Can onNext may be signalled before onSubscribe returns?

[onacit]

Can the subscriber.onNext(item) be invoked before the subscriber.onSubscribe(subscription) returns, e.g. the subscriber invokes subscription.request(n) within the onSubscribe(subscription) method?

void onSubscribe(Subscription subscription) {
    subscription.request(1);
    // the publisher may start (asynchronously) publishing items to the subscriber
    // which means the onNext(item) may be invoked before this method returns
    log.debug("end-of-on-subscribe");
}

void onNext(T item) {
    log.debug("next");
}

(If logging is synchronized,)
Can next be logged before the end-of-on-subscribe?

[akarnokd]

Yes and no.

Rule §3.2 implies it is allowed. Rule §1.3 forbids overlapping concurrent invocation between onSubscribe and onNext. Thus synchronous invocation should be allowed in theory.

In practice, you may have to defer request calls happening while onSubscribe is running and reissue them after. Especially if you are mixing implementations from different vendors or your own.