ProtectSystem= and other sandboxing options require a user namespace in
order to work as user units (the user manager does not run as root and
thus without a user namespace it is unable to perform mounts).
This reverts commit 552c30eae4.
It has caused various problems; for example, MPD wasn't able to write
the pid_file (which was already mitigated by commit a4e4217204).
And apparently, the socket file created in the same directory by
mpd.socket disappears when mpd.service (re)creates the directory. I
could not reproduce this problem with 247.3, but maybe this is a bug
in older systemd versions?
Until we figure out why this happens, let's remove the
RuntimeDirectory directive. A future MPD version may be launched as
regular user, not as root, which will eliminate one major problem with
RuntimeDirectory.
This way, MPD can reliably detect whether it was started as systemd
service, which is better than checking sd_booted(), which only checks
whether systemd manages all services, but still MPD could be started
manually.
On linux-rt, kernel IRQ threads are configured with priority=50, and
this change configures MPD somewhat below that priority, leaving some
room for other programs to be configured in between.
Closes https://github.com/MusicPlayerDaemon/MPD/issues/643
The user unit omits the "ProtectKernelModules" setting which fails
with modular kernels:
Failed at step CAPABILITIES spawning /usr/bin/mpd: Operation not permitted
It is unfortunate that systemd (version 232) is unable to reduce its
own capabilities, because this requires us to split system and user
units.
https://bugs.musicpd.org/view.php?id=4608
