bug#22039: [PATCH] 'guix system reconfigure' must start/restart/stop services

[Carlo Zancanaro]

Hey Ludo’,

> From the POV of the Shepherd, services carry no semantics.

In Guix we have as much information as possible about the 
services. We should be know which services should be upgraded 
automatically, which ones we should prompt the user to upgrade, 
and which ones are never safe to upgrade. Maybe we could add a 
"restart-strategy" to the shepherd-service object?

> Thus I think it’s reasonable to print a message along the lines 
> of:
>
>   The following services were upgraded: …
>   Please run “herd restart SERVICE” to stop, upgrade, and 
>   restart services that were not automatically upgraded.
>
> WDYT?

The main reasons I'm not super happy with this are that it's not 
discoverable (which is bad for new users), and it requires 
interaction (so cannot be an unattended upgrade). In particular 
for discoverability, some of our services don't take advantage of 
the Shepherd's ability to have multiple "provision" values. For 
instance, I just have to know that to restart wicd I have to run 
"herd restart networking".

Maybe this should be a separate ticket. Replacing the services and 
printing a generic message will still be an improvement on what 
Guix currently does, and I don't want to hold that up just because 
I think we can do better.

Carlo