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`docker run -a stdin` confusing behaviour #14528

Open gwynforthewyn opened 2 years ago

gwynforthewyn commented 2 years ago

File: engine/reference/commandline/run.md

Picking up from this slack thread https://dockercommunity.slack.com/archives/C7GKACWDV/p1649427806814209, the behaviour of docker run -a stdin seems is a bit confusing.

The current docker run docs show that the --interactive flag will "Keep STDIN open even if not attached". In order to attach STDIN without the --interactive flag, the docs show that --attach stdin is available.

The expectation was that docker run -a stdin -a stdout -a stderr busybox cat < foo.txt would be equivalent to docker run -i busybox cat < foo.txt. However, docker run without the -i flag but with stdin/stderr/stdout attached displays nothing to stdout.

After investigation, it looks historically like there was confusion about how --interactive would behave when -a stdin was not specified (https://github.com/moby/moby/pull/12401). There's some discussion across several issues (https://github.com/moby/moby/pull/14390, https://github.com/moby/moby/pull/14417) about whether there were use-cases for --interactive when stdin wasn't attached, with the conclusion that the right way to fix was to enforce that -a stdin is silently enforced whenever --interactive is specified.

My read here is that the initial use-case for the -a stdin flag was to pair with the --interactive flag (again, cf. this diff https://github.com/moby/moby/pull/12401/files), and that's always happening since PR 14417.

However, that leaves -a stdin now as a cli option without clear purpose. It appears to me that specifying docker run -a stdin -a stdout -a stderr busybox cat < foo.txt results in stdin being attached, then the handles are being detached before any output can happen. I expected the handles to be blocked open until after the command output is returned.

I'm not sure what the best docs fix here is; maybe -a stdin has use-cases that aren't obvious to me here? I'm happy to help answer questions as best I can to try and find a way to clarify the intended behaviour.

docker-robott commented 1 year ago

There hasn't been any activity on this issue for a long time. If the problem is still relevant, mark the issue as fresh with a /remove-lifecycle stale comment. If not, this issue will be closed in 14 days. This helps our maintainers focus on the active issues.

Prevent issues from auto-closing with a /lifecycle frozen comment.

/lifecycle stale

gwynforthewyn commented 1 year ago

/remove-lifecycle stale

gwynforthewyn commented 1 year ago

/lifecycle frozen

I'm keeping this one open because this really took a decent amount of investigation to understand, and I'm just a community member who worked with another community member trying to figure this behaviour out, which took a fair bit of time.

I'm really not sure what the right fix is, as I had said, but the confusing issue is there.