gettext.c: detect the vsnprintf bug at runtime

Bug 6530 [1] in glibc causes "git show v0.99.6~1" to fail with error
"your vsnprintf is broken". The workaround avoids that, but it
corrupts system error messages in non-C locales.

The bug has been fixed since 2.17. We could know running glibc version
with gnu_get_libc_version(). But version is not a sure way to detect
the bug because downstream may back port the fix to older versions. Do
a runtime test that immitates the call flow that leads to "your
vsnprintf is broken". Only enable the workaround if the test fails.

Tested on Gentoo Linux, glibc 2.16.0 and 2.17, amd64.

[1] http://sourceware.org/bugzilla/show_bug.cgi?id=6530

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gettext.c

@@ -29,6 +29,17 @@ int use_gettext_poison(void)
 #endif
 
 #ifndef NO_GETTEXT
+static int test_vsnprintf(const char *fmt, ...)
+{
+	char buf[26];
+	int ret;
+	va_list ap;
+	va_start(ap, fmt);
+	ret = vsnprintf(buf, sizeof(buf), fmt, ap);
+	va_end(ap);
+	return ret;
+}
+
 static const char *charset;
 static void init_gettext_charset(const char *domain)
 {
@@ -99,9 +110,7 @@ static void init_gettext_charset(const char *domain)
 	   $ LANGUAGE= LANG=de_DE.utf8 ./test
 	   test: Kein passendes Ger?t gefunden
 
-	   In the long term we should probably see about getting that
-	   vsnprintf bug in glibc fixed, and audit our code so it won't
-	   fall apart under a non-C locale.
+	   The vsnprintf bug has been fixed since glibc 2.17.
 
 	   Then we could simply set LC_CTYPE from the environment, which would
 	   make things like the external perror(3) messages work.
@@ -115,7 +124,9 @@ static void init_gettext_charset(const char *domain)
 	setlocale(LC_CTYPE, "");
 	charset = locale_charset();
 	bind_textdomain_codeset(domain, charset);
-	setlocale(LC_CTYPE, "C");
+	/* the string is taken from v0.99.6~1 */
+	if (test_vsnprintf("%.*s", 13, "David_K\345gedal") < 0)
+		setlocale(LC_CTYPE, "C");
 }
 
 void git_setup_gettext(void)