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

[Lennart Poettering]

On Mon, 06.08.12 16:52, Daniel P. Berrange (berrange at redhat.com) wrote:

> 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 ?

This is fundamentally difficult to implement, simply because restarting
a service also means that the services binding to it need restarting
too. And the ordering of that gets impossible if the stop/start sequence
is "atomic" because it is done internally in the service, and cannot be
split into two steps that we can order freely against each other.

So, I fear we cannot really add this for you. As Kay suggested
"systemctl kill" is probably your best choice here, or maybe "systemctl
reload".

Lennart

-- 
Lennart Poettering - Red Hat, Inc.