Re: Question About Git V2 Protocol & SHA256

From: Jeff King <peff@peff.net>
Date: Sep 30, 2020, 00:46 UTC
Message-ID: <20200930004630.GA623061@coredump.intra.peff.net>
In-Reply-To: <20200929224356.GH1392312@camp.crustytoothpaste.net>

On Tue, Sep 29, 2020 at 10:43:56PM +0000, brian m. carlson wrote:

Show 18 quoted lines

> On 2020-09-29 at 22:13:11, Jeff King wrote:
> > On Tue, Sep 29, 2020 at 01:17:59AM +0000, Wu, Zhichen wrote:
> > > 2. I see v2 has a capability called “object-format” that provides SHA1
> > >    option. I’m wondering if that capability will be the only way for
> > >    client and server to start using SHA256? Or put it as another word,
> > >    will v2 protocol be the prerequisite of SHA256?
> > 
> > I think it would be impossible to handle object-format via v1, because
> > the v1 protocol writes the ref advertisement before any capabilities are
> > negotiated. So I think v1 must implicitly remain sha1-only (and a sha256
> > repository on the server side would need to either reject a v1 client,
> > or back-translate as it would for a v2 client which asks for sha1).
> 
> I don't think that's the case.  You can indeed use v1 with SHA-256, but
> if you have a SHA-1-only Git, it will choke because the object ID is
> longer than it expects.  If you want to negotiate the algorithm when we
> support both and the client can't deal with translating the initial ref
> advertisement, then yes, you'll need v2.

I agree that we _could_ just dump sha256 within a v1 protocol and wait for the client to choke. But that seems like an awfully lousy user experience. By contrast, assuming that the client wants sha1 means that either:

  - we can reject it with a sensible ERR message that tells the user
    what is going on

  - we can serve them by talking in terms of sha1 if we're willing to
    do the extra conversion work server-side (and/or have a cache of
    sha1-format objects)

The only thing we lose is that a recent client who understands sha256 wouldn't be able to contact us and do a sha256-over-v1 transaction. But why would they want to do so?

This is all hypothetical at this point, though, right? I tried finding details in the hash transition document, but protocol changes are listed as out of scope there. It does say that sha256 servers may just reject sha1 clients; but even so I'd prefer if we could do it with a nice message (i.e., my bullet one above).

My suggestion does also require that we have a v2 receive-pack protocol, which does not yet exist (but following the blueprint for fetch, I don't expect it to be a big deal).

-Peff