docs: public final void registerSession(Session session, EventType event) {
  		if (session.isClosed() && event != EventType.UNREGISTER) {
  			return;
  		}
  		Reactor reactor = (Reactor) session.getAttribute(REACTOR_ATTRIBUTE);
  
  		if (reactor == null) {
  			reactor = nextReactor();
  			final Reactor oldReactor = (Reactor) session.setAttributeIfAbsent(
  					REACTOR_ATTRIBUTE, reactor);
  			if (oldReactor != null) {
  				reactor = oldReactor;
  			}
  		}
  		reactor.registerSession(session, event);
  	}