[systemd-devel] Re-exec()ing services for 'systemctl restart' ?

[Daniel P. Berrange]

For libvirt, we (will soon) have a daemon (virtlockd) which maintains
exclusive fcntl() based locks on disk images/devices, on behalf of both
libvirtd and any running QEMU or LXC instances. This is a safety critical
daemon (hence separate from libvirtd), to the extent that if the daemon
stops / crashes, the entire host should be immediately fenced using a
kernel watchdog and/or hardware power control device.

We still want to be able to restart this daemon during RPM upgrades to
newer versions, but we can't use a normal stop+start sequence, because
that will loose locks for any active VMs. Thus the daemon has the ability
to re-exec() itself triggered by SIGUSR1, preserving its critical state.
I've read the manpages for .service, .exec, etc but I've not seen any
reference to changing config such that

  # systemctl restart virtdlockd.service

will simply send SIGUSR1 to the process, instead of stopping it and then
starting it again. Obviously I could make the RPM %post send SIGUSR1
directly and ignore systemctl, but that doesn't help admins who just
expect to use systemctl. So I want to know if there is a recommended
way to handle this kind of use case ?

Regards,
Daniel
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