Re: [PATCH v2 0/4] Add proto v2 archive command with HTTP support

From: Josh Steadmon <steadmon@google.com>
Date: Sep 27, 2018, 18:30 UTC
Message-ID: <20180927183036.GA55647@google.com>
In-Reply-To: <CAGZ79kaBvHwUUf0rXeAwBY-M+Oi9JjsQnLs4v3FAvx4a9ZRbSA@mail.gmail.com>

On 2018.09.27 11:20, Stefan Beller wrote:

Show 16 quoted lines

> On Wed, Sep 26, 2018 at 6:25 PM Josh Steadmon <steadmon@google.com> wrote:
> >
> > This is the second version of my series to add a new protocol v2 command
> > for archiving, with support for HTTP(S).
> >
> > NEEDSWORK: a server built with this series is not backwards-compatible
> > with clients that set GIT_PROTOCOL=version=2 or configure
> > protocol.version=2. The old client will unconditionally send "argument
> > ..." packet lines, which breaks the server's expectations of a
> > "command=archive" request,
> 
> So if an old client sets protocol to v2, it would only apply that
> protocol version
> to fetch, not archive, so it would start following a v0 conversation, but
> as the protocol version is set, it would be transmitted to the server.
> This sounds like a bug in the client?

Yeah, basically. We're telling the server we support v2, even if the specific operation we're trying to do doesn't have a v2 implementation on the client. So this is going to make it ugly to replace existing commands.

Show 11 quoted lines

> >  while the server's capability advertisement
> > in turn breaks the clients expectation of either an ACK or NACK.
> 
> Could a modern client send either another protocol version (3?)
> or a special capability along the protocol version ("fixed_archive")
> 
> > I've been discussing workarounds for this with Jonathan Nieder, but
> > please let me know if you have any suggestions for v3 of this series.
> 
> Care to open the discussion to the list? What are the different
> approaches, what are the pros/cons?

Jonathan suggested something along the lines of what you said above, adding a new field in GIT_PROTOCOL. So we'd send something like "version=2:archive_version=2" and have the server detect the latter.

I'm not sure if that's the best way to go about this since I'm not familiar with the version detection code for other parts of the system. I worry that it will lead us down the path of having to specify a version for every command that we eventually convert to protocol v2. On the other hand, I don't see any other way to work around this, at least in the archive case. We can't peek at the client's transmissions on the server, because v2 requires that the server speaks first...