git was intended to be single-thread executable.
    +    `localtime(3)' can be used in such codebase for cleaner code.
    +
    +    Overtime, we're employing multithread in our code base.
    +
    +    Let's phase out `gmtime(3)' in favour of `localtime_r(3)'.
    +
    +    Signed-off-by: Doan Tran Cong Danh <congdanhqx@gmail.com>
     
      ## archive-zip.c ##
     @@ archive-zip.c: static void write_zip_trailer(const struct object_id *oid)
5:  2c39f9a04f ! 3:  33a67eb377 mingw: use {gm,local}time_s as backend for {gm,local}time_r
    @@ Commit message
         Signed-off-by: Doan Tran Cong Danh <congdanhqx@gmail.com>
     
      ## compat/mingw.c ##
    -@@
    - #include "../git-compat-util.h"
    - #include "win32.h"
    - #include <conio.h>
    -+#include <errno.h>
    - #include <wchar.h>
    - #include "../strbuf.h"
    - #include "../run-command.h"
     @@ compat/mingw.c: int pipe(int filedes[2])
      
      struct tm *gmtime_r(const time_t *timep, struct tm *result)