[PATCH 1/2] t5332-multi-pack-reuse.sh: demonstrate duplicate packing failure

In the multi-pack reuse code, there are two paths for reusing the on-disk representation of an object, handled by:

  - builtin/pack-objects.c::write_reused_pack_one()
  - builtin/pack-objects.c::write_reused_pack_verbatim()

The former is responsible for copying the bytes for a single object out of an existing source pack. The latter does the same but for a region of objects aligned at eword_t boundaries.

Demonstrate a bug whereby write_reused_pack_verbatim() can be tricked into writing out objects from some source pack, even when those objects were selected from a different source pack in the MIDX bitmap.

When the caller wants at least one of the objects in that region, pack-objects will write the same object twice as a result of this bug. In the other case where the caller doesn't want any of the objects in the region of interest, we will write out objects that weren't requested.

Demonstrate this bug by creating two packs, where the preferred one of those packs contains a single object which also appears in the main (non-preferred) pack. A separate bug[^1] prevents us from triggering the main bug when the duplicated object is the last one in the main pack, but any earlier object will suffice.

We could fix that separate bug, but the following commit will simplify write_reused_pack_verbatim() and only call it on the preferred pack, so doing so would have little point.

[^1]: Because write_reused_pack_verbatim() only reuses bits in the range

    off_t pack_start_off = pack_pos_to_offset(reuse_packfile->p, 0);
    off_t pack_end_off = pack_pos_to_offset(reuse_packfile->p,
                                            pos - reuse_packfile->bitmap_pos);

    written += pos - reuse_packfile->bitmap_pos;

    /* We're recording one chunk, not one object. */
    record_reused_object(pack_start_off,
                         pack_start_off - (hashfile_total(out) - pack_start));

  , or in other words excluding the object beginning at position 'pos -
  reuse_packfile->bitmap_pos' in the source pack. But since
  reuse_packfile->bitmap_pos is '1' in the non-preferred pack
  (accounting for the single-object pack which is preferred), we don't
  actually copy the bytes from the last object.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 t/t5332-multi-pack-reuse.sh | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)