Proposal: allow docker attach for non-running containers

[aanand]

In Compose (specifically docker-compose run), instead of duplicating all of Docker's considerable client-side attach logic, we'd like to start shelling out to docker attach instead, so we have better parity across all platforms (especially Windows).

However, there's currently client-side code which prohibits a user from docker attach-ing to a non-running container. This is a showstopper: to avoid race conditions, the order of operations must be create, attach, start.

I'd like to either remove this logic or add a flag to skip it.

[thaJeztah]

ping @crosbymichael @mlaventure PTAL

[tonistiigi]

Switched labels as this is allowed in API already. Does this have to be cli flag or can't you use the client API? Public client.ContainerAttach() method doesn't have this limitation.

[aanand]

@tonistiigi We can't use the Go API, because Compose is written in Python! Shelling out to docker attach is the only way I can see to re-use Engine's client-side attach code from Compose.

[thaJeztah]

Perhaps @tonistiigi meant the /container/(id or name)/attach/ endpoint, which is used by client.ContainerAttach()? https://github.com/docker/docker/blob/3ea762b9f6ba256cf51bd2c35988f0c48bccf0b0/client/container_attach.go#L14-L34, there's also a web socket endpoint; https://docs.docker.com/engine/reference/api/docker_remote_api_v1.24/#/attach-to-a-container-websocket

[aanand]

Well, those API endpoints are what we're using right now. The problem is that replicating all of the interactive functionality of docker run/attach/exec still involves a great deal of client-side logic (some of which is detailed here).

All of that logic has been reimplemented in Python, which has inevitably become a huge time sink and a source of many bugs over the years. It's also currently POSIX-only, because attaching interactively on Windows involves a completely different set of system calls.

Accordingly, over time, we'd like to remove it and rely on the Docker client binary instead.

[thaJeztah]

Ah, clear. (I think ideally, we should have an improved API to make it easier to use)

[tonistiigi]

@aanand How about docker start -a, wouldn't that work for you?

[aanand]

@tonistiigi I didn't know about docker start -a! That will probably work, yes. I'll close this and reopen if it turns out it doesn't work for any reason. Thanks.