pack-objects: fix tree_depth and layer invariants

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.

That doesn't happen in our test scripts, because the repositories they
use are so small, but it's easy to trigger by running:

  echo HEAD | git pack-objects --revs --delta-islands --stdout >/dev/null

in any reasonably-sized repo (like git.git).

We can fix it by always growing the array to match pack->nr_alloc, not
pack->nr_objects. Likewise for the "layer" array from fe0ac2fb7f
(pack-objects: move 'layer' into 'struct packing_data', 2018-08-16),
which has the same bug.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>