Comment 6 for bug 1641464

I agree that the charm shouldn't start the service if it is not configured, and should probably attempt to stop the service if it was previously started and is now missing relations or configuration etc.

I manually attempted to start/stop the openstack-nova-compute.service when unconfigured and as expected systemctl will hang, because the service definition contains a Timeout:

/etc/systemd/system/multi-user.target.wants/openstack-nova-compute.service

TimeoutStartSec=0

Which overrides the system wide defaults.

Changing this to another value (tested with 15, perhaps 60 seconds would be better) means the hooks will fire, and may error due to an unfinished configuration / incomplete relation(s) state but these will attempt to resolve themselves - making this (modifying timeout in service definition file) a convenient work-around until the charm is fixed.