src/main/java/io/reactivex/internal/util/EndConsumerHelper.java
public static boolean validate(Subscription upstream, Subscription next, Class<?> subscriber) {
        ObjectHelper.requireNonNull(next, "next is null");
        if (upstream != null) {
            next.cancel();
            if (upstream != SubscriptionHelper.CANCELLED) {
                reportDoubleSubscription(subscriber);
            }
            return false;
        }
        return true;
    }
public static boolean validate(Subscription upstream, Subscription next, Class<?> subscriber) {
        ObjectHelper.requireNonNull(next, "next is null");
        if (upstream != null) {
            next.cancel();
            if (upstream != SubscriptionHelper.CANCELLED) {
                reportDoubleSubscription(subscriber);
            }
            return false;
        }
        return true;
    }
Ensures that the upstream Subscription is null and returns true, otherwise
cancels the next Subscription and if the upstream is not the shared
cancelled instance, reports a ProtocolViolationException due to
multiple subscribe attempts.
@param upstream the upstream current value
@param next the Subscription to check for nullness and cancel if necessary
@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 upstream was non null