[PATCH] notes: Fix note_tree_consolidate not to break the note_tree structure

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@glandium.org>
---
 notes.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)