bug#58149: Letting clients warn about old daemons

[Ludovic Courtès]

Hi,

(Cc: Mathieu O. + Maxim.)

Matthieu Haefele <matthieu.haefele@cnrs.fr> skribis:

> I worked intensively with guix this summer, I did not get any
> deprecation warnings, I am quite sure. And I still do not get any. So
> might be a good idea to check the deprecation machinery.
>
> (base) mhaefele@mdlspc113:~ $ guix shell python-numpy python coreutils
> (base) mhaefele@mdlspc113:~ $ echo $GUIX_ENVIRONMENT
> /gnu/store/cy8y10jfnbq5y2r16i13q04h1lii428a-profile

Indeed.  I’ve just realized that commit
f9c62b23cc88541756656b3ec602ce987828d906, which added that deprecation
warning, will actually only fire with daemons dating back from before
2018 (the date at which ‘PROTOCOL_VERSION’ was last updated in
‘worker-protocol.hh’).  Going back to this issue at hand, it won’t
report a daemon that lacks lzip support.

Mathieu, Maxim: I think we need a finer-grain mechanism here, or maybe a
new builtin that would let a client ask the daemon for supported
features.

Thoughts?

Ludo’.