[0/2] pack-objects: fix a pair of MIDX bitmap-related races

Message

Taylor Blau May 13, 2022, 4:23 p.m. UTC

This series fixes two races related to using multi-pack bitmaps in pack-objects.

The crux of both is that we call `is_pack_valid()` far too late, leaving us in a
situation where `pack-objects` committed to using objects from a specific pack
in the MIDX bitmap, without having actually opened those packs. So if those
packs are removed in the background (e.g., due to a simultaneous repack),
any ongoing clones or fetches will see this error message:

    remote: Enumerating objects: 1498802, done.
    remote: fatal: packfile ./objects/pack/pack-$HASH.pack cannot be accessed
    remote: aborting due to possible repository corruption on the remote side.

See 4c08018204 (pack-objects: protect against disappearing packs, 2011-10-14)
and bab919bd44 (pack-bitmap: check pack validity when opening bitmap,
2015-03-26) for previous treatment of similar issues.

Taylor Blau (2):
  pack-bitmap: check preferred pack validity when opening MIDX bitmap
  builtin/pack-objects.c: ensure pack validity from MIDX bitmap objects

 builtin/pack-objects.c |  8 ++++++--
 pack-bitmap.c          | 19 +++++++++++++++++--
 2 files changed, 23 insertions(+), 4 deletions(-)