compat/snprintf: don't look at va_list twice

If you define SNPRINTF_RETURNS_BOGUS, we use a special
git_vsnprintf wrapper assumes that vsnprintf returns "-1"
instead of the number of characters that you would need to
store the result.

To do this, it invokes vsnprintf multiple times, growing a
heap buffer until we have enough space to hold the result.
However, this means we evaluate the va_list parameter
multiple times, which is generally a bad thing (it may be
modified by calls to vsnprintf, yielding undefined
behavior).

Instead, we must va_copy it and hand the copy to vsnprintf,
so we always have a pristine va_list.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/snprintf.c

@@ -19,11 +19,14 @@
 #undef vsnprintf
 int git_vsnprintf(char *str, size_t maxsize, const char *format, va_list ap)
 {
+	va_list cp;
 	char *s;
 	int ret = -1;
 
 	if (maxsize > 0) {
-		ret = vsnprintf(str, maxsize-SNPRINTF_SIZE_CORR, format, ap);
+		va_copy(cp, ap);
+		ret = vsnprintf(str, maxsize-SNPRINTF_SIZE_CORR, format, cp);
+		va_end(cp);
 		if (ret == maxsize-1)
 			ret = -1;
 		/* Windows does not NUL-terminate if result fills buffer */
@@ -42,7 +45,9 @@ int git_vsnprintf(char *str, size_t maxsize, const char *format, va_list ap)
 		if (! str)
 			break;
 		s = str;
-		ret = vsnprintf(str, maxsize-SNPRINTF_SIZE_CORR, format, ap);
+		va_copy(cp, ap);
+		ret = vsnprintf(str, maxsize-SNPRINTF_SIZE_CORR, format, cp);
+		va_end(cp);
 		if (ret == maxsize-1)
 			ret = -1;
 	}