private void abort0(Throwable cause) {
            DownstreamSubscription<T> currentSubscription = subscription;
            if (currentSubscription != null) {
                currentSubscription.abort(cause);
                return;
            }

            final DownstreamSubscription<T> newSubscription = new DownstreamSubscription<>(
                    this, AbortingSubscriber.get(cause), processor, ImmediateEventExecutor.INSTANCE,
                    false, false);

            if (!subscribe0(newSubscription)) {
                currentSubscription = subscription;
                assert currentSubscription != null;
                currentSubscription.abort(cause);
            }
        }
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