Graceful shutdown request signal

[Maria Matejka]

Hello Daniel, 

On 21 June 2023 01:03:50 CEST, "Daniel Gröber" <dxld at darkboxed.org> wrote:
>Hi Erin,
>
>On Tue, Jun 20, 2023 at 08:20:50PM +0200, Erin Shepherd wrote:
>> I run bird on a system which uses systemd as a service supervisor, and
>> would like to implement graceful restart in a way which works well with
>> it.
>
>I'm also interested in getting this working. I'm wondering how graceful
>restart is supposed to behave to begin with though. Last time I just tried
>`birdc graceful restart` I was surprised that this actually makes bird exit
>with rv=0 instead of ... well actually restarting.
>
>Is that normal or a bug in the Debian packaging/systemd service?

You're supposed to start BIRD again on your terms. It's a misleading command wording, kind of. I don't know whether it's a good idea to actually exec from the shutdown cycle. Will think about it. It may be a nice feature.

>I suppose if you're supposed to use graceful restart just before rebooting
>the system it makes sense but at the time I just wanted to do a full
>restart to update the running bird executable without causing traffic
>disruption and was rather surprised when it didn't come back up.

Yes, these two different use cases were what we were choosing from when implementing this.


>> In particular, what I'd like to do is:
>>  • If I restart the bird service (e.g. for an upgrade), Bird performs a graceful restart
>>  • If I manually stop bird (systemctl stop bird2), shutdown the system, or any other action which is liable to cause a longer period of downtime, perform a graceful restart (for BGP at least)
>
>When you want to tickle special behaviour out of systemd I found it best to
>not try and do everything in one unit. Using the dependency system you can
>stop another unit before bird.service gets stopped, this can then call
>birdc graceful restart, like this:
>
>bird-graceful-restart.service:
>
>    [Unit]
>    After=bird.service
>    Requires=bird.service
>    [Service]
>    Type=oneshot
>    RemainAfterExit=yes
>    ExecStop=birdc graceful restart
>    [Install]
>    WantedBy=bird.service
>
>That way you can enable/disable system wide graceful bird restart by
>`systemctl enable bird-graceful-restart.service`.
>
>The BindsTo+After means stopping bird.service (which is part of restart)
>will first stop bird-graceful-restart.service, which can then casually
>signal bird to perform graceful restart without a race (I
>think). WanteBy=bird.service is needed as ExecStop will only run when a
>service has actually been started.
>
>Also some special handling for the case where bird is already gone (crashed
>or exited) and birdc would fail might be needed. Excercise for the reader :)
>
>Doing some quick testing this actually seems to do what I want. Somewhat
>unexpectedly `systemctl restart bird.servce` with the above enabled does
>then actually do what you'd expect since restart is just stop+start. Bird
>will exit on getting the graceful restart command so systemd can't try to
>stop it some more but the subsequent start will make it come back up. Neat.
>
>On system shutdown only the stop part will happen so bird exiting is
>actully useful now and that too should work as expected.

If there was somebody to implement this additional Systemd unit and provide it as a patch, we'd happily merge it.

Thank you for all the ideas!
Maria

-- 
Maria Matejka (she/her) | BIRD Team Leader | CZ.NIC, z.s.p.o.