src/main/java/io/reactivex/internal/util/EndConsumerHelper.java
public static boolean setOnce(AtomicReference<Subscription> upstream, Subscription next, Class<?> subscriber) {
        ObjectHelper.requireNonNull(next, "next is null");
        if (!upstream.compareAndSet(null, next)) {
            next.cancel();
            if (upstream.get() != SubscriptionHelper.CANCELLED) {
                reportDoubleSubscription(subscriber);
            }
            return false;
        }
        return true;
    }
public static boolean setOnce(AtomicReference<Subscription> upstream, Subscription next, Class<?> subscriber) {
        ObjectHelper.requireNonNull(next, "next is null");
        if (!upstream.compareAndSet(null, next)) {
            next.cancel();
            if (upstream.get() != SubscriptionHelper.CANCELLED) {
                reportDoubleSubscription(subscriber);
            }
            return false;
        }
        return true;
    }
Atomically updates the target upstream AtomicReference from null to the non-null
next Subscription, otherwise cancels next and reports a ProtocolViolationException
if the AtomicReference doesn't contain the shared cancelled indicator.
@param upstream the target AtomicReference to update
@param next the Subscription to set on it atomically
@param subscriber the class of the consumer to have a personalized
error message if the upstream already contains a non-cancelled Subscription.
@return true if successful, false if the content of the AtomicReference was non null