src/main/java/io/reactivex/processors/MulticastProcessor.java
public void start() {
        if (SubscriptionHelper.setOnce(upstream, EmptySubscription.INSTANCE)) {
            queue = new SpscArrayQueue<T>(bufferSize);
        }
    }
public void start() {
        if (SubscriptionHelper.setOnce(upstream, EmptySubscription.INSTANCE)) {
            queue = new SpscArrayQueue<T>(bufferSize);
        }
    }
Initializes this Processor by setting an upstream Subscription that
ignores request amounts, uses a fixed buffer
and allows using the onXXX and offer methods
afterwards.