def future_add_done_callback(future, callback):
    """Arrange to call ``callback`` when ``future`` is complete.

    ``callback`` is invoked with one argument, the ``future``.

    If ``future`` is already done, ``callback`` is invoked immediately.
    This may differ from the behavior of ``Future.add_done_callback``,
    which makes no such guarantee.

    .. versionadded:: 5.0
    """
    if future.done():
        callback(future)
    else:
        future.add_done_callback(callback)
