Graceful shutdown request signal

[Alexander Zubkov]

Hello Maria,

Regarding restarts, I think the killer feature might be some sort of
restart, when bird execs a new binary, keeping all the file descriptors
open and its state somehow. So the new instance could transparently catch
up with all the running sessions, etc. It can serialize the internal state
somehow and then reinitialize it from that. But I'm afraid it would require
a great effort to implement something like that.

On Wed, Jun 21, 2023, 08:58 Maria Matejka via Bird-users <
bird-users at network.cz> wrote:

> 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.
>
>
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