Cuirass actors

[Ludovic Courtès]

Hello Guix!

My (useful? misguided? questionable?) quest around Cuirass has led me
closer to some of the goals mentioned in my previous message¹ and in the
‘TODO’ file².

The ‘wip-actors’ branch³, which I plan to merge soon, splits activities
of the ‘cuirass register’ process among several actors.  Quoth ‘base.scm’:

  - The "channel updater" is responsible for updating Git checkouts for
    channels.  There's a single instance of this actor; it limits
    concurrent Git updates.

  - The "evaluator" spawns evaluations of jobsets for the given channel
    instances, again limiting the number of concurrent evaluations.

  - The "builder" spawns derivation builds.  There are currently two
    implementations: the local builder sends build requests to the local
    'guix-daemon' process, while the remote build delegates builds to
    'cuirass remote-server'.

  - Each jobset as an associated "monitor"; it requests channel updates,
    evaluations, and builds to the actors above.  It also receives requests
    such as evaluation triggers that can come, for example, from the
    /jobset/NAME/hook/evaluate HTTP endpoint.

  - The "jobset" registry is a directory that maps jobset names to their
    monitor.

In addition, ‘cuirass register’ implements a “bridge”: it listens for
connections on a Unix-domain socket, which allows ‘cuirass web’ to send
it commands.

This is used for instance to implement the /jobset/NAME/hook/evaluate
HTTP endpoint, which lets users trigger an evaluation of the given
jobset.

With this actor split, one could implement another “builder” backend,
for instance one that talks to a Build Coordinator process.  It’s also
obviously close to the programming model encouraged by Goblins, which
should make eventual migration easier (and when that happens, if Goblins
provides an inter-process transport, we’ll no longer need the custom
“bridge” and we’ll be able to move actors from one process to another
much more easily.)

Feedback welcome!

Ludo’.

¹ https://lists.gnu.org/archive/html/guix-devel/2023-07/msg00096.html
² 
https://git.savannah.gnu.org/cgit/guix/guix-cuirass.git/tree/TODO?id=9b227abd29b15e7e25c54a71c524e7b26252a270
³ https://git.savannah.gnu.org/cgit/guix/guix-cuirass.git/log/?h=wip-actors