@Override
    public final CompletableFuture<Void> whenComplete() {
        return completionFuture;
    }
Sets the specified {@code subscription} for the current stream.

@return the {@link SubscriptionImpl} which is used in actual subscription. If it's not the specified
        {@code subscription}, it means the current stream is subscribed by other {@link Subscriber}
        or aborted.
core/src/main/java/com/linecorp/armeria/common/stream/CancellableStreamMessage.java