Add mksnpath which allows you to specify the output buffer

This is just vsnprintf's but additionally calls cleanup_path() on the
result. To be used as alternatives to mkpath() where the buffer for the
created path may not be reused by subsequent calls of the same formatting
function.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -480,6 +480,9 @@ extern int check_repository_format(void);
 #define DATA_CHANGED    0x0020
 #define TYPE_CHANGED    0x0040
 
+extern char *mksnpath(char *buf, size_t n, const char *fmt, ...)
+	__attribute__((format (printf, 3, 4)));
+
 /* Return a statically allocated filename matching the sha1 signature */
 extern char *mkpath(const char *fmt, ...) __attribute__((format (printf, 1, 2)));
 extern char *git_path(const char *fmt, ...) __attribute__((format (printf, 1, 2)));

path.c

@@ -32,6 +32,21 @@ static char *cleanup_path(char *path)
 	return path;
 }
 
+char *mksnpath(char *buf, size_t n, const char *fmt, ...)
+{
+	va_list args;
+	unsigned len;
+
+	va_start(args, fmt);
+	len = vsnprintf(buf, n, fmt, args);
+	va_end(args);
+	if (len >= n) {
+		snprintf(buf, n, bad_path);
+		return buf;
+	}
+	return cleanup_path(buf);
+}
+
 char *mkpath(const char *fmt, ...)
 {
 	va_list args;