[Shepherd] Service monitoring agent

[Ludovic Courtès]

Hello Guix!

I recently pushed a Shepherd branch called ‘wip-service-monitor’:

  https://git.savannah.gnu.org/cgit/shepherd.git/log/?h=wip-service-monitor

It creates a “service monitoring agent”—code that runs in a separate
fiber and encapsulates the current state of services: registered,
running, etc.

Currently, state is managed directly within <service> objects, for
instance with the ‘running’ slot; this approach would replace that.  The
expected benefits are: improved clarity through separation of state from
config, and finer-grain view of service state.

The first visible application (and original motivation) is a
representation of services that are “being started”.  Currently, we can
only distinguish between “running” and “stopped” services; the
monitoring agent has been augmented to handle a “starting” state.  It
provides a synchronization mechanism that makes sure that (start S)
blocks until S has been started, in case it was already being started
(that’s the semantics we had pre-0.9 but 0.9 broke that.)

The service monitor is a fiber that answers requests on a dedicated
channel, similar to a client/server model with remote procedure calls
(RPCs).  That way, all accesses to service state are serialized.

At this point, there’s still state in <service>, such as respawn times,
that we could move to the monitor.

Feedback welcome!

Ludo’.