Clarify intent of rule 1.9

[NiteshKant]

Rule 1.9 defines that onSubscribe() on a Subscriber should be called before calling any other method on that Subscriber. However, this does not clarify that if onSubscribe() is signalled asynchronously then there should be a happens-before relationship between subscribe() and signalling of onSubscribe().
In absence of such a guarantee, non-final fields initialized in a constructor have no guarantee to be visible inside onSubscribe().

Considering a simple example:

public class UnsafeSubscriber implements Subscriber {
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?

Fixes #486

[viktorklang]

@NiteshKant Your Subscriber is not a valid subscriber since it violates 2:13: https://github.com/reactive-streams/reactive-streams-jvm#2.13

[NiteshKant]

@viktorklang yikes, yes. Throwing IllegalArgumentException isn't the crux of the problem though. Once we get to an agreement on #486 about whether this PR make sense, I wil fix the commit message as:

if (duplicateOnSubscribe) {
        System.out.println("Duplicate onSubscribe() calls.");
}