do not write out index when status does not have to

Some codepaths, such as "git status" and "git commit --dry-run",
tried to opportunisticly refresh the index and write the result
out.  But they did so without checking if there was actually any
change that needs to be written out.

Noticed by Jeff King and Daniel at Rutgers.edu

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/builtin/commit.c b/builtin/commit.c
index 80c621d..da18287 100644 (file)
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -334,9 +334,13 @@ static char *prepare_index(int argc, const char **argv, const char *prefix, int
        if (!pathspec || !*pathspec) {
                fd = hold_locked_index(&index_lock, 1);
                refresh_cache_or_die(refresh_flags);
-               if (write_cache(fd, active_cache, active_nr) ||
-                   commit_locked_index(&index_lock))
-                       die("unable to write new_index file");
+               if (active_cache_changed) {
+                       if (write_cache(fd, active_cache, active_nr) ||
+                           commit_locked_index(&index_lock))
+                               die("unable to write new_index file");
+               } else {
+                       rollback_lock_file(&index_lock);
+               }
                commit_style = COMMIT_AS_IS;
                return get_index_file();
        }
@@ -1067,9 +1071,11 @@ int cmd_status(int argc, const char **argv, const char *prefix)
 
        fd = hold_locked_index(&index_lock, 0);
        if (0 <= fd) {
-               if (!write_cache(fd, active_cache, active_nr))
+               if (active_cache_changed &&
+                   !write_cache(fd, active_cache, active_nr))
                        commit_locked_index(&index_lock);
-               rollback_lock_file(&index_lock);
+               else
+                       rollback_lock_file(&index_lock);
        }
 
        s.is_initial = get_sha1(s.reference, sha1) ? 1 : 0;