def engine(func):
    """Callback-oriented decorator for asynchronous generators.

    This is an older interface; for new code that does not need to be
    compatible with versions of Tornado older than 3.0 the
    `coroutine` decorator is recommended instead.

    This decorator is similar to `coroutine`, except it does not
    return a `.Future` and the ``callback`` argument is not treated
    specially.

    In most cases, functions decorated with `engine` should take
    a ``callback`` argument and invoke it with their result when
    they are finished.  One notable exception is the
    `~tornado.web.RequestHandler` :ref:`HTTP verb methods <verbs>`,
    which use ``self.finish()`` in place of a callback argument.
    """
    func = _make_coroutine_wrapper(func, replace_callback=False)

    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        future = func(*args, **kwargs)

        def final_callback(future):
            if future.result() is not None:
                raise ReturnValueIgnoredError(
                    "@gen.engine functions cannot return values: %r" %
                    (future.result(),))
        # The engine interface doesn't give us any way to return
        # errors but to raise them into the stack context.
        # Save the stack context here to use when the Future has resolved.
        future.add_done_callback(stack_context.wrap(final_callback))
    return wrapper