[2.1.15] map method called with null

[boris-petrov]

I have something like:

observable.concatMapMaybe(...).map(x -> ...);

This x is null sometimes (our tests reproduce it consistently in one case). Didn't happen on 2.1.14. I can't give more information for now, just to let you know there is a problem. If you can't figure it out soon, I'll try to create some reproduction.

[akarnokd]

Version 2.1.15 did not touch any of the operators you listed and without code demonstrating the problem (you say you have a test reliably failing), we can't do much about it.

[boris-petrov]

OK, I tried debugging with what I thought the problem might be, no luck so far. Here's a couple of stacktraces in the meantime if you can figure out something from them:

NPE in my code as the value is null
        at io.reactivex.internal.operators.observable.ObservableMap$MapObserver.onNext(ObservableMap.java:59)
        at io.reactivex.internal.operators.mixed.ObservableConcatMapMaybe$ConcatMapMaybeMainObserver.drain(ObservableConcatMapMaybe.java:256)
        at io.reactivex.internal.operators.mixed.ObservableConcatMapMaybe$ConcatMapMaybeMainObserver.innerSuccess(ObservableConcatMapMaybe.java:165)
        at io.reactivex.internal.operators.mixed.ObservableConcatMapMaybe$ConcatMapMaybeMainObserver$ConcatMapMaybeObserver.onSuccess(ObservableConcatMapMaybe.java:290)
        at io.reactivex.internal.operators.single.SingleFlatMapMaybe$FlatMapMaybeObserver.onSuccess(SingleFlatMapMaybe.java:117)
        at io.reactivex.internal.operators.maybe.MaybeJust.subscribeActual(MaybeJust.java:36)
        at io.reactivex.Maybe.subscribe(Maybe.java:4096)
        at io.reactivex.internal.operators.single.SingleFlatMapMaybe$FlatMapSingleObserver.onSuccess(SingleFlatMapMaybe.java:89)
        at io.reactivex.internal.operators.single.SingleMap$MapSingleObserver.onSuccess(SingleMap.java:64)
        at io.reactivex.internal.operators.single.SingleMap$MapSingleObserver.onSuccess(SingleMap.java:64)
        at io.reactivex.internal.operators.single.SingleDoOnSuccess$DoOnSuccess.onSuccess(SingleDoOnSuccess.java:59)
        at io.reactivex.internal.operators.single.SingleMap$MapSingleObserver.onSuccess(SingleMap.java:64)
        at io.reactivex.internal.operators.observable.ObservableToListSingle$ToListObserver.onComplete(ObservableToListSingle.java:113)
        at io.reactivex.internal.operators.observable.ObservableFlatMap$MergeObserver.drainLoop(ObservableFlatMap.java:378)
        at io.reactivex.internal.operators.observable.ObservableFlatMap$MergeObserver.drain(ObservableFlatMap.java:327)
        at io.reactivex.internal.operators.observable.ObservableFlatMap$MergeObserver.onComplete(ObservableFlatMap.java:304)
        at io.reactivex.internal.observers.BasicFuseableObserver.onComplete(BasicFuseableObserver.java:119)
        at io.reactivex.internal.operators.mixed.SingleFlatMapObservable$FlatMapObserver.onComplete(SingleFlatMapObservable.java:79)
        at io.reactivex.observers.SerializedObserver.onComplete(SerializedObserver.java:181)
        at io.reactivex.internal.operators.observable.ObservableConcatMap$SourceObserver.drain(ObservableConcatMap.java:200)
        at io.reactivex.internal.operators.observable.ObservableConcatMap$SourceObserver.onComplete(ObservableConcatMap.java:146)
        at io.reactivex.internal.operators.observable.ObservableTake$TakeObserver.onComplete(ObservableTake.java:84)
        at io.reactivex.internal.observers.BasicFuseableObserver.onComplete(BasicFuseableObserver.java:119)
        at io.reactivex.internal.operators.observable.ObservableFlatMap$MergeObserver.drainLoop(ObservableFlatMap.java:378)
        at io.reactivex.internal.operators.observable.ObservableFlatMap$MergeObserver.drain(ObservableFlatMap.java:327)
        at io.reactivex.internal.operators.observable.ObservableFlatMap$InnerObserver.onComplete(ObservableFlatMap.java:590)
        at io.reactivex.internal.operators.mixed.SingleFlatMapObservable$FlatMapObserver.onComplete(SingleFlatMapObservable.java:79)
        at io.reactivex.internal.operators.observable.ObservableCreate$CreateEmitter.onComplete(ObservableCreate.java:98)
my code that calls ObservableEmitter.onComplete

NPE in my code as the value is null
        at io.reactivex.internal.operators.observable.ObservableMap$MapObserver.onNext(ObservableMap.java:59)
        at io.reactivex.internal.operators.mixed.ObservableConcatMapMaybe$ConcatMapMaybeMainObserver.drain(ObservableConcatMapMaybe.java:256)
        at io.reactivex.internal.operators.mixed.ObservableConcatMapMaybe$ConcatMapMaybeMainObserver.onNext(ObservableConcatMapMaybe.java:124)
        at io.reactivex.observers.SerializedObserver.onNext(SerializedObserver.java:113)
        at io.reactivex.internal.operators.observable.ObservableConcatMap$SourceObserver$InnerObserver.onNext(ObservableConcatMap.java:247)
        at io.reactivex.internal.observers.DeferredScalarDisposable.complete(DeferredScalarDisposable.java:82)
        at io.reactivex.internal.operators.single.SingleToObservable$SingleToObservableObserver.onSuccess(SingleToObservable.java:73)
        at io.reactivex.internal.operators.single.SingleMap$MapSingleObserver.onSuccess(SingleMap.java:64)
        at io.reactivex.internal.operators.single.SingleJust.subscribeActual(SingleJust.java:30)
        at io.reactivex.Single.subscribe(Single.java:3313)
        at io.reactivex.internal.operators.single.SingleMap.subscribeActual(SingleMap.java:34)
        at io.reactivex.Single.subscribe(Single.java:3313)
        at io.reactivex.internal.operators.single.SingleToObservable.subscribeActual(SingleToObservable.java:36)
        at io.reactivex.Observable.subscribe(Observable.java:12036)
        at io.reactivex.internal.operators.observable.ObservableConcatMap$SourceObserver.drain(ObservableConcatMap.java:218)
        at io.reactivex.internal.operators.observable.ObservableConcatMap$SourceObserver.onNext(ObservableConcatMap.java:128)
        at io.reactivex.internal.operators.mixed.SingleFlatMapObservable$FlatMapObserver.onNext(SingleFlatMapObservable.java:69)
        at io.reactivex.internal.operators.observable.ObservableCreate$CreateEmitter.onNext(ObservableCreate.java:67)
my code that calls ObservableEmitter.onNext

[akarnokd]

Is dispose/cancel involved in your flows?

[boris-petrov]

Well, we do have dispose here and there but they're in completely different places in the code. It IS possible that a dispose could reach this piece of code but... not sure, it is quite complex. I could try removing all dispose calls and checking if that fixes it?

[akarnokd]

I've identified a race condition which can happen when the flow is getting disposed the same time the Maybe succeeds, resulting in a null item being emitted. It would be great if you could check if there is a dispose/cancel happening in your test as well.

[akarnokd]

Can you verify the crash is resolved with the latest snapshot (give it an hour and check https://oss.jfrog.org/libs-snapshot/io/reactivex/rxjava2/rxjava/2.2.0-SNAPSHOT/ for binaries for today)?

repositories {
    maven { url 'https://oss.jfrog.org/libs-snapshot' }
}

dependencies {
    compile 'io.reactivex.rxjava2:rxjava:2.2.0-SNAPSHOT'
}

[boris-petrov]

Sure, I'll try it out, thanks for the quick response, as always! Its funny how I seem to always be catching some horrible edge-case race conditions like this one and the one before it that you've mentioned in the PR. :D

[boris-petrov]

@akarnokd - I can confirm that 2.2.0-SNAPSHOT of today seemingly fixes the issue (I don't get the exception running our tests).

[akarnokd]

Great. How urgent is for you to have this fix in a release version (next would be in a month)?

[boris-petrov]

Well, I can’t upgrade as this is a big regression in our case so we’ll stay on 2.1.14 until the next release. Not a big deal but releases are free, so why not release a quick patch version?

[akarnokd]

I'd wait a couple of days as most bugs are reported next to a release.

@vanniktech @artem-zinnatullin @davidmoten would you be fine with 2.1.16 as a patch release next Tuesday?

[vanniktech]

Sure. Releases are free.