s6-rc as user service manager

[Ihor Antonov <ihor@antonovs.family>]

Kicking off another thread because it is slightly different from UX
related questions.

I am trying to get s6-rc set up as a user service manager (similar how
systemd allows user's to manage their own services with systemctl --user
start bla). 
This is useful for example for starting user's dbus, pipewire,
xdg-desktop-portal services, and other stuff that is strictly user
related.

This usecase is geared towards a desktop/laptop.

There are number of challengers that I am facing with this task and I am
not sure how to best solve. I will try to explain those as a set of
scenarios as I amnot yet sure how make it more systematic:

# Scenario 1

It is clear that in the very beginning we have to start svscan on an
empty dir, then run s6-rc-init on it. Lets's assume svscan dir is tmpfs 
and is empty when machine boots. User's shell is configured in
such way that a new login shell spawns s6-svscan on a scan dir and then
runs s6-rc-init. Then necessary services can be started.

User starts their computer,logs in, uses s6-rc to start services
and everything is fine. (or is it?)


# Scenario 2

User logs out their session. Svscan process dies with
the login shell that spawned it. All services supervised by svscan die
too.
If machine is shutting down then we don't care, but it would've been
nice to shut down serices in reverse dependency order.

But machine is not shutting down and user logs in again.
The Scandir is not empty, it contains 
all the links to services that were previously orchestrated with s6-rc.
Login shell spawns svscan process on a scandir and all services start up
immediately, out of order, and therfore some of them fail. Oneshot
service that were intdended to be "once per boot" may run again too.

This is a problem.

# Scenario 3 

A user is logged in on tty1, presses CTRL-ALT-F2 and switches to tty2
and logs in again. A login shell tries to run svscan on a scandir,
svscan fail with error that another process is already running there.
Same happens with s6-rc-init. 

There is s6-svok that tests if a service dir is supervised, but there 
are no similar test tools for scandir and live dir. 
Login shell can ignore errors, but this is bad practice since an error
might happen in other legitimate situation.


# Scenario 4

User has 2 login shells available, tty1 and tty2. User logs out of tty1
and svscan process spawned by tty1 login shell dies, leaving session on
tty2 without neccessary services.



# Problems summary

- Login shell doesn't seem like a good place for user's service manager.
  What is a good place then?

- How to solve "Run service manager once per user"? Such that service
  manager is spawned with the first login and exits correctly with the
  last login?

- Is the amount of login shells a good measure for user's activity? Are
  the other cases when service manager is needed on a desktop machine?
  Like when we have 0 login shells but we need user's service manager?

- Minor: a test utility for svscan dir would be nice
- Minor: a test utility for live dir would be nice
  Checking lock files from shell is not fun and not reliable too