Re: [PATCH 1/3] pack-objects: fix tree_depth and layer invariants

>
> Commit 108f530385 (pack-objects: move tree_depth into 'struct
> packing_data', 2018-08-16) dynamically manages a tree_depth array in
> packing_data that maintains one of these invariants:
>
>   1. tree_depth is NULL (i.e., the requested options don't require us to
>      track tree depths)
>
>   2. tree_depth is non-NULL and has as many entries as the "objects"
>      array
>
> We maintain (2) by:
>
>   a. When the objects array grows, grow tree_depth to the same size
>      (unless it's NULL, in which case we can leave it).
>
>   b. When a caller asks to set a depth via oe_set_tree_depth(), if
>      tree_depth is NULL we allocate it.
>
> But in (b), we use the number of stored objects, _not_ the allocated
> size of the objects array. So we can run into a situation like this:
>
>   1. packlist_alloc() needs to store the Nth object, so it grows the
>      objects array to M, where M > N.
>
>   2. oe_set_tree_depth() wants to store a depth, so it allocates an
>      array of length N. Now we've violated our invariant.
>
>   3. packlist_alloc() needs to store the N+1th object. But it _doesn't_
>      grow the objects array, since N <= M still holds. We try to assign
>      to tree_depth[N+1], which is out of bounds.