Considering a simple example:

public class UnsafeSubscriber implements Subscriber<String> {
    private boolean duplicateOnSubscribe = false;

    @Override
    public void onSubscribe(final Subscription s) {
        if (duplicateOnSubscribe) {
            throw new IllegalStateException("Duplicate onSubscribe() calls.");
        }
        duplicateOnSubscribe = true;
    }

    @Override
    public void onNext(final String s) {

    }

    @Override
    public void onError(final Throwable t) {

    }

    @Override
    public void onComplete() {

    }
}

If an UnsafeSubscriber instance is created in a different thread than the one that invokes onSubscribe() (true for an asynchronous Publisher), according to the java memory model, this statement inside onSubscribe():

if (duplicateOnSubscribe) {

is guaranteed to compute to false if and only if the instance is published safely between these threads. None of the rules in the specifications establish a happens-before relationship between Publisher#subscribe() and Subscriber#onSubscribe(). So, the usage above can be categorized as unsafe. In a more convoluted form, the assignment:

private boolean duplicateOnSubscribe = false;

can be interleaved with

duplicateOnSubscribe = true; such that duplicateOnSubscribe is set to false later.

Has this been considered before or am I missing something?