Here is a bugfix for the recently-landed-in-next generation v2 topic
(ak/corrected-commit-date).

This was occasionally hitting us when computing new commit-graphs on
existing repositories with the new bits. It was very hard to reproduce, and
it turns out to be due to not parsing commits before accessing generation
number data. Doing so in the right place demonstrates the bug of recomputing
the corrected commit date even for commits in lower layers with computed
values.

The fix is split into these steps:

 1. Parse commits more often before accessing their data. (This allows the
    bug to be demonstrated in the test suite.)
 2. Check the full commit-graph chain for generation data chunks.
 3. Don't compute corrected commit dates if the lower layers do not support
    them.
 4. Parse the commit-graph file more often.

Thanks, -Stolee

Updates in v2
=============

 * Fixed some typos or other clarifications in commit messages.

 * The loop assigning read_generation_data is skipped if they already all
   agree with value 1.

 * I split compute_generation_numbers into two methods. This essentially
   splits the previous patch 4 into patches 4 & 5 here. The new patch 4 just
   splits the logic as-is, then the new patch 5 does the re-initialization
   of generation values when in the upgrade scenario.

Derrick Stolee (6):
  commit-graph: use repo_parse_commit
  commit-graph: always parse before commit_graph_data_at()
  commit-graph: validate layers for generation data
  commit-graph: compute generations separately
  commit-graph: be extra careful about mixed generations
  commit-graph: prepare commit graph

 commit-graph.c          | 138 +++++++++++++++++++++++++++++-----------
 commit.h                |   5 +-
 t/t5318-commit-graph.sh |  21 ++++++
 3 files changed, 125 insertions(+), 39 deletions(-)

base-commit: 5a3b130cad0d5c770f766e3af6d32b41766374c0

Submitted-As: https://lore.kernel.org/git/pull.850.v2.git.1612234883.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.850.git.1612199707.gitgitgadget@gmail.com