connector-proxy: close stdout when child stdout closes

[mdibaiee]

Description:

• Here is an attempt to ensure that as soon as the child process exits, we close stdout of connector-proxy
• However... this does not work. From my tests, I think the closing of the stdout is successful in connector-proxy, but my current understanding is that docker run does not propagate this closing of the stdout and keeps an open stdout even if the entrypoint closes its stdout. I tested with and without --init flag. Still investigating...

Workflow steps:

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Documentation links affected:

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[mdibaiee]

Okay, I can confirm the issue is in behaviour of docker run, here is a rundown of the issue:

You can test the behaviour of docker run using a busybox container, to see how it handles stdin:

The example below does not print "test":

echo "test" | docker run busybox "head"

I tried attaching stdin, but still no "test":

echo "test" | docker run --attach stdin --attach stdout busybox "head"

By reading the the code, it seems that stdin is actually not opened at all unless --interactive is specified, and --interactive is problematic, because it "Keeps STDIN open even if not attached". In practice though, it seems to keep stdout open as well.

This prints "test":

echo "test" | docker run --interactive busybox "head"

I verified this PR by running a pull RPC using cdk, and then going inside the container. I can see that the code is correctly replacing stdout file descriptor of the process with /dev/null:

docker/connector $ cd /proc/1/fd
/proc/1/fd $ ls -lha
lr-x------ 1 nonroot nonroot 64 Sep 15 15:08 0 -> 'pipe:[35608139]'
lr-x------ 1 nonroot nonroot 64 Sep 15 15:08 1 -> /dev/null

However stdout of docker run stays open...

This is yet another reason for us to switch to using a network socket for communicating with the connector instead of stdio. cc @jgraettinger