commit-graph: fix corruption during generation v2 upgrade

This brief series resolves a bug where a commit-graph would become corrupt when upgrading from generation number v1 to v2, as originally reported in [1].

Some speculation occurred as to what might be causing that bug in the thread beginning at [1], until the problem was explained in more detail by Will Chandler in [2].

The crux of the issue, as is described in [2] and [3], is that the commit_graph_data slab is reused for read and write operations involving the commit-graph, leading to situations where data computed in preparation of a write is clobbered by a read of existing data.

The first patch demonstrates the issue, and the second patch prepares to fix it by introducing a helper function. The crux of the issue is described and fixed in the third patch.

[1]: https://lore.kernel.org/git/[email protected]/
[2]: https://lore.kernel.org/git/[email protected]/
[3]: https://lore.kernel.org/git/[email protected]/

Taylor Blau (3):
t5318: demonstrate commit-graph generation v2 corruption
commit-graph: introduce repo_find_commit_pos_in_graph()
commit-graph: fix corrupt upgrade from generation v1 to v2

 bloom.c                 | 10 +++++-----
 commit-graph.c          | 12 +++++++++---
 commit-graph.h          | 15 +++++++++++++++
 t/t5318-commit-graph.sh | 27 +++++++++++++++++++++++++++
 4 files changed, 56 insertions(+), 8 deletions(-)

2.37.0.1.g1379af2e9d