* <p>To avoid races between threads doing release and acquire, we transition to the final state
     * in two steps. One thread will successfully CAS from RUNNING to COMPLETING, that thread will
     * then set the result of the computation, and only then transition to COMPLETED, CANCELLED, or
     * INTERRUPTED.
     *
     * <p>We don't use the integer argument passed between acquire methods so we pass around a -1
     * everywhere.
     */