private void request0(long n) {
        if (obj == null) {
            return;
        }

        if (n <= 0) {
            onError(new IllegalArgumentException(
                    "n: " + n + " (expected: > 0, see Reactive Streams specification rule 3.9)"));
            return;
        }

        final T item = obj;
        obj = null;
        onNext(item);
        onComplete();
    }
A {@link FixedStreamMessage} that only publishes one object.
core/src/main/java/com/linecorp/armeria/internal/common/stream/OneElementFixedStreamMessage.java