[PATCH] mingw: use modern strftime implementation if possible

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: =?UTF-8?q?Matthias=20A=C3=9Fhauer?= <mha1993@live.de>

Microsoft introduced a new "Universal C Runtime Library" (UCRT) with
Visual Studio 2015. The UCRT comes with a new strftime() implementation
that supports more date formats. We link git against the older
"Microsoft Visual C Runtime Library" (MSVCRT), so to use the UCRT
strftime() we need to load it from ucrtbase.dll using
DECLARE_PROC_ADDR()/INIT_PROC_ADDR().

Most supported Windows systems should have recieved the UCRT via Windows
update, but in some cases only MSVCRT might be available. In that case
we fall back to using that implementation.

With this change, it is possible to use e.g. the `%g` and `%V` date
format specifiers, e.g.

	git show -s --format=%cd --date=format:‘%g.%V’ HEAD

Without this change, the user would see this error message on Windows:

	fatal: invalid strftime format: '‘%g.%V’'

This fixes https://github.com/git-for-windows/git/issues/2495

Signed-off-by: Matthias Aßhauer <mha1993@live.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
    Use a modern strftime() on Windows when available
    
    This is another contribution that came in via Git for Windows.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-753%2Fdscho%2Fmingw-modern-strftime-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-753/dscho/mingw-modern-strftime-v1
Pull-Request: https://github.com/git/git/pull/753

 compat/mingw.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/compat/mingw.c b/compat/mingw.c
index d14065d60ec..2136744af35 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -964,7 +964,16 @@ int mingw_utime (const char *file_name, const struct utimbuf *times)
 size_t mingw_strftime(char *s, size_t max,
 		      const char *format, const struct tm *tm)
 {
-	size_t ret = strftime(s, max, format, tm);
+	/* a pointer to the original strftime in case we can't find the UCRT version */
+	static size_t (*fallback)(char *, size_t, const char *, const struct tm *) = strftime;
+	size_t ret;
+	DECLARE_PROC_ADDR(ucrtbase.dll, size_t, strftime, char *, size_t,
+		const char *, const struct tm *);
+
+	if (INIT_PROC_ADDR(strftime))
+		ret = strftime(s, max, format, tm);
+	else
+		ret = fallback(s, max, format, tm);
 
 	if (!ret && errno == EINVAL)
 		die("invalid strftime format: '%s'", format);

base-commit: 9fadedd637b312089337d73c3ed8447e9f0aa775
-- 
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