containerAttachStream's stderrEvent is emitted on wrong stream

[knaerzche]

I'm trying to capture demultiplexed stderr and stdout streams from a running container with following example code:

$container = await(
    $client->containerCreate(
        [
            'Image' => 'alpine',
            'Tty' => false,
            'Cmd' => [
                'sh',
                '-c',
                'i=0; while true; do echo "OUT $i"; echo "ERR $i" 1>&2; i=$((i+1)); sleep 1; done'
            ],
        ]
    )
);

$stream = $client
    ->containerAttachStream(
        $container['Id'],
        stream:true,
        stderrEvent: 'stderr',
    )
    ->on('data', function ($data) {
        echo 'stdout: ' . $data . "\n";
    })
    ->on('stderr', function ($data) {
        echo 'stderr: ' . $data . "\n";
    }
);


await(
    $client
        ->containerStart($container['Id']
    )
);

There's never anything logged for 'stderr' event and I'm not sure that ever worked.
The reason behind is that ReadableDemultiplexStream::containerAttachStream returns an UnwrapReadableStream:`

...
        // first inspect container to check TTY setting, then attach with appropriate log parser
        return \React\Promise\Stream\unwrapReadable($this->containerInspect($container)->then(function ($info) use ($url, $browser, $parser, $stderrEvent) {
            $stream = $parser->parsePlainStream($browser->requestStreaming('POST', $url));

            if (!$info['Config']['Tty']) {
                $stream = $parser->demultiplexStream($stream, $stderrEvent);
            }

            return $stream;
        }));
....

If I understand things correctly this is "only" a proxy stream, not the actual stream.
I understand you want to check Tty setting before using a ReadableDemultiplexStream. The problem is that the stream event listeners are subcribing to events of UnwrapReadableStream which in turn only emits (hardcoded) data, end, error and close events: https://github.com/reactphp/promise-stream/blob/f5fff26ec6ac4ea89168e49554a304bfedae1e7f/src/UnwrapReadableStream.php#L54 and there seems no way to add more.

One possible solution is to not use an promise, but await:

        // first inspect container to check TTY setting, then attach with appropriate log parser
        $info = await($this->containerInspect($container));
        $stream = $parser->parsePlainStream($browser->requestStreaming('POST', $url));

        if (!$info['Config']['Tty']) {
            $stream = $parser->demultiplexStream($stream, $stderrEvent);
        }

        return $stream;

But clue/reactphp-docker does currently not depend on react/async - so I'm not sure thats a correct fix for that issue.
I have no idea how to proper fix it or how to get the actual stream which emitts stderr events in my client code.