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