Bug#992990: systemd: Does not clean up user session

[Ansgar]

On Thu, 2021-08-26 at 09:13 +0200, Michael Biebl wrote:
> In your case, dbus-daemon is *not* started as a user service, but as
> a 
> regular process within a login session, so there can be multiple
> ones.

This looks incorrect: dbus was started with `--address=systemd` and
`--systemd-activation`. So it looks like it was started via systemd as
a user service.

> systemd does have a facility `KillUserProcesses=yes` which kills all 
> processes of such a login session, when the user logs out.
> The upstream default is "yes" but in Debian we had complaints that
> this 
> killed processes like screen, so we patch it to "no" as default in
> Debian.

And with `KillUserProcesses=no` any process still left in the user's
login session will keep the login alive (in "closing" state) and the
user's `systemd --user` instance and thus dbus will be kept running.

Ansgar