expectNoEvent does not fail on emission

[stefnoten-aca]

Expected Behavior

expectNoEvent fails when a emission happens

Actual Behavior

expectNoEvent does not fail on an emission

Steps to Reproduce

@Test
void shouldFailButDoesNot() {
    StepVerifier.withVirtualTime(() -> Flux.never().startWith("a"))
                .expectSubscription()
                .expectNoEvent(Duration.ofSeconds(1)) // should fail
                .expectNext("a")
                .verifyTimeout(Duration.ofSeconds(1));
}

Also incorrectly succeeds:

• Use StepVerifier.create instead of withVirtualTime
• Use thenCancel().verify() instead of verifyTimeout (apparently a known case since #1440)

Your Environment

• Reactor version(s) used: reactor-core and reactor-test 3.4.24 (also reproducible on 3.5.2)
• JVM version (java -version): Temurin-17.0.4+8
• OS and version (eg uname -a): Darwin Kernel Version 21.6.0

[OlegDokuka]

@stefnoten-aca Thanks for finding that!

Unfortunately, no promises of a fix 😔. The issue is related to the StepVerifier design and what you observe could be a design flaw.

The problem is that expectNoEven triggers VirtualTimeScheduler to progress by 1 second. Since the startWith has nothing to do with schedulers, the progress will basically do nothing, hence no events will be produced during the expectNoEvents stage. Unfortunately, we don't have a mechanism to put the expectNoEvents stage back in the queue and wait for another event and check whether the event arrived during expectation or not.

All that said - try to redesign your test and use a different approach to verify event time.

One possible workaround could be the use of elapsed operator in combination with predictable virtual time

@Test
    void shouldFailButDoesNot() {
        StepVerifier.withVirtualTime(() -> Flux.never().startWith("a").elapsed())
                    .expectSubscription()
                    .expectNoEvent(Duration.ofSeconds(1)) // should fail
                    .expectNextMatches(e -> e.getT2().equals("a") && e.getT1() >= 1000)
                    .verifyTimeout(Duration.ofSeconds(1));
    }