How to make systemd unit files work in all supported distros?

[Nathan Cutler <ncutler@suse.cz>]

In master we currently have an issue with the systemd unit files in that 
they contain hard-coded paths that are specific to RH/CentOS/Fedora. 
Other distros can and do have the executables in different places.

I opened a bug[1] for this, but before I go off trying to fix it I would 
like to solicit your feedback on the following possible approaches I 
came up with (and/or "turn me on" to a different approach):

(1) Rely on PATH. Basically, instead of running executables directly, 
run them by "/bin/sh -c", which summons the system PATH to help find the 
executables. See https://github.com/ceph/ceph/pull/6803 for an example 
of this.

(2) Set environment variables in /etc/sysconfig/ceph. Systemd has an 
EnvironmentFile directive which reads environment variables from a file. 
If this were pointed to /etc/sysconfig/ceph (as it is in several unit 
files already), we could set the distro-specific parts of the paths 
there and write the paths similar to how we do it in the spec file.

(3) Generate unit files at build time. In this approach, the unit files 
would exist in the source tree as templates (e.g. ceph-osd@.service.in) 
and these would get transformed into the "real" unit files at build 
time. The ceph-detect-init utility could be used to determine the distro 
and the Makefile logic would then fill in the templates as needed for 
the distro.

Of these three approaches, only (3) would seem to be general enough to 
work nicely for all distros. (For example, EnvironmentFile will need to 
be set to /etc/default/ceph on Debianesque systems and there is no 
obvious way to set *this* path as an environment variable.)

Any thoughts on this? Thanks in advance!

[1] http://tracker.ceph.com/issues/14687

-- 
Nathan Cutler
Software Engineer Distributed Storage
SUSE LINUX, s.r.o.
Tel.: +420 284 084 037