def _loopbackAsyncBody(server, serverToClient, client, clientToServer,
                       pumpPolicy):
    """
    Transfer bytes from the output queue of each protocol to the input of the other.

    @param server: The protocol instance representing the server-side of this
    connection.

    @param serverToClient: The L{_LoopbackQueue} holding the server's output.

    @param client: The protocol instance representing the client-side of this
    connection.

    @param clientToServer: The L{_LoopbackQueue} holding the client's output.

    @param pumpPolicy: See L{loopbackAsync}.

    @return: A L{Deferred} which fires when the connection has been closed and
        both sides have received notification of this.
    """
    def pump(source, q, target):
        sent = False
        if q:
            pumpPolicy(q, target)
            sent = True
        if sent and not q:
            # A write buffer has now been emptied.  Give any producer on that
            # side an opportunity to produce more data.
            source.transport._pollProducer()

        return sent

    while 1:
        disconnect = clientSent = serverSent = False

        # Deliver the data which has been written.
        serverSent = pump(server, serverToClient, client)
        clientSent = pump(client, clientToServer, server)

        if not clientSent and not serverSent:
            # Neither side wrote any data.  Wait for some new data to be added
            # before trying to do anything further.
            d = defer.Deferred()
            clientToServer._notificationDeferred = d
            serverToClient._notificationDeferred = d
            d.addCallback(
                _loopbackAsyncContinue,
                server, serverToClient, client, clientToServer, pumpPolicy)
            return d
        if serverToClient.disconnect:
            # The server wants to drop the connection.  Flush any remaining
            # data it has.
            disconnect = True
            pump(server, serverToClient, client)
        elif clientToServer.disconnect:
            # The client wants to drop the connection.  Flush any remaining
            # data it has.
            disconnect = True
            pump(client, clientToServer, server)
        if disconnect:
            # Someone wanted to disconnect, so okay, the connection is gone.
            server.connectionLost(failure.Failure(main.CONNECTION_DONE))
            client.connectionLost(failure.Failure(main.CONNECTION_DONE))
            return defer.succeed(None)

    Transfer bytes from the output queue of each protocol to the input of the other.

    @param server: The protocol instance representing the server-side of this
    connection.

    @param serverToClient: The L{_LoopbackQueue} holding the server's output.

    @param client: The protocol instance representing the client-side of this
    connection.

    @param clientToServer: The L{_LoopbackQueue} holding the client's output.

    @param pumpPolicy: See L{loopbackAsync}.

    @return: A L{Deferred} which fires when the connection has been closed and
        both sides have received notification of this.
    
stackoverflow/venv/lib/python3.6/site-packages/twisted/protocols/loopback.py