private static boolean isReproducibleScalarType(Publisher<Object> publisher) {
        if (publisher instanceof ScalarCallable) {
            final String className = publisher.getClass().getName();
            return !className.equals(FLUX_ERROR_SUPPLIED) && !className.equals(MONO_ERROR_SUPPLIED);
        }
        return false;
    }
Returns whether the specified {@link Publisher} is reproducible {@link Fuseable.ScalarCallable} such as
{@link Flux#empty()}, {@link Flux#just(Object)}, {@link Flux#error(Throwable)},
{@link Mono#empty()}, {@link Mono#just(Object)} and {@link Mono#error(Throwable)}.
reactor3/src/main/java/com/linecorp/armeria/common/reactor3/RequestContextHooks.java