[PATCH 0/2] Objects treated as missing despite being present, due to race with geometric repacking

When an object is found in multiple packs that are in a multi-pack-index,
and a subsequent geometric repacking creates a new multi-pack-index and
removes the pack that was considered the owner of the object in the old
multi-pack-index, then an already-running process that had opened the old
multi-pack-index and hadn't yet opened the removed packfile will not be able
to access the object -- lookups will return it as missing. Additionally,
replay has a separate bug where a missing object causes a SIGSEGV rather
than an error message.

This appears to affect a very small percentage of git operations in
production since it is a tiny window, but I've found evidence of it
occurring in at least eight distinct server-side operations, covering seven
different git commands:

git operation                        symptom
-----------------------------------  -----------------------------
git replay (server-side rebase)      SIGSEGV (this series, 1/2)
git merge-tree                       spurious read-miss failure
git diff (raw and tree-vs-tree)      spurious read-miss failure
git rev-list --count                 spurious read-miss failure
git merge-base                       spurious read-miss failure
object/rev resolution (rev-parse,    spurious read-miss failure
  cat-file)
repository repair (fsck/repack)      spurious read-miss failure


There are also commands that could be changing behavior without throwing an
error -- e.g. object negotiation thinking an object doesn't exist and
instead negotiating based on an older common commit, or cat-file --batch
reporting that some objects don't exist.

This series fixes the replay bug first, since it's simpler; investigating
it, together with my other recent repacking work, is what led me to the
underlying multi-pack-index issue that 2/2 addresses.

Elijah Newren (2):
  replay: fail gracefully when a merge input is unreadable
  packfile: recover when a multi-pack-index names a removed pack

 odb/source-packed.c         | 29 +++++++++++++++++++++++++++
 replay.c                    |  7 +++++++
 t/t3650-replay-basics.sh    | 35 ++++++++++++++++++++++++++++++++
 t/t5319-multi-pack-index.sh | 40 +++++++++++++++++++++++++++++++++++++
 4 files changed, 111 insertions(+)


base-commit: 18e66859d87fb4b76599f73460b54f0848c76b16
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2207%2Fnewren%2Fmidx-removed-pack-recovery-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2207/newren/midx-removed-pack-recovery-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/2207
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