Re: git clone of empty repositories doesn't preserve hash

From: Junio C Hamano <gitster@pobox.com>
Date: Apr 5, 2023, 20:40 UTC
Message-ID: <xmqqa5zmukp5.fsf@gitster.g>
In-Reply-To: <20230405200153.GA525125@coredump.intra.peff.net>

Jeff King <peff@peff.net> writes:

Show 36 quoted lines

> Yeah, we send a special capability line in that case. If you do:
>
>   GIT_TRACE_PACKET=1 git clone a b
>
> you can see that upload-pack indicates that ls-refs understands the
> "unborn" capability:
>
>   packet:  upload-pack> version 2
>   packet:  upload-pack> agent=git/2.40.0.824.g7b678b1f643
>   packet:  upload-pack> ls-refs=unborn
>   packet:  upload-pack> fetch=shallow wait-for-done
>   packet:  upload-pack> server-option
>   packet:  upload-pack> object-format=sha256
>   packet:  upload-pack> object-info
>   packet:  upload-pack> 0000
>
> And then clone asks for it say "yes, I also understand unborn":
>
>   packet:        clone> command=ls-refs
>   packet:        clone> agent=git/2.40.0.824.g7b678b1f643
>   packet:        clone> object-format=sha256
>   packet:        clone> 0001
>   packet:        clone> peel
>   packet:        clone> symrefs
>   packet:        clone> unborn
>   packet:        clone> ref-prefix HEAD
>   packet:        clone> ref-prefix refs/heads/
>   packet:        clone> ref-prefix refs/tags/
>   packet:        clone> 0000
>
> And then upload-pack can send us the extra information:
>
>   packet:  upload-pack> unborn HEAD symref-target:refs/heads/maestro
>   packet:  upload-pack> 0000
>
> I think we'd need to do something similar here for object-format.

I guess we only need to touch "git clone" then. Without being asked, it advertsizes object-format=sha256 already, and when the maestro repository is prepared without --object-format=sha256, upload-pack advertises object-format=sha1 instead. So it probably is just the matter of capturing it and using it to populate the extensions.objectformat with an appropriate value.