Notes API: prune_notes(): Prune notes that belong to non-existing objects

When an object is made unreachable by Git, any notes that annotate that object
are not automagically made unreachable, since all notes are always trivially
reachable from a notes ref. In order to remove notes for non-existing objects,
we therefore need to add functionality for traversing the notes tree and
explicitly removing references to notes that annotate non-reachable objects.
Thus the notes objects themselves also become unreachable, and are removed
by a later garbage collect.

prune_notes() performs this traversal (by using for_each_note() internally),
and removes the notes in question from the notes tree.

Note that the effect of prune_notes() is not persistent unless a subsequent
call to write_notes_tree() is made.

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

notes.c

@@ -749,6 +749,29 @@ static int write_each_note(const unsigned char *object_sha1,
 		write_each_note_helper(d->root, note_path, mode, note_sha1);
 }
 
+struct note_delete_list {
+	struct note_delete_list *next;
+	const unsigned char *sha1;
+};
+
+static int prune_notes_helper(const unsigned char *object_sha1,
+		const unsigned char *note_sha1, char *note_path,
+		void *cb_data)
+{
+	struct note_delete_list **l = (struct note_delete_list **) cb_data;
+	struct note_delete_list *n;
+
+	if (has_sha1_file(object_sha1))
+		return 0; /* nothing to do for this note */
+
+	/* failed to find object => prune this note */
+	n = (struct note_delete_list *) xmalloc(sizeof(*n));
+	n->next = *l;
+	n->sha1 = object_sha1;
+	*l = n;
+	return 0;
+}
+
 int combine_notes_concatenate(unsigned char *cur_sha1,
 		const unsigned char *new_sha1)
 {
@@ -922,6 +945,22 @@ int write_notes_tree(struct notes_tree *t, unsigned char *result)
 	return ret;
 }
 
+void prune_notes(struct notes_tree *t)
+{
+	struct note_delete_list *l = NULL;
+
+	if (!t)
+		t = &default_notes_tree;
+	assert(t->initialized);
+
+	for_each_note(t, 0, prune_notes_helper, &l);
+
+	while (l) {
+		remove_note(t, l->sha1);
+		l = l->next;
+	}
+}
+
 void free_notes(struct notes_tree *t)
 {
 	if (!t)

notes.h

@@ -161,6 +161,18 @@ int for_each_note(struct notes_tree *t, int flags, each_note_fn fn,
  */
 int write_notes_tree(struct notes_tree *t, unsigned char *result);
 
+/*
+ * Remove all notes annotating non-existing objects from the given notes tree
+ *
+ * All notes in the given notes_tree that are associated with objects that no
+ * longer exist in the database, are removed from the notes tree.
+ *
+ * IMPORTANT: The changes made by prune_notes() to the given notes_tree
+ * structure are not persistent until a subsequent call to write_notes_tree()
+ * returns zero.
+ */
+void prune_notes(struct notes_tree *t);
+
 /*
  * Free (and de-initialize) the given notes_tree structure
  *