def _stream_output(process):
    """Stream the output of a process to stdout

    This function takes an existing process that will be polled for output. Only stdout
    will be polled and sent to sys.stdout.

    Args:
        process(subprocess.Popen): a process that has been started with
            stdout=PIPE and stderr=STDOUT

    Returns (int): process exit code
    """
    exit_code = None

    while exit_code is None:
        stdout = process.stdout.readline().decode("utf-8")
        sys.stdout.write(stdout)
        exit_code = process.poll()

    if exit_code != 0:
        raise RuntimeError("Process exited with code: %s" % exit_code)

    return exit_code