nixos/console: fix console setting reloading

[rnhmjoj]

Motivation for this change

Hope you enjoy long commit messages.

It's a dull and boring day, it's cold outside and I'm stuck at home: let
me tell you the story of systemd-vconsole-setup.

In the beginnings of NixOS1, systemd-vconsole-setup was a powerful
sysinit.target unit, installed and running at boot to set up fonts
keyboard layouts and even colors of the virtual consoles. If needed, the
service would also be restarted after a configuration change, consoles
were happy and everything was good, well, almost.

Since the service had no way to specify the dependency "ttys are ready",
modesetting could sometimes happen after systemd-vconsole-setup had
started, leaving the console in a broken state. So abbradar worked
around that by putting a systemd-udev-settle After=.

In the meanwhile, probably realizing their mistake, systemd added a
shiny udev rule to start the systemd-udev-settle at the right time2.
However, the rule bypassed systemd by directly running the binary
systemd-udev-settle, and the service - though still installed - fell
into disuse.

Two years would pass before a good samaritan, seeing the poor jobless
systemd-udev-settle service, decided to give it the coup de grâs3 by
unlisting it from the installed units.
This, combined with another bug, caused quite a commotion4 in NixOS;
to see why remember the fact that WantedBy= in upstream units doesn't
work5, so it had to be added manually in cc54211, but while systemd
removed it, the NixOS unit continued to install and restart the service,
making a lot of fuss when switching configuration.

After at least thee different tentative fixes, deedrah realised6 what
the root cause was and fpletz put the final nail7 in the coffin of
systemd-udev-settle. The service would never see the light of a boot
again, NixOS would not restart it all the time but thanks to udev
consoles would still get their pretty fonts and playful colors.

The En..

..no, wait! You should ask what came of systemd-udev-settle, first.
And why is the service even around if udev is doing all the work?

Udev-settle, like the deceitful snake that he is, laid hidden for years.
He looks innocuous doesn't it? A little hack. Only until it leaves his
den and a poor user8 drops dead. Obviously, it serves no purpose, as
the service is not part of the boot process anymore, so let's remove it
for good!

About the service, it may not be useful at boot, but it can be started
to pick up changes in vconsole.conf and set the consoles accordingly.
But wait, this doesn't work anymore: the service is never started at
boot (remember f76d2aa), so switch-to-configuration.pl will not restart
it. Fortunately it can be repaired: here I install a new unit which
does nothing on start, but restarts the real service when reloaded.
This perfectly reproduces the original behavior, hopefully without the
original bugs too.

The End?

Things done

• Tested using sandboxing (nix.useSandbox on NixOS)
• Tested console.font and console.packages trigger a reload
• Tested reloading doesn't mess with Xorg server
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[aanderse]

+1 for storytelling.

[rnhmjoj]

Just in case, I tested that reloading doesn't break the desktop sessions with gdm (which used to be a problem in the past).