After a note is removed, note_tree_consolidate is called to eliminate some useless nodes. The typical case is that if you had an int_node with 2 PTR_TYPE_NOTEs in it, and remove one of them, then the PTR_TYPE_INTERNAL pointer in the parent tree can be replaced with the remaining PTR_TYPE_NOTE. This works fine when PTR_TYPE_NOTEs are involved, but falls flat when other types are involved. To put things in more practical terms, let's say we start from an empty notes tree, and add 3 notes: - one for a sha1 that starts with 424 - one for a sha1 that starts with 428 - one for a sha1 that starts with 4c To keep track of this, note_tree.root will have a PTR_TYPE_INTERNAL at a[4], pointing to an int_node*. In turn, that int_node* will have a PTR_TYPE_NOTE at a[0xc], pointing to the leaf_node* with the key and value, and a PTR_TYPE_INTERNAL at a[2], pointing to another int_node*. That other int_node* will have 2 PTR_TYPE_NOTE, one at a[4] and the other at a[8]. When looking for the note for the sha1 starting with 428, get_note() will recurse through (simplified) root.a[4].a[2].a[8]. Now, if we remove the note for the sha1 that starts with 4c, we're left with a int_node* with only one PTR_TYPE_INTERNAL entry in it. After note_tree_consolidate runs, root.a[4] now points to what used to be pointed at by root.a[4].a[2]. Which means looking up for the note for the sha1 starting with 428 now fails because there is nothing at root.a[4].a[2] anymore: there is only root.a[4].a[4] and root.a[4].a[8], which don't match the expected structure for the lookup. So if all there is left in an int_node* is a PTR_TYPE_INTERNAL pointer, we can't safely remove it. I think the same applies for PTR_TYPE_SUBTREE pointers. IOW, only PTR_TYPE_NOTEs are safe to be moved to the parent int_node*. This doesn't have a practical effect on git because all that happens after a remove_note is a write_notes_tree, which just iterates the entire note tree, but this affects anything using libgit.a that would try to do lookups after removing notes. Signed-off-by: Mike Hommey <mh@xxxxxxxxxxxx> --- notes.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/notes.c b/notes.c index 2bab961ac..542563b28 100644 --- a/notes.c +++ b/notes.c @@ -153,8 +153,8 @@ static struct leaf_node *note_tree_find(struct notes_tree *t, * How to consolidate an int_node: * If there are > 1 non-NULL entries, give up and return non-zero. * Otherwise replace the int_node at the given index in the given parent node - * with the only entry (or a NULL entry if no entries) from the given tree, - * and return 0. + * with the only NOTE entry (or a NULL entry if no entries) from the given + * tree, and return 0. */ static int note_tree_consolidate(struct int_node *tree, struct int_node *parent, unsigned char index) @@ -173,6 +173,8 @@ static int note_tree_consolidate(struct int_node *tree, } } + if (p && (GET_PTR_TYPE(p) != PTR_TYPE_NOTE)) + return -2; /* replace tree with p in parent[index] */ parent->a[index] = p; free(tree); -- 2.12.1.dirty