What is the correct way to run fluidsynth as a service on multi-seat system?

[crusader-mike]

I am using Debian-based distro (Pop!_OS 22.04 LTS). fluidsynth v2.2.5 is prepackaged. I want fluidsynth to run as a service (so that I don't need to start it manually every time I want to play a MIDI-using game). Wrt services package (only) includes /usr/lib/systemd/user/fluidsynth.service file which (in addition to some online discussions) suggests that I am supposed to use --user services.

So:

systemctl --user enable -now fluidsynth

everything works fine. Reboot, login:

journalctl --user -e -u fluidsynth
...
Nov 22 14:52:05 numenor systemd[2394]: Starting FluidSynth Daemon...
Nov 22 14:52:06 numenor fluidsynth[2404]: ALSA lib seq_hw.c:466:(snd_seq_hw_open) open /dev/snd/seq failed: Permission denied
Nov 22 14:52:06 numenor fluidsynth[2404]: fluidsynth: error: Error opening ALSA sequencer
Nov 22 14:52:06 numenor fluidsynth[2404]: Failed to create the MIDI thread; no MIDI input
Nov 22 14:52:06 numenor fluidsynth[2404]: will be available. You can access the synthesizer
Nov 22 14:52:06 numenor fluidsynth[2404]: through the console.
Nov 22 14:52:06 numenor systemd[2394]: Started FluidSynth Daemon.
Nov 22 14:52:06 numenor fluidsynth[2404]: fluidsynth: Using PulseAudio driver
Nov 22 14:52:06 numenor fluidsynth[2404]: fluidsynth: warning: Failed to set thread to high priority

why we can't access /dev/snd/seq? getfacl /dev/snd/seq shows we have explicit entry for my user granting access. Stop (currently-non-working) service, start it again:

systemctl --user restart fluidsynth

everything works fine. So, after I finished logging in -- service starts just fine.

After digging in this a bit I found a workaround -- adding my user to audio group: sudo usermod -a -G audio $(whoami). But apparently this is bad: https://wiki.ubuntu.com/Audio/TheAudioGroup

According to this logind at some point during creation of active login session modifies ACLs of /dev/snd/* devices to permit access for active user. I still don't know if fluidsync fails because it gets started too early or because it gets started in non-active session by systemd (probably former because manually starting it works).

What is the correct way to do it?

Also, lets say I'll get it to work somehow. What will happen if I go back to login screen and login as another user (that has fluidsynth service enabled) -- we will have two fluidsync instances. How will this work -- logind will change ACLs of sound devices, what it means for fluidsynth that already has sound device open? How is this supposed to work?

[crusader-mike]

Basically, I need to be able to tell systemd that given service has to wait for udev to finish executing /lib/udev/rules.d/73-seat-late.rules....

[crusader-mike]

For future reference:

(improper) workaround for failure to start on login:
	sudo usermod -a -G audio $(whoami)

workaround for inability to stop:
	manually install fluidsynth 2.3.1 (anything >= 2.2.7)

workaround for "Read-only file system" error:
	systemctl edit --user fluidsynth
		override ProtectHome -> no
	systemctl --user daemon-reload

systemctl --user enable -now fluidsynth
journalctl --user -e -u fluidsynth
systemctl --user status fluidsynth

places of interest:
	/usr/lib/systemd/user/fluidsynth.service
	/etc/default/fluidsynth
	~/.config/fluidsynth

login udev rules (that needs to complete before service start):
	/lib/udev/rules.d/70-uaccess.rules
	/lib/udev/rules.d/73-seat-late.rules