docs: protected boolean registerAllRequestsProcessedListener(NotificationListener listener) throws IOException {
  		checkNotNull(listener);
  
  		synchronized (listenerLock) {
  			if (allRequestsProcessedListener == null) {
  				// There was a race with the processing of the last outstanding request
  				if (requestsNotReturned.get() == 0) {
  					return false;
  				}
  
  				allRequestsProcessedListener = listener;
  
  				return true;
  			}
  		}
  
  		throw new IllegalStateException("Already subscribed.");
  	}