Re: [PATCH 1/5] http-protocol.txt: document SHA-256 "want"/"have" format

[Junio C Hamano <gitster@pobox.com>]

"brian m. carlson" <sandals@crustytoothpaste.net> writes:

> On 2020-08-14 at 17:28:27, Junio C Hamano wrote:
>> Martin Ågren <martin.agren@gmail.com> writes:
>> 
>> > Document that in SHA-1 repositories, we use SHA-1 for "want"s and
>> > "have"s, and in SHA-256 repositories, we use SHA-256.
>> 
>> Ehh, doesn't this directly contradict the transition plan of "on the
>> wire everything will use SHA-1 version for now?"
>
> SHA-256 repositories interoperate currently using SHA-256 object IDs.
> It was originally intended that we wouldn't update the protocol, but
> that leads to much of the testsuite failing since it's impossible to
> move objects from one place to another.
>
> If we wanted to be more pedantically correct and optimize for the
> future, we could say that the values use the format negotiated by the
> "object-format" protocol extension and SHA-1 otherwise.

Yup.  I think a reasonable evolution path is

    0) everything on the wire is SHA-1 and no local operation knows
       SHA-256 (i.e. a few releases ago)

    1) local operations are either SHA-1 or SHA-256 but not both.
       On the wire, only protocol for SHA-1 repositories are
       defined, so SHA-256 repositories cannot talk with anybody
       using any official protocol, but a "borked" SHA-1 protocol
       that naturally extends the object names width exists and
       SHA-256 repositories can interoperate with each other.  This
       will be a backward compatibility nightmare, as Git from
       SHA-256 repository that tries to talk to SHA-1 repository
       will fail but without grace (i.e. the current situation).

    2) on-the-wire protocol gains just one new capability to safely
       unleash SHA-256 repositories to talk to the wider world.  The
       "borked" SHA-1 protocol above will become official when the
       object-format=sha256 capability is negotiated by both ends.
       At this stage, SHA-256 repositories still cannot talk with
       SHA-1 repositories, but at least they can talk among
       themselves as long as they use new-enough version of Git that
       knows about the new capability.

    3) on-the-fly SHA-1 vs SHA-256 migration gets implemented.
       SHA-256 reposotories trying to talk to somebody else, after
       discovering that the other end lacks object-format=sha256
       capability, on-the-fly converts its SHA-256 objecst to SHA-1
       and vice versa.  Between SHA-256 repositories, the capability
       above in 2) will allow native conversation with SHA-256.

Reaching 3) may be a lot of work, but at least we should get to 2)
to be able to safely let SHA-256 repositories to talk to the outside
world (yes, I consider it OK for SHA-256 repositories talking among
themselves in a private setting in the current state, and it would
be a good milestone and also test towards the eventual goal of
reaching 3), and with much smaller effort.

Thanks.