NullPointerException in MultiConcatMapOp

[markusdlugi]

Context

We tried to update our application to Quarkus 3.7.0 which is shipping the latest Mutiny version 2.5.5, but one of our unit tests is failing due to a NPE in Mutiny.

Description

The exception stack trace:

java.lang.NullPointerException: Cannot invoke "java.util.concurrent.Flow$Subscription.request(long)" because "this.innerUpstream" is null

	at io.smallrye.mutiny.operators.multi.MultiConcatMapOp$MainSubscriber.request(MultiConcatMapOp.java:250)
	at io.smallrye.mutiny.helpers.test.AssertSubscriber.request(AssertSubscriber.java:711)
	at io.smallrye.mutiny.helpers.test.AssertSubscriber.awaitNextItemEvents(AssertSubscriber.java:614)
	at io.smallrye.mutiny.helpers.test.AssertSubscriber.awaitNextItems(AssertSubscriber.java:395)
	at io.smallrye.mutiny.helpers.test.AssertSubscriber.awaitNextItems(AssertSubscriber.java:373)
	at com.acme.ReproducerQuarkusTest.testNullPointerException(ReproducerQuarkusTest.java:41)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at io.quarkus.test.junit.QuarkusTestExtension.runExtensionMethod(QuarkusTestExtension.java:1013)
	at io.quarkus.test.junit.QuarkusTestExtension.interceptTestMethod(QuarkusTestExtension.java:827)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)

Additional details

I was able to create a minimal reproducer:

@Inject
Vertx vertx;

public Multi<Long> nullPointerException() {
	return Multi.createFrom().ticks().every(Duration.ofSeconds(30))
			.onItem().call(i -> vertx.executeBlocking(() -> null));
}

Tested in a @QuarkusTest using:

@Test
void testNullPointerException() {
	testee.nullPointerException().subscribe().withSubscriber(AssertSubscriber.create(1))
			.awaitNextItems(1, Duration.ofSeconds(3));
}

The test passes sometimes, but sometimes it doesn't, so maybe some kind of race condition?

[jponge]

Thanks, we'll have a look in the coming days

[jponge]

There's indeed a very narrow chance the first inner upstream hasn't arrived yet.

The fix will be a simple null check. I'm trying to reproduce this in pure Mutiny (no Quarkus, no Vert.x, but this isn't easy 🤣)

[jponge]

I even found a deadlock, thanks @markusdlugi

[markusdlugi]

Perfect, thanks a lot for the quick fix!