Advertise hg client capabilities to hg server

Matt Mackall mpm at
Thu Jan 30 23:22:59 CST 2014

On Tue, 2014-01-28 at 12:37 +0400, Anton Gogolev wrote:
> Hi,
> I'm entertaining the idea of how useful would it be for hg server to know
> a thing or two about the client that is connecting to it.

To date, we've intentionally kept negotiation with the server very
one-sided. The server reports what it's capable of, the client makes all
the decisions. This is especially useful in situations like hgweb.cgi
where the server retains no state between requests. It also means
there's no question of a client/server mismatch as clients have to be
capable of talking to servers with zero capabilities.

> How can this be useful, you ask? For one, this would allow server to
> enforce a client to enable specific extensions (like projrc, which is
> useful in a corporate environment) or warn about outdated version or
> possible problems with repository on a local machine (when cloning a
> repository created on a case-sensitive file system to a Windows machine).
> This might introduce slight security problems with exposing a list of
> enabled extensions to the server, but this can be solved with a client-
> side configuration knob, which would default to existing, "anonymous"
> behavior.
> What do you think of that?

It's not a completely awful idea but I think it can still be done in a
server-tells-client fashion. For instance, the server can advertise that
it requires-extensions.hgeol or recommends-extensions.projrc and the
client can pass that info along to the user.

Mathematics is the supreme nostalgia of our time.

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