refs.c: commit_packed_refs to return a meaningful errno on failure

Making errno when returning from commit_packed_refs() meaningful,
which should fix

 * a bug in "git clone" where it prints strerror(errno) based on
   errno, despite errno possibly being zero and potentially having
   been clobbered by that point
 * the same kind of bug in "git pack-refs"

and prepares for repack_without_refs() to get a meaningful
error message when commit_packed_refs() fails without falling into
the same bug.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Acked-by: Michael Haggerty <mhagger@alum.mit.edu>

refs.c

@@ -2239,11 +2239,16 @@ int lock_packed_refs(int flags)
 	return 0;
 }
 
+/*
+ * Commit the packed refs changes.
+ * On error we must make sure that errno contains a meaningful value.
+ */
 int commit_packed_refs(void)
 {
 	struct packed_ref_cache *packed_ref_cache =
 		get_packed_ref_cache(&ref_cache);
 	int error = 0;
+	int save_errno = 0;
 
 	if (!packed_ref_cache->lock)
 		die("internal error: packed-refs not locked");
@@ -2253,10 +2258,13 @@ int commit_packed_refs(void)
 	do_for_each_entry_in_dir(get_packed_ref_dir(packed_ref_cache),
 				 0, write_packed_entry_fn,
 				 &packed_ref_cache->lock->fd);
-	if (commit_lock_file(packed_ref_cache->lock))
+	if (commit_lock_file(packed_ref_cache->lock)) {
+		save_errno = errno;
 		error = -1;
+	}
 	packed_ref_cache->lock = NULL;
 	release_packed_ref_cache(packed_ref_cache);
+	errno = save_errno;
 	return error;
 }
 

refs.h

@@ -98,6 +98,7 @@ extern void add_packed_ref(const char *refname, const unsigned char *sha1);
  * Write the current version of the packed refs cache from memory to
  * disk.  The packed-refs file must already be locked for writing (see
  * lock_packed_refs()).  Return zero on success.
+ * Sets errno to something meaningful on error.
  */
 extern int commit_packed_refs(void);