systemd units should not be placed in /etc/systemd (cannot mask a service)

[Trent Lloyd]

snapd should not place generated unit files in /etc/systemd and instead place them in another location.

The systemd documentation notes under the "Table 1. Load path when running in system mode (--system)." section of https://www.freedesktop.org/software/systemd/man/systemd.unit.html#id-1.8.4

/usr/local/lib/systemd/system System units installed by the administrator
/usr/lib/systemd/system System units installed by the distribution package manager
run/systemd/generator Generated units with medium priority (see normal-dir in systemd.generator(7))

In practice this causes problems, for example "systemctl mask" does not work:
root@skc:~# systemctl mask snap.maas.supervisor.service
Failed to mask unit: File /etc/systemd/system/snap.maas.supervisor.service already exists.

There are 2 main solutions I envisage to this problem. Firstly it should just install them to the normal package manager location of /usr/lib/systemd/system. Given that snapd currently prefixes all units with "snap." that is not likely to cause a conflict with other system package managers.

Secondly a potentially better or alternative solution would be to harness the systemd.generator functionality which is natively designed to have "non-native" configuration converted to systemd units - and is automatically run on boot and daemon-reload (which is required to pickup generated config files anyway). And would store them in the above generator location instead.

Documentation: https://www.freedesktop.org/software/systemd/man/systemd.generator.html#

This was discovered when looking for a workaround to Bug #1868908 snap update causes disabled systemd service to start again