lock.unlock();
        }
    }

    @Nullable
    @Override
    public ResponseProducer selectResponseProducer(int id, HttpExchange exchange) {
        lock.lock();
        try {
            if (current != null) {
                ResponseProducer producer = current.selectResponseProducer(id, exchange);
                if (producer != null) {
                    return producer;
                }

     *
     * @return null when this handler is not expecting any further requests.
     */
    @Nullable
    ResponseProducer selectResponseProducer(int id, HttpExchange exchange);

    boolean expecting(HttpExchange exchange);

    /**

            }
            for (TrackingHttpHandler handler : handlers) {
                ResponseProducer responseProducer = handler.selectResponseProducer(id, httpExchange);
                if (responseProducer != null) {
                    return responseProducer;
                }
            }

        ResourceHandlerWrapper handler = selectPending(notReceived, path);
        return handler != null;
    }

    @Override
    public ResponseProducer selectResponseProducer(int id, HttpExchange exchange) {
        ResourceHandlerWrapper handler;
        lock.lock();
        try {
            if (notReceived.isEmpty()) {
                // barrier open, let it travel on