Re: [PATCH 07/15] commit-graph: new filter ver. that fixes murmur3

From: Junio C Hamano <gitster@pobox.com>
Date: Oct 9, 2023, 19:52 UTC
Message-ID: <xmqqr0m3y3o2.fsf@gitster.g>
In-Reply-To: <ZSRVJ3iASRaDGc80@nand.local>

Taylor Blau <me@ttaylorr.com> writes:

Show 15 quoted lines

> However, I am not entirely sure I agree with you that this is a "new"
> issue. At least in the sense that Git (on 'master') does not currently
> know how to deal with Bloom filters that have different settings across
> commit-graph layers.
>
> IOW, you could produce this problem today using the test you wrote in
> <20201015132147.GB24954@szeder.dev> using different values of the
> GIT_BLOOM_SETTINGS environment variables as a proxy for different values
> of the commitGraph.changedPathsVersion configuration variable introduced
> in this series.
>
> So I think that this series makes it easier to fall into that trap, but
> the trap itself is not new. I think a reasonable stopgap (which IIUC you
> have suggested earlier) is to prevent writing a new commit-graph layer
> with a different hash version than the previous layer.

What we probably want more urgently than that stopgap is to perhaps teach the code pretend as if commit-graph did not exist when we detect multiple layers use different hash versions (or perhaps only use the base layer and ignore the rest as an anti-pessimization), to protect correctness first, no?