write_locked_index(): add flag to avoid writing unchanged index

We have several callers like

	if (active_cache_changed && write_locked_index(...))
		handle_error();
	rollback_lock_file(...);

where the final rollback is needed because "!active_cache_changed"
shortcuts the if-expression. There are also a few variants of this,
including some if-else constructs that make it more clear when the
explicit rollback is really needed.

Teach `write_locked_index()` to take a new flag SKIP_IF_UNCHANGED and
simplify the callers. Leave the most complicated of the callers (in
builtin/update-index.c) unchanged. Rewriting it to use this new flag
would end up duplicating logic.

We could have made the new flag behave the other way round
("FORCE_WRITE"), but that could break existing users behind their backs.
Let's take the more conservative approach. We can still migrate existing
callers to use our new flag. Later we might even be able to flip the
default, possibly without entirely ignoring the risk to in-flight or
out-of-tree topics.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

10 files changed, 37 insertions, 40 deletions

diff --git a/builtin/add.c b/builtin/add.c
index bf01d89e2..9e5a80cc6 100644
--- a/builtin/add.c
+++ b/builtin/add.c
@@ -534,10 +534,9 @@ int cmd_add(int argc, const char **argv, const char *prefix)
 	unplug_bulk_checkin();
 
 finish:
-	if (active_cache_changed) {
-		if (write_locked_index(&the_index, &lock_file, COMMIT_LOCK))
-			die(_("Unable to write new index file"));
-	}
+	if (write_locked_index(&the_index, &lock_file,
+			       COMMIT_LOCK | SKIP_IF_UNCHANGED))
+		die(_("Unable to write new index file"));
 
 	UNLEAK(pathspec);
 	UNLEAK(dir);
diff --git a/builtin/commit.c b/builtin/commit.c
index e8e8d13be..ff6dac4b9 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -389,13 +389,9 @@ static const char *prepare_index(int argc, const char **argv, const char *prefix
 		if (active_cache_changed
 		    || !cache_tree_fully_valid(active_cache_tree))
 			update_main_cache_tree(WRITE_TREE_SILENT);
-		if (active_cache_changed) {
-			if (write_locked_index(&the_index, &index_lock,
-					       COMMIT_LOCK))
-				die(_("unable to write new_index file"));
-		} else {
-			rollback_lock_file(&index_lock);
-		}
+		if (write_locked_index(&the_index, &index_lock,
+				       COMMIT_LOCK | SKIP_IF_UNCHANGED))
+			die(_("unable to write new_index file"));
 		commit_style = COMMIT_AS_IS;
 		ret = get_index_file();
 		goto out;
diff --git a/builtin/merge.c b/builtin/merge.c
index 92ba99a1a..7efa3c041 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -651,10 +651,9 @@ static int try_merge_strategy(const char *strategy, struct commit_list *common,
 
 	hold_locked_index(&lock, LOCK_DIE_ON_ERROR);
 	refresh_cache(REFRESH_QUIET);
-	if (active_cache_changed &&
-	    write_locked_index(&the_index, &lock, COMMIT_LOCK))
+	if (write_locked_index(&the_index, &lock,
+			       COMMIT_LOCK | SKIP_IF_UNCHANGED))
 		return error(_("Unable to write index."));
-	rollback_lock_file(&lock);
 
 	if (!strcmp(strategy, "recursive") || !strcmp(strategy, "subtree")) {
 		int clean, x;
@@ -691,10 +690,9 @@ static int try_merge_strategy(const char *strategy, struct commit_list *common,
 				remoteheads->item, reversed, &result);
 		if (clean < 0)
 			exit(128);
-		if (active_cache_changed &&
-		    write_locked_index(&the_index, &lock, COMMIT_LOCK))
+		if (write_locked_index(&the_index, &lock,
+				       COMMIT_LOCK | SKIP_IF_UNCHANGED))
 			die (_("unable to write %s"), get_index_file());
-		rollback_lock_file(&lock);
 		return clean ? 0 : 1;
 	} else {
 		return try_merge_command(strategy, xopts_nr, xopts,
@@ -810,10 +808,9 @@ static int merge_trivial(struct commit *head, struct commit_list *remoteheads)
 
 	hold_locked_index(&lock, LOCK_DIE_ON_ERROR);
 	refresh_cache(REFRESH_QUIET);
-	if (active_cache_changed &&
-	    write_locked_index(&the_index, &lock, COMMIT_LOCK))
+	if (write_locked_index(&the_index, &lock,
+			       COMMIT_LOCK | SKIP_IF_UNCHANGED))
 		return error(_("Unable to write index."));
-	rollback_lock_file(&lock);
 
 	write_tree_trivial(&result_tree);
 	printf(_("Wonderful.\n"));
diff --git a/builtin/mv.c b/builtin/mv.c
index cf3684d90..ae013d6d2 100644
--- a/builtin/mv.c
+++ b/builtin/mv.c
@@ -293,8 +293,8 @@ int cmd_mv(int argc, const char **argv, const char *prefix)
 	if (gitmodules_modified)
 		stage_updated_gitmodules(&the_index);
 
-	if (active_cache_changed &&
-	    write_locked_index(&the_index, &lock_file, COMMIT_LOCK))
+	if (write_locked_index(&the_index, &lock_file,
+			       COMMIT_LOCK | SKIP_IF_UNCHANGED))
 		die(_("Unable to write new index file"));
 
 	return 0;
diff --git a/builtin/rm.c b/builtin/rm.c
index 4a2fcca27..5d59a0aa6 100644
--- a/builtin/rm.c
+++ b/builtin/rm.c
@@ -385,10 +385,9 @@ int cmd_rm(int argc, const char **argv, const char *prefix)
 			stage_updated_gitmodules(&the_index);
 	}
 
-	if (active_cache_changed) {
-		if (write_locked_index(&the_index, &lock_file, COMMIT_LOCK))
-			die(_("Unable to write new index file"));
-	}
+	if (write_locked_index(&the_index, &lock_file,
+			       COMMIT_LOCK | SKIP_IF_UNCHANGED))
+		die(_("Unable to write new index file"));
 
 	return 0;
 }
diff --git a/cache.h b/cache.h
index 21fbcc241..5c880318e 100644
--- a/cache.h
+++ b/cache.h
@@ -599,6 +599,7 @@ extern int read_index_unmerged(struct index_state *);
 
 /* For use with `write_locked_index()`. */
 #define COMMIT_LOCK		(1 << 0)
+#define SKIP_IF_UNCHANGED	(1 << 1)
 
 /*
  * Write the index while holding an already-taken lock. Close the lock,
@@ -615,6 +616,9 @@ extern int read_index_unmerged(struct index_state *);
  * With `COMMIT_LOCK`, the lock is always committed or rolled back.
  * Without it, the lock is closed, but neither committed nor rolled
  * back.
+ *
+ * If `SKIP_IF_UNCHANGED` is given and the index is unchanged, nothing
+ * is written (and the lock is rolled back if `COMMIT_LOCK` is given).
  */
 extern int write_locked_index(struct index_state *, struct lock_file *lock, unsigned flags);
 
diff --git a/merge-recursive.c b/merge-recursive.c
index 129577987..2f232ad3b 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -2223,10 +2223,9 @@ int merge_recursive_generic(struct merge_options *o,
 		return clean;
 	}
 
-	if (active_cache_changed &&
-	    write_locked_index(&the_index, &lock, COMMIT_LOCK))
+	if (write_locked_index(&the_index, &lock,
+			       COMMIT_LOCK | SKIP_IF_UNCHANGED))
 		return err(o, _("Unable to write index."));
-	rollback_lock_file(&lock);
 
 	return clean ? 0 : 1;
 }
diff --git a/read-cache.c b/read-cache.c
index 28bf0db9d..e2a939a5d 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -2538,6 +2538,12 @@ int write_locked_index(struct index_state *istate, struct lock_file *lock,
 	int new_shared_index, ret;
 	struct split_index *si = istate->split_index;
 
+	if ((flags & SKIP_IF_UNCHANGED) && !istate->cache_changed) {
+		if (flags & COMMIT_LOCK)
+			rollback_lock_file(lock);
+		return 0;
+	}
+
 	if (istate->fsmonitor_last_update)
 		fill_fsmonitor_bitmap(istate);
 
diff --git a/rerere.c b/rerere.c
index 79203c6c1..ea24d4c2f 100644
--- a/rerere.c
+++ b/rerere.c
@@ -719,11 +719,9 @@ static void update_paths(struct string_list *update)
 			item->string);
 	}
 
-	if (active_cache_changed) {
-		if (write_locked_index(&the_index, &index_lock, COMMIT_LOCK))
-			die("Unable to write new index file");
-	} else
-		rollback_lock_file(&index_lock);
+	if (write_locked_index(&the_index, &index_lock,
+			       COMMIT_LOCK | SKIP_IF_UNCHANGED))
+		die("Unable to write new index file");
 }
 
 static void remove_variant(struct rerere_id *id)
diff --git a/sequencer.c b/sequencer.c
index 548ad997b..6ee251849 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -517,15 +517,14 @@ static int do_recursive_merge(struct commit *base, struct commit *next,
 		return clean;
 	}
 
-	if (active_cache_changed &&
-	    write_locked_index(&the_index, &index_lock, COMMIT_LOCK))
+	if (write_locked_index(&the_index, &index_lock,
+			       COMMIT_LOCK | SKIP_IF_UNCHANGED))
 		/*
 		 * TRANSLATORS: %s will be "revert", "cherry-pick" or
 		 * "rebase -i".
 		 */
 		return error(_("%s: Unable to write new index file"),
 			_(action_name(opts)));
-	rollback_lock_file(&index_lock);
 
 	if (!clean)
 		append_conflicts_hint(msgbuf);
@@ -1713,13 +1712,13 @@ static int read_and_refresh_cache(struct replay_opts *opts)
 			_(action_name(opts)));
 	}
 	refresh_index(&the_index, REFRESH_QUIET|REFRESH_UNMERGED, NULL, NULL, NULL);
-	if (the_index.cache_changed && index_fd >= 0) {
-		if (write_locked_index(&the_index, &index_lock, COMMIT_LOCK)) {
+	if (index_fd >= 0) {
+		if (write_locked_index(&the_index, &index_lock,
+				       COMMIT_LOCK | SKIP_IF_UNCHANGED)) {
 			return error(_("git %s: failed to refresh the index"),
 				_(action_name(opts)));
 		}
 	}
-	rollback_lock_file(&index_lock);
 	return 0;
 }