Multiple errors in zip'ed observables with flatMap

While reading Error handling on wiki I realized that in a zip operator with a multiple network calls, it’s possible to receive a crash with an UndeliverableException. I tried to reproduce this behavior:

RxJava version: 2.2.0

Reproduce code:

    @Test
    public void testUncaughtException() throws InterruptedException {
        Observable first = Observable.create(e -> {
            System.out.println("first");
            throw new HelperException("first exception");
        });

        Observable second = Observable.create(e -> {
            System.out.println("second");
            throw new HelperException("second exception");
        });

        List<Observable<?>> observableList = new ArrayList<>();
        observableList.add(first);
        observableList.add(second);

        Observable.zip(observableList, objects -> "result")
                .subscribeOn(Schedulers.io())
                .subscribe(
                        System.out::println,
                        t -> {
                            System.out.println("exception caught!");
                        }
                );
        Thread.sleep(2000);
    }

The output as expected:

first
exception caught!
second
io.reactivex.exceptions.UndeliverableException: ... HelperException: second exception ...

And the second test:

    @Test
    public void testUncaughtExceptionWithFlatMap() throws InterruptedException {
        Observable testObservable = Observable.create(e -> e.onNext(""))
                .flatMap((Function<Object, ObservableSource<?>>) o -> {

                    Observable first = Observable.create(e -> {
                        System.out.println("first");
                        throw new HelperException("first exception");
                    });

                    Observable second = Observable.create(e -> {
                        System.out.println("second");
                        throw new HelperException("second exception");
                    });

                    List<Observable<?>> observableList = new ArrayList<>();
                    observableList.add(first);
                    observableList.add(second);

                    return Observable.zip(observableList, objects -> "result");
                });

        testObservable
                .subscribeOn(Schedulers.io())
                .subscribe(
                        System.out::println,
                        t -> System.out.println("exception caught!")
                );

        Thread.sleep(2000);
    }

And I expected an UndeliverableException too, but the output is:

first
exception caught!

Is this behavior correct? Why there is no UndeliverableException in the second test?

Thanks!

[akarnokd]

Correction. In both cases, you shouldn’t see any undeliverable exceptions. What happens is that flatMap issues a cancel when it detects the error which then stops the subscription to the second source. I’ll investigate this further.

[radityagumay]

@akarnokd thanks for explanations!

i am currently using onErrorResumeNext to tweak that part. so, whenever error occurs in observable i return default value.