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

"brian m. carlson" [off-list ref] writes:

On 2020-08-14 at 17:28:27, Junio C Hamano wrote:

quoted

Martin Ågren [off-list ref] writes:

quoted

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.