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From: Junio C Hamano <gitster@pobox.com>
To: "brian m. carlson" <sandals@crustytoothpaste.net>
Cc: "Martin Ågren" <martin.agren@gmail.com>,
	git@vger.kernel.org, "Derrick Stolee" <stolee@gmail.com>
Subject: Re: [PATCH 1/5] http-protocol.txt: document SHA-256 "want"/"have" format
Date: Fri, 14 Aug 2020 13:39:37 -0700	[thread overview]
Message-ID: <xmqqo8ndufli.fsf@gitster.c.googlers.com> (raw)
In-Reply-To: <20200814202347.GN8085@camp.crustytoothpaste.net> (brian m. carlson's message of "Fri, 14 Aug 2020 20:23:47 +0000")

"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.


  parent reply	other threads:[~2020-08-14 20:39 UTC|newest]

Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-13 22:48 [PATCH 0/2] Documentation updates for SHA-256 brian m. carlson
2020-08-13 22:49 ` [PATCH 1/2] docs: document SHA-256 pack and indices brian m. carlson
2020-08-14  2:26   ` Derrick Stolee
2020-08-13 22:49 ` [PATCH 2/2] docs: fix step in transition plan brian m. carlson
2020-08-14 12:21   ` Martin Ågren
2020-08-14  2:33 ` [PATCH 0/2] Documentation updates for SHA-256 Derrick Stolee
2020-08-14  4:47   ` Junio C Hamano
2020-08-14 20:20     ` brian m. carlson
2020-08-14 20:25       ` Junio C Hamano
2020-08-14 12:21 ` [PATCH 0/5] more SHA-256 documentation Martin Ågren
2020-08-14 12:21   ` [PATCH 1/5] http-protocol.txt: document SHA-256 "want"/"have" format Martin Ågren
2020-08-14 17:28     ` Junio C Hamano
2020-08-14 20:23       ` brian m. carlson
2020-08-14 20:32         ` Martin Ågren
2020-08-14 20:39         ` Junio C Hamano [this message]
2020-08-14 20:47           ` Junio C Hamano
2020-08-14 12:21   ` [PATCH 2/5] index-format.txt: document SHA-256 index format Martin Ågren
2020-08-14 12:28     ` Derrick Stolee
2020-08-14 14:05       ` Martin Ågren
2020-08-14 12:21   ` [PATCH 3/5] protocol-capabilities.txt: clarify "allow-x-sha1-in-want" re SHA-256 Martin Ågren
2020-08-14 12:31     ` Derrick Stolee
2020-08-14 14:05       ` Martin Ågren
2020-08-14 17:33     ` Junio C Hamano
2020-08-14 20:35       ` Martin Ågren
2020-08-14 20:43         ` Junio C Hamano
2020-08-14 12:21   ` [PATCH 4/5] shallow.txt: document SHA-256 shallow format Martin Ågren
2020-08-14 12:21   ` [PATCH 5/5] commit-graph-format.txt: fix "Hash Version" description Martin Ågren
2020-08-14 12:37     ` Derrick Stolee
2020-08-14 14:10       ` Martin Ågren
2020-08-14 20:28   ` [PATCH 0/5] more SHA-256 documentation brian m. carlson
2020-08-15 16:05   ` [PATCH v2 0/4] " Martin Ågren
2020-08-15 16:05     ` [PATCH v2 1/4] http-protocol.txt: document SHA-256 "want"/"have" format Martin Ågren
2020-08-15 16:06     ` [PATCH v2 2/4] index-format.txt: document SHA-256 index format Martin Ågren
2020-08-15 16:06     ` [PATCH v2 3/4] protocol-capabilities.txt: clarify "allow-x-sha1-in-want" re SHA-256 Martin Ågren
2020-08-15 16:06     ` [PATCH v2 4/4] shallow.txt: document SHA-256 shallow format Martin Ågren

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