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

[Daniel P. Berrange]

On Mon, Aug 06, 2012 at 06:04:11PM +0200, Kay Sievers wrote:
> On Mon, Aug 6, 2012 at 5:52 PM, 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 ?
> 
> $ systemctl reload ... ?

I thought about reload, but using that to re-exec the daemon seemed
a little evil to me, since it was really for just reloading config
files.

> or with the signal speficied:
> 
> $ systemctl kill ...

True, I guess I could use that.

I'm infering from your response that there's no way to customize what
'restart' does, as you can do with stop/start/reload/etc.

Daniel
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