after scratching my head for a long time to make systemd behave when
running jack as a service, I thought I'd share my findings:
Example service file:
Description=JACK Audio Connection Kit
So it's easy to start a service as a user, but that user's environment
is not automatically pulled in. That means even though user nettings
does have the necessary permissions in /etc/security/limits.conf, you
still need to spell them out for systemd.
Another thing to note is that at least on current raspbian, the
time-sync.target is broken. Its promise is to only be triggered after
ntp has worked its magic and the system time is monotonous. However,
I've seen time jumps which combined with cheap USB hardware or the
builtin Raspi sound will hang jack. Hence the ugly sleep.
Final goodness: systemd might try to clean up after you. So when you
have a jackd service started in your name, all is well, until you have
logged in somehow and then closed the last shell, at which point the
janitor from hell will happily clear /dev/shm and with it all your jack
semaphores, leading to instant dumpster fire and silence.
This magic can be avoided by setting
After clearing these hurdles, I've been quite happy with how systemd
handles jack clients. The usecase here is a number of clients running
in an embedded appliance, and systemd takes care of restoring them when
I cause them to crash, which is good. Not that they crash, but it's good
to have defense in depth.
Figuring out how to order services is hard because you cannot assume a
service is really ready when systemd has fired it off. So I've used a
ExecStartPre script that blocks until a downstream jack port has
appeared in those cases where I want to start clients in a particular
order (matters most for the one job that needs to connect all the ports,
so I let it wait on a test port for each client it needs to connect).
Old-time SysV user here, and granted, systemd has given me many a WTF
moment, but it does its job. Unfortunately, it does too many other jobs
as well, but hey...
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