[PATCH v2 2/3] pack-bitmap.c: avoid uninitialized `pack_int_id` during reuse

When performing multi-pack reuse, reuse_partial_packfile_from_bitmap() is responsible for generating an array of bitmapped_pack structs from which to perform reuse.

In the multi-pack case, we loop over the MIDXs packs and copy the result of calling `nth_bitmapped_pack()` to construct the list of reusable paths.

But we may also want to do pack-reuse over a single pack, either because we only had one pack to perform reuse over (in the case of single-pack bitmaps), or because we explicitly asked to do single pack reuse even with a MIDX[^1].

When this is the case, the array we generate of reusable packs contains only a single element, which is either (a) the pack attached to the single-pack bitmap, or (b) the MIDX's preferred pack.

In 795006fff4 (pack-bitmap: gracefully handle missing BTMP chunks, 2024-04-15), we refactored the reuse_partial_packfile_from_bitmap() function and stopped assigning the pack_int_id field when reusing only the MIDX's preferred pack. This results in an uninitialized read down in try_partial_reuse() like so:

    ==7474==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x55c5cd191dde in try_partial_reuse pack-bitmap.c:1887:8
    #1 0x55c5cd191dde in reuse_partial_packfile_from_bitmap_1 pack-bitmap.c:2001:8
    #2 0x55c5cd191dde in reuse_partial_packfile_from_bitmap pack-bitmap.c:2105:3
    #3 0x55c5cce0bd0e in get_object_list_from_bitmap builtin/pack-objects.c:4043:3
    #4 0x55c5cce0bd0e in get_object_list builtin/pack-objects.c:4156:27
    #5 0x55c5cce0bd0e in cmd_pack_objects builtin/pack-objects.c:4596:3
    #6 0x55c5ccc8fac8 in run_builtin git.c:474:11

which happens when try_partial_reuse() tries to call midx_pair_to_pack_pos() when it tries to reject cross-pack deltas.

Avoid the uninitialized read by ensuring that the pack_int_id field is set in the single-pack reuse case by setting it to either the MIDX preferred pack's pack_int_id, or '0', in the case of single-pack bitmaps. In the latter case, we never read the pack_int_id field, so the choice of '0' is arbitrary.

Guard against further regressions in this area by adding a test which ensures that we do not throw out deltas from the preferred pack as "cross-pack" due to an uninitialized pack_int_id.

[^1]: This can happen for a couple of reasons, either because the
  repository is configured with 'pack.allowPackReuse=(true|single)', or
  because the MIDX was generated prior to the introduction of the BTMP
  chunk, which contains information necessary to perform multi-pack
  reuse.

Reported-by: Kyle Lippincott <spectral@google.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 pack-bitmap.c               | 10 ++++++++++
 t/t5332-multi-pack-reuse.sh | 26 ++++++++++++++++++++++++++
 2 files changed, 36 insertions(+)