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