Exception not propagated on parallel execution with blocking zip function

When using the zip operator on an Observable or Single along with a blocking call, there apparently isn’t a way to propagate a single error consistently since mergeArrayDelayError() or similar all accept a single data type.

In this example, I have two Single, executed in parallel on a different thread, which are then passed into a zip function and then synchronously executed through a blockingGet(). What I’d expect is that the exception is thrown by this latter method. What happens though is that the exception is passed to the default uncaught exception handler from RxJavaPlugins.onError().

Am I using the wrong approach or is there an issue with error propagation here? RxJava 2.1.3 on Android.

Here’s the (failing) JUnit test:

    @Test(expected = InterruptedException.class)
    public void testParallelZip_exceptionNotPropagated() throws Exception {
        Single<Object> allPeopleSource = Single.fromCallable(() -> {
            throw new InterruptedException();
        }).subscribeOn(Schedulers.io());
        Single<String> idsSource = Single.fromCallable(() -> "second_single").subscribeOn(Schedulers.io());

        Single.zip(allPeopleSource, idsSource, (o, s) -> "result").blockingGet();
    }

And here’s the failure crash log:

java.lang.Exception: Unexpected exception, expected<java.lang.InterruptedException> but was<java.lang.RuntimeException>

	at org.junit.internal.runners.statements.ExpectException.evaluate(ExpectException.java:28)
	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:117)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:42)
	at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:262)
	at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:84)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)
Caused by: java.lang.RuntimeException: java.lang.InterruptedException
	at io.reactivex.internal.util.ExceptionHelper.wrapOrThrow(ExceptionHelper.java:45)
	at io.reactivex.internal.observers.BlockingMultiObserver.blockingGet(BlockingMultiObserver.java:91)
	at io.reactivex.Single.blockingGet(Single.java:2154)
	at [...].RxUtilsTest.testParallelZip_exceptionNotPropagated(RxUtilsTest.java:70)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.internal.runners.statements.ExpectException.evaluate(ExpectException.java:19)
	... 20 more
Caused by: java.lang.InterruptedException
	at com.teamwork.data.util.RxUtilsTest.lambda$testParallelZip_exceptionNotPropagated$1(RxUtilsTest.java:66)
	at io.reactivex.internal.operators.single.SingleFromCallable.subscribeActual(SingleFromCallable.java:35)
	at io.reactivex.Single.subscribe(Single.java:2700)
	at io.reactivex.internal.operators.single.SingleSubscribeOn$SubscribeOnObserver.run(SingleSubscribeOn.java:89)
	at io.reactivex.Scheduler$DisposeTask.run(Scheduler.java:452)
	at io.reactivex.internal.schedulers.ScheduledRunnable.run(ScheduledRunnable.java:61)
	at io.reactivex.internal.schedulers.ScheduledRunnable.call(ScheduledRunnable.java:52)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at java.lang.Thread.run(Thread.java:745)

[marcosalis]

@akarnokd, unfortunately I can’t reproduce the issue outside of the production code of the app I’m working on. What seems to be happening is that there is some kind of weird interaction with the RxJava scheduler threads, their interrupted state and the fact that the Observable seems to be disposed right after it’s created. I will need to investigate this further, in the meantime I’m closing the issue as this doesn’t seem to happen when using Single instead of Observable (I guess because of the different implementation of SingleFromCallable.subscribeActual, which never calls to RxJavaPlugins.onError).

[akarnokd]

So what remains, @marcosalis, that you post the code that causes that exception so I can reproduce the issue.