[PATCH 0/4] midx: verify MIDX checksum before reusing

Here is a small series that came out of a corruption we noticed at GitHub. The corruption is described in the third patch, but the gist is this:

  - After writing a MIDX, we experience a disk failure that corrupts the
    contents of the repository's MIDX.

  - We later go to write another MIDX, notice that one is already there, and
    then blindly reuse its data without checking its validity, resulting in a
    MIDX that contains corrupt data, but whose trailing checksum matches the
    preceding contents.

This series checks the validity of the trailing checksum when reusing a MIDX while writing a new one, as well as a few other related fix-ups. That should be sufficient to avoid propagating corrupt data forward while writing a MIDX.

We could go further here and run 'git multi-pack-index verify' on the existing MIDX while writing a new one, but the point of this reuse is to avoid having to re-compute/verify the contents of an existing MIDX. So a more lightweight option is pursued here, but alternatives are discussed in 3/4.

Thanks in advance for your review.

Taylor Blau (4):
  csum-file: introduce checksum_valid()
  commit-graph: rewrite to use checksum_valid()
  midx: don't reuse corrupt MIDXs when writing
  midx: report checksum mismatches during 'verify'

 commit-graph.c              | 14 ++++++--------
 csum-file.c                 | 16 ++++++++++++++++
 csum-file.h                 |  3 +++
 midx.c                      | 13 +++++++++++++
 pack-check.c                | 11 +----------
 t/t5319-multi-pack-index.sh | 13 +++++++++++++
 6 files changed, 52 insertions(+), 18 deletions(-)