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

Jeff King [off-list ref] writes:

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.

Ouch.  I see counting and allocationg is hard (I think I spotted a
bug in another area that comes from the same "count while filtering
and then allocate" pattern during this cycle).  Thanks for spotting.