private void doCleanupIfLastSubscription() {
            if (isClosed() && duplicator.unsubscribed == 0 && downstreamSubscriptions.isEmpty()) {
                // Because the duplicator is closed, we know that unsubscribed will not be incremented
                // anymore and are guaranteed that the last unsubscribed downstream will run this cleanup logic.
                state = State.ABORTED;
                doCancelUpstreamSubscription();
                signals.clear(null);
            }
        }
Returns the default {@link EventExecutor} which will be used when a user subscribes to a child
stream using {@link StreamMessage#subscribe(Subscriber, SubscriptionOption...)}.
core/src/main/java/com/linecorp/armeria/common/stream/DefaultStreamMessageDuplicator.java