Re: [PATCH] setup: default hash algo to GIT_HASH_DEFAULT

["brian m. carlson" <sandals@crustytoothpaste.net>]

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On 2026-11-08 at 04:42:56, Junio C Hamano wrote:
> "brian m. carlson" <sandals@crustytoothpaste.net> writes:
> 
> > When we have no other hash algorithm to choose, we set no hash
> > algorithm, which ends up with us defaulting to SHA-1.  Instead, default
> > to GIT_HASH_DEFAULT.  This means that we will choose SHA-256 in breaking
> > changes mode and in Git 3.0, which is what we want.
> >
> > Reported-by: Adam Dinwoodie <adam@dinwoodie.org>
> > Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
> > ---
> >  setup.c         |  2 ++
> >  t/t0001-init.sh | 16 ++++++++++++++++
> >  2 files changed, 18 insertions(+)
> 
> With Git 3.0, we promised that SHA-1/SHA-256 interop will not have
> to be ready, and that SHA-256 would become the default.  And this
> patch would be a good thing to have for the second promise.
> 
> But didn't we also discuss "empty/initial repository whose hash
> algorithm is undecided can take a push of either SHA-1 or SHA-256
> payload and that first push can decide and freeze the hash algorithm
> used in the repository" or something along that line?

We discussed that as a possibility, but it requires protocol changes and
nobody has stepped up to implement that.  I think Peff was thinking
about maybe implementing that, but I don't think anyone came to a
decision.

> I am wondering if we want to reserve 'HASH_UNKNOWN' for such a "was
> just created, not quite finished initializing yet" state.

I think we concluded that we'd need to initialize a repository to _some_
hash algorithm even if we had the limbo state because otherwise people
could `git init` and then work locally and we need to have some existing
hash algorithm for that case.  That would be true even with bare
repositories.

So I think the limbo functionality, if we implemented it, would be a
separate option (maybe `extensions.potentialObjectFormat`?) and this
patch should go in either way.
-- 
brian m. carlson (they/them)
Toronto, Ontario, CA
Speaking for myself only

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