How to return a concrete value and a promise when implementing a server

[edevil]

I wanted to implement the server side of a capnproto schema that has a method that returns 2 values. The first value is a capability that can be used to send some data, and the second value is a promise of something that depends on this received data.

Is this possible to do with this library?

Example schema:

interface EventDispatcher {
  dispatch @0 (metadata :Metadata) -> (data :ByteStream, response :Response);
}

interface ByteStream {
  write @0 (bytes :Data) -> stream;
  end @1 ();
}

I've tried using a PromiseFulfillerPair but if I return this no progress is made, and the rpc just stalls:

    class ByteStreamImpl(example_capnp.ByteStream.Server):
        def __init__(self, promise):
            super().__init__()
            self.data = b''
            self.promise = promise

        def write(self, bytes, _context, **kwargs):
            self.data += bytes
            return None

        def end(self, _context, **kwargs):
            self.promise.fulfill()

    class DispatcherImpl(example_capnp.EventDispatcher.Server):
        def dispatch(self, metadata, _context, **kwargs):

            p = capnp.PromiseFulfillerPair()
            bstream = ByteStreamImpl(p)

            def set_result(_res):
                response = example_capnp.Response()
                response.outcome = len(bstream.data)
                _context.results.response = response

            _context.results.data = bstream
            return p.promise.then(set_result)

Am I doing something wrong or is this just not possible?
Thanks.