def wait_fd_write(fd, timeout=None):
    """Suspends handling of the current request until there is nothing more
    to be written on file descriptor.

    May be called several times to add more file descriptors to the set to be watched.

    * http://uwsgi-docs.readthedocs.io/en/latest/Async.html#waiting-for-i-o

    :param int fd: File descriptor number.

    :param int timeout: Timeout. Default:  infinite.

    :rtype: bytes|str

    :raises IOError: If unable to read.
    """
