Troubles with demuxStream called from docker.run

[q0rban]

I am trying to pass in a stdout and stderr array of streams from docker.run() in dockerode. Something like this:

    const stdout = new stream.PassThrough();
    const stderr = new stream.PassThrough();
    await docker.run(someimage, ['some', 'command'], [stdout, stderr]);

When I pass things in this way, nothing gets output to stdout or stderr streams. However, when I do the following, I do get the output I expect:

    const stdout = new stream.PassThrough();
    await docker.run(someimage, ['some', 'command'], stdout);

I've tracked this down to demuxStream in docker-modem, but I'm having trouble making sense of how it's supposed to work. In my testing, nextDataLength is always a very large integer that is much larger than the buffer.length, so the conditional statement that then writes to the streams is never called. I don't really understand buffers much, so I'm not sure if I'm just implementing something incorrectly, or if there is a bug.

I was wondering if it had something to do with endianess, as I am on an arm processor, but I had a coworker try on an intel processor and they had the same issue.

[q0rban]

Banging my head on this some more, I decided to hack the code in demuxStream, specifically changing this:

    } else {
      if (buffer.length >= nextDataLength) {
        var content = bufferSlice(nextDataLength);
        if (nextDataType === 1) {
          stdout.write(content);
        } else {
          stderr.write(content);
        }
        nextDataType = null;
        // It's possible we got a "data" that contains multiple messages
        // Process the next one
        processData();
      }
    }

to this:

    } else {
      var content = bufferSlice(nextDataLength);
      if (nextDataType === 1) {
        stdout.write(content);
      } else {
        stderr.write(content);
      }
      nextDataType = null;
      if (buffer.length >= nextDataLength) {
        // It's possible we got a "data" that contains multiple messages
        // Process the next one
        processData();
      }
    }

With this change, I actually received output in stderr! The output I received was this:

pen 'tugboat': no such pool\r\n

If I run the same command using docker run ... the stderr is:

cannot open 'tugboat': no such pool

So the first 8 characters are missing from the stream with my hack. Does this give any clues as to what might be wrong?