The commit-graph file format has some shortcomings that were discussed
on-list:

 1. It doesn't use the 4-byte format ID from the_hash_algo.

 2. There is no way to change the reachability index from generation numbers
    to corrected commit date [1].

 3. The unused byte in the format could be used to signal the file is
    incremental, but current clients ignore the value even if it is
    non-zero.

This series adds a new version (2) to the commit-graph file. The fifth byte
already specified the file format, so existing clients will gracefully
respond to files with a different version number. The only real change now
is that the header takes 12 bytes instead of 8, due to using the 4-byte
format ID for the hash algorithm.

The new bytes reserved for the reachability index version and incremental
file formats are now expected to be equal to the defaults. When we update
these values to be flexible in the future, if a client understands
commit-graph v2 but not those new values, then it will fail gracefully.

This series is based on ab/commit-graph-write-progress and bc/sha-256.

Thanks, -Stolee

[1]
https://public-inbox.org/git/6367e30a-1b3a-4fe9-611b-d931f51effef@gmail.com/

Derrick Stolee (6):
  commit-graph: return with errors during write
  commit-graph: collapse parameters into flags
  commit-graph: create new version flags
  commit-graph: add --version=<n> option
  commit-graph: implement file format version 2
  commit-graph: test verifying a corrupt v2 header

 Documentation/git-commit-graph.txt            |   3 +
 .../technical/commit-graph-format.txt         |  26 ++-
 builtin/commit-graph.c                        |  43 +++--
 builtin/commit.c                              |   5 +-
 builtin/gc.c                                  |   7 +-
 commit-graph.c                                | 158 +++++++++++++-----
 commit-graph.h                                |  16 +-
 t/t5318-commit-graph.sh                       |  42 ++++-
 8 files changed, 233 insertions(+), 67 deletions(-)

base-commit: 91b3ce35eeb93be1f4406e25ccdc4ab983a8e5af

Submitted-As: https://public-inbox.org/git/pull.112.git.gitgitgadget@gmail.com