Re: [PATCH 0/3] commit-graph: fix corruption during generation v2 upgrade

On 7/12/2022 7:10 PM, Taylor Blau wrote:

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.

Thanks, all, for identifying the scenario that causes this breakage.
It was frustrating that we had reports of corruption without knowing
where it was coming from.

This solution looks like the best we can do right now, although it
hints that some refactoring would be good to distinguish between
"commits as read" and "commits to be written" during this upgrade
phase. Something to think about, but not to block this fix.

Thanks,
-Stolee