notes: Don't create (empty) commit when removing non-existing notes

Extend remove_note() in the notes API to return whether or not a note was
actually removed. Use this in 'git notes remove' to skip the creation of
a notes commit when no notes were actually removed.

Also add a test illustrating the change in behavior.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/notes.c

@@ -769,6 +769,7 @@ static int remove_cmd(int argc, const char **argv, const char *prefix)
 	const char *object_ref;
 	struct notes_tree *t;
 	unsigned char object[20];
+	int retval;
 
 	argc = parse_options(argc, argv, prefix, options,
 			     git_notes_remove_usage, 0);
@@ -785,12 +786,17 @@ static int remove_cmd(int argc, const char **argv, const char *prefix)
 
 	t = init_notes_check("remove");
 
-	fprintf(stderr, "Removing note for object %s\n", sha1_to_hex(object));
-	remove_note(t, object);
+	retval = remove_note(t, object);
+	if (retval)
+		fprintf(stderr, "Object %s has no note\n", sha1_to_hex(object));
+	else {
+		fprintf(stderr, "Removing note for object %s\n",
+			sha1_to_hex(object));
 
-	commit_notes(t, "Notes removed by 'git notes remove'");
+		commit_notes(t, "Notes removed by 'git notes remove'");
+	}
 	free_notes(t);
-	return 0;
+	return retval;
 }
 
 static int prune(int argc, const char **argv, const char *prefix)

notes.c

@@ -263,11 +263,13 @@ static int note_tree_consolidate(struct int_node *tree,
  * To remove a leaf_node:
  * Search to the tree location appropriate for the given leaf_node's key:
  * - If location does not hold a matching entry, abort and do nothing.
+ * - Copy the matching entry's value into the given entry.
  * - Replace the matching leaf_node with a NULL entry (and free the leaf_node).
  * - Consolidate int_nodes repeatedly, while walking up the tree towards root.
  */
-static void note_tree_remove(struct notes_tree *t, struct int_node *tree,
-		unsigned char n, struct leaf_node *entry)
+static void note_tree_remove(struct notes_tree *t,
+		struct int_node *tree, unsigned char n,
+		struct leaf_node *entry)
 {
 	struct leaf_node *l;
 	struct int_node *parent_stack[20];
@@ -282,6 +284,7 @@ static void note_tree_remove(struct notes_tree *t, struct int_node *tree,
 		return; /* key mismatch, nothing to remove */
 
 	/* we have found a matching entry */
+	hashcpy(entry->val_sha1, l->val_sha1);
 	free(l);
 	*p = SET_PTR_TYPE(NULL, PTR_TYPE_NULL);
 
@@ -1003,17 +1006,20 @@ void add_note(struct notes_tree *t, const unsigned char *object_sha1,
 	note_tree_insert(t, t->root, 0, l, PTR_TYPE_NOTE, combine_notes);
 }
 
-void remove_note(struct notes_tree *t, const unsigned char *object_sha1)
+int remove_note(struct notes_tree *t, const unsigned char *object_sha1)
 {
 	struct leaf_node l;
 
 	if (!t)
 		t = &default_notes_tree;
 	assert(t->initialized);
-	t->dirty = 1;
 	hashcpy(l.key_sha1, object_sha1);
 	hashclr(l.val_sha1);
 	note_tree_remove(t, t->root, 0, &l);
+	if (is_null_sha1(l.val_sha1)) // no note was removed
+		return 1;
+	t->dirty = 1;
+	return 0;
 }
 
 const unsigned char *get_note(struct notes_tree *t,

notes.h

@@ -89,8 +89,10 @@ void add_note(struct notes_tree *t, const unsigned char *object_sha1,
  * IMPORTANT: The changes made by remove_note() to the given notes_tree
  * structure are not persistent until a subsequent call to write_notes_tree()
  * returns zero.
+ *
+ * Return 0 if a note was removed; 1 if there was no note to remove.
  */
-void remove_note(struct notes_tree *t, const unsigned char *object_sha1);
+int remove_note(struct notes_tree *t, const unsigned char *object_sha1);
 
 /*
  * Get the note object SHA1 containing the note data for the given object

t/t3301-notes.sh

@@ -365,6 +365,13 @@ c18dc024e14f08d18d14eea0d747ff692d66d6a3 1584215f1d29c65e99c6c6848626553fdd07fd7
 c9c6af7f78bc47490dbf3e822cf2f3c24d4b9061 268048bfb8a1fb38e703baceb8ab235421bf80c5
 EOF
 
+test_expect_success 'removing non-existing note should not create new commit' '
+	git rev-parse --verify refs/notes/commits > before_commit &&
+	test_must_fail git notes remove HEAD^ &&
+	git rev-parse --verify refs/notes/commits > after_commit &&
+	test_cmp before_commit after_commit
+'
+
 test_expect_success 'list notes with "git notes list"' '
 	git notes list > output &&
 	test_cmp expect output