Re: [PATCH 1/3] show-index: implement automatic hash detection

From: Patrick Steinhardt <ps@pks.im>
Date: Jan 21, 2026, 08:09 UTC
Message-ID: <aXCJp_rGPetsXE8J@pks.im>
In-Reply-To: <xmqqzf68yx75.fsf@gitster.g>

On Tue, Jan 20, 2026 at 10:07:42AM -0800, Junio C Hamano wrote:

> Shreyansh Paliwal <shreyanshpaliwalcmsmn@gmail.com> writes:
> > @@ -71,6 +60,40 @@ int cmd_show_index(int argc,

[snip]

> By the way, what happens if we find SHA-256 also broken and end up
> choosing another hash function that is 256-bit wide in the next hash
> revamp?

Yeah, agreed. The index unfortunately does not carry sufficient info to clearly identify the hash function that is in use, and second-guessing via the hash length doesn't really seem like a sensible solution to me. If we cannot tell for sure what the hash is, then we should rather ask the user to specify the object format. And in fact we already do that, as we have the `--object-format=` option for git-show-index(1).

I think if we wanted to fix properly this we should rather introduce index v5 with a header that encodes the hash used by it. Like that we wouldn't have to guess anymore. Whether the hassle is worth it might be a different question though.

Patrick