[Swarm mode] Define signals used to stop containers for updates

[mvdstam]

I'm not sure, but I suspect docker does a docker stop (ie. sending a SIGTERM to a container, and then a SIGKILL after waiting 10 seconds) when performing "rolling updates" on a service. With some images, like [https://hub.docker.com/_/httpd/](those powered by apache2), this is actually a destructive (non-graceful) way to stop containers, since apache needs a SIGWINCH to actually shutdown in a graceful way.

It would be awesome if we could define a signal used to stop services when creating them, for example:

docker service create --stop-signal SIGWINCH --name my-apache-container httpd:latest

[thaJeztah]

Thanks for suggesting; docker service does not have all options that are present on docker run, but we're planning to add more options in future releases (although not all options will find their way). I added this issue to #25303, which is used to keep track on progress; you can subscribe to that issue to stay op to date.

[mvdstam]

Cheers @thaJeztah!

[pnickolov]

My $.02 - consider making the signal # to use for stopping a container/service part of the Dockerfile rather than being part of the command to create the container/service.

The Dockerfile sets the boundary and the termination signal expected of the installed software is part of the boundary, not of any particular instance. This way, the knowledge belongs where it is defined (i.e., with the software being encapsulated); it will travel together with the image, just like the exposed ports and command.

[vingrad]

+1

[thaJeztah]

I realised I didn't mention that here, but although you cannot override the stop-signal when creating a service, if an image should always be stopped with a specific stop-signal; you can define the stop-signal in the Dockerfile (using the STOPSIGNAL command), e.g.;

FROM httpd:2.2
STOPSIGNAL WINCH

Docker services will take that into account when stopping the container;

INFO[0120] Container 52aca76dd0c25e733f43121b4097103aae5f2dd125a36c083407d5b61eb8ae50
failed to exit within 10 seconds of signal 28 - using the force

[aluzzardi]

I think --stop-signal should make it into the ServiceSpec (/cc @ehazlett)

[ehazlett]

@aluzzardi Yes I agree. Here is a tracking issue to get feedback: #30634

[thaJeztah]

@ehazlett were already tracking missing options through #25303 😇

[ehazlett]

Oh well in that case... :)

[ehazlett]

Oops didn't mean to close :)

[yongtang]

Added a PR #30754 and a SwarmKit PR moby/swarmkit#1924 for --stop-signal.