[bug#32408] [PATCH shepherd] Allow replacement of services

[ludo@gnu.org (Ludovic Courtès)]

Hello!

Carlo Zancanaro <carlo@zancanaro.id.au> skribis:

> I've attached an updated patch. I couldn't think of any unwanted
> consequences, so I took your idea of making register-services handle
> most of the details of replacement. With my patch, something like
>>   herd eval root '(register-services (load "a.scm") (load
>> "b.scm"))'
> will deal with a conflict by either replacing the old service (if it's
> not running), arranging for the old service to be replaced when it's
> stopped, or raising an error. This seems like a sensible way for
> things to function.

Awesome.  Thus, the only thing we need to change in ‘guix system
reconfigure’ is to make the ‘register-services’ call unconditional
(currently it’s limited to services that are stopped.)

> Despite my desire to deal with the race condition, I haven't done
> anything about it in this patch. The modification of %services that
> was done in register-services was already racy, and I don't think this
> patch will make it worse. If it hasn't been a problem up until now,
> then I don't think this will make it a problem.

Yeah, sounds reasonable.

> From 9ec5c0000e9a45441417a6ee4138cdcbf1b1f2b2 Mon Sep 17 00:00:00 2001
> From: Carlo Zancanaro <carlo@zancanaro.id.au>
> Date: Thu, 9 Aug 2018 22:30:38 +1000
> Subject: [PATCH] service: Add a replacement slot for delayed service
>  replacement.
>
> * modules/shepherd/service.scm (<service>): Add replacement slot
> (replace-service): New procedure.
> (stop): Call replace-service after stopping a service.
> (register-services): Replace existing services where possible, setting the new
> replacement slot if they are currently running.
> * tests/replacement.sh: Add a test for it.
> * Makefile.am (TESTS): Add the new test.
> * doc/shepherd.texi (Slots of services): Document it.

Awesome, please push!  And sorry about the delay.

Thank you,
Ludo’.