Graceful shutdown request signal

[Maria Matejka]

Hello Alexander,

you're right, catching up with all the running session and restoring the 
full internal state would be a nightmare. I think it is doable but too 
difficult (and labour-costly) to get it right. Regarding the need for 
graceful restart support anyway, I don't see much of a good reason to 
prefer this over a regular graceful restart.

Maria

On 6/21/23 13:41, Alexander Zubkov wrote:
> 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.
>
-- 
Maria Matejka (she/her) | BIRD Team Leader | CZ.NIC, z.s.p.o.
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