code: private void lockEvent(EventId eventId) {
  		Lockable<V> eventWrapper = sharedBuffer.getEvent(eventId);
  		checkState(
  			eventWrapper != null,
  			"Referring to non existent event with id %s",
  			eventId);
  		eventWrapper.lock();
  		sharedBuffer.upsertEvent(eventId, eventWrapper);
  	}
docs: Increases the reference counter for the given event so that it is not
  accidentally removed.
  
  @param eventId id of the entry