Without linger, if you log in, a user manager (systemd --user if I'm not using the right terminology) will fire up for your session and exit after all of the processes have exited.
When linger is enabled, you're supposed to already have a user manager running, either at boot (which is a really suboptimal thing to happen), from one being started when linger was enabled (by someone else), or the one that was running for your login when you enabled linger.
However, if linger is enabled and your user manager exits for some reason, it will not be restarted and now you have no user manager. The only way I've found to get one back is to disable linger, log out, and log back in.
To reproduce, make sure your test user is logged out, linger is enabled for them, and their user manager is running. Have your test user log in (ssh is fine) and kill -9 -1, or just have root kill their user manager. Have them log back in. For me this gets the following:
root 13020 1.4 0.0 177728 9704 ? Ss 18:45 00:00:00 sshd: tester2 [priv]
tester2 13023 0.0 0.0 177728 4780 ? S 18:45 00:00:00 sshd: tester2@pts/2
tester2 13024 0.3 0.0 133428 4944 pts/2 Ss+ 18:45 00:00:00 -bash
(i.e. no user manager is running at all). /var/lib/systemd/linger has:
-rw-r--r--. 1 root root 0 Aug 30 18:43 tester2
And of course then you can't do systemd-run:
> systemd-run --user true
Failed to create bus connection: Connection refused
If at this point you enable linger (doesn't matter who does it), they still won't have a user manager. If they disable linger, log out and log back in, _then_ they will have a user manager, and can then enable linger and run jobs in scopes.
This makes linger rather less useful, since a user can easily get into a situation where they can't run a job in a different scope to bypass KillUserProcesses=yes. But of course, it's required in that situation, so there's no real way out....
What I don't quite understand is why lingering isn't an attribute on the scope, with the user manager exiting when no more lingering scopes exist, and of course have the manager always starting when the first user session is brought up (instead of at boot, which just causes problems when your home directories are on a network). I'm sure it was designed this way for a reason; I just haven't been able to understand it.
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