Re: [PATCH v2 2/6] commit-graph: always parse before commit_graph_data_at()

Derrick Stolee wrote:

There is a subtle failure happening when computing corrected commit
dates with --split enabled. It requires a base layer needing the
generation_data_overflow chunk. Then, the next layer on top
erroneously thinks it needs an overflow chunk due to a bug leading
to recalculating all reachable generation numbers. The output of
the failure is

  BUG: commit-graph.c:1912: expected to write 8 bytes to
  chunk 47444f56, but wrote 0 instead

At Google, we're running into a commit-graph issue that appears to
have also arrived as part of this last week's rollout.  This one is a
bit worse --- it is reproducible for affected users and stops them
from being able to do day-to-day development:

  $ git pull
  remote: Finding sources: 100% (33/33)
  remote: Total 33 (delta 18), reused 33 (delta 18)
  Unpacking objects: 100% (33/33), 27.44 KiB | 460.00 KiB/s, done.
  From https://example.com/path/to/repo
     05ba0d775..279e4e6d0  master     -> origin/master
  BUG: commit-reach.c:64: bad generation skip     29e3 >      628 at 62abdabd1be00ebadbf73061ecf72b35042338e3
  error: merge died of signal 6

"git commit-graph verify" agrees that the generation numbers are wrong:

  $ git commit-graph verify
  commit-graph generation for commit 4290b2214cdf50263118322735347d151715a272 is 3 != 1586
  Verifying commits in commit graph: 100% (1/1), done.
  commit-graph generation for commit b6c73a8472c7cb503cce0668849150a4b4329230 is 1576 != 10724
  Verifying commits in commit graph: 100% (10/10), done.
  Verifying commits in commit graph: 100% (88/88), done.
  Verifying commits in commit graph: 100% (208/208), done.
  Verifying commits in commit graph: 100% (592/592), done.
  Verifying commits in commit graph: 100% (1567/1567), done.
  Verifying commits in commit graph: 100% (8358/8358), done.

We have some examples of repositories that were corrupted this way,
but we didn't catch them in the act of corruption --- it started
happening to several users with this release so we immediately rolled
back.

Questions:

- is this likely to be due to the same cause, or is it orthogonal?

- what is the recommended way to recover from this state?  "git fsck"
  shows the repositories to have no problems.  "git help commit-graph"
  doesn't show a command for users to use; is
  `rm -fr .git/objects/info/commit-graphs/` the recommended recovery
  command?

- is there configuration or a patch we can roll out to help affected
  users recover from this state?

Thanks,
Jonathan