@Override
    final void request(long n) {
        final SubscriptionImpl downstreamSubscription = this.downstreamSubscription;
        assert downstreamSubscription != null;

        if (downstreamSubscription.needsDirectInvocation()) {
            doRequest(n);
        } else {
            downstreamSubscription.executor().execute(() -> doRequest(n));
        }
    }
Closes the deferred stream without setting a delegate.

@throws IllegalStateException if the delegate has been set already or
                              if {@link #close()} or {@link #close(Throwable)} was called already.
core/src/main/java/com/linecorp/armeria/common/stream/DeferredStreamMessage.java