Error is not propagated. Mono hangs when StackOverflowError is thrown in map or flatMap

[0v1se]

Expected behavior

StackOverFlow should be thrown

Actual behavior

java.lang.IllegalStateException: Timeout on blocking read for 5000 MILLISECONDS

Steps to reproduce

run strange method provided:

private void exception() {
    exception();
}

public void strange() {
    CompletableFuture<Integer> future = new CompletableFuture<>();
    future.complete(1);
    Mono<Integer> simple = Mono.fromFuture(future);//Mono.just(1)

    simple.map(r -> {
        exception();
        return r;
    }).block(Duration.ofSeconds(5));
}

Reactor Core version

3.1.5.RELEASE

JVM version (e.g. java -version)

java version "1.8.0_162"
Java(TM) SE Runtime Environment (build 1.8.0_162-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.162-b12, mixed mode)

[smaldini]

StackOverflowError is considered a fatal error and falls into the general catch-all error from Hooks.onErrorDropped(consumer). We might want to clarify that @simonbasle

[simonbasle]

It is expected that a fatal error like StackOverflowError isn't propagated as an onError signal. However, what is not good in your case is that you don't see any trace of the error.

Looks like the error was swallowed by the CompletableFuture.whenComplete method :(

We made it so Mono.fromFuture will now explicitly "drop" the error (see the Hooks.onErrorDropped consumer). Additionally, the default behaviour of the error drop hook is now to log the dropped exception additionally to bubbling it up.

As a result, what you'd see after the fix is a timeout exception + logs of the stack overflow error.

[0v1se]

Ok. Thanks.

Also, such behaviour can be observed not only when CompletableFuture is used. It was only an example