while (true) {
        Listener listener = future.listeners;
        if (update == listener) {
          return listener;
        }
        if (casListeners(future, listener, update)) {
          return listener;
        }
      }
    }

    /** Performs a GAS operation on the {@link #waiters} field. */
    @Override
    Waiter gasWaiters(AbstractFuture<?> future, Waiter update) {
      while (true) {

  // operation in the constructor.
  //
  // 1. In the listener that performs the callback. In this case it is fine since inputFuture is
  //    assigned prior to calling addListener, and addListener happens-before any invocation of the
  //    listener. Notably, this means that 'volatile' is unnecessary to make 'inputFuture' visible
  //    to the listener.
  //

      return ticker;
    }
    return recordsTime ? Ticker.systemTicker() : NULL_TICKER;
  }

  /**
   * Specifies a listener instance that caches should notify each time an entry is removed for any
   * {@linkplain RemovalCause reason}. Each cache created by this builder will invoke this listener
   * as part of the routine maintenance described in the class documentation above.
   *