void fireComplete() throws IOException {
        final FutureCallback<Void> callback = callbackRef.getAndSet(null);
        if (callback != null) {
            callback.completed(null);
        }
    }
TODO: to be replaced by core functionality
httpclient5/src/main/java/org/apache/hc/client5/http/impl/compat/ClassicToAsyncResponseConsumer.java