2.x: fix flatMap emitting the terminal exception indicator on cancel

[akarnokd]

This PR fixes the case when Flowable.flatMap and Observable.flatMap would emit the ExceptionHelper.TERMINTED indicator exception to downstream due to emission-cancellation race.

Reported on StackOverflow.

[codecov]

Codecov Report

Merging #5188 into 2.x will decrease coverage by 0.04%.
The diff coverage is 73.33%.

@@             Coverage Diff              @@
##                2.x    #5188      +/-   ##
============================================
- Coverage     95.98%   95.94%   -0.05%     
- Complexity     5665     5667       +2     
============================================
  Files           621      621              
  Lines         40340    40346       +6     
  Branches       5616     5620       +4     
============================================
- Hits          38722    38711      -11     
- Misses          638      653      +15     
- Partials        980      982       +2

Impacted Files	Coverage Δ	Complexity Δ
...x/internal/operators/flowable/FlowableFlatMap.java	88.09% <71.42%> (+0.09%)	4 <0> (ø)	❌
...ternal/operators/observable/ObservableFlatMap.java	83.93% <75%> (+1.15%)	3 <0> (ø)	❌
...a/io/reactivex/processors/SerializedProcessor.java	89.36% <0%> (-8.52%)	26% <0%> (-2%)
...in/java/io/reactivex/subjects/BehaviorSubject.java	83.93% <0%> (-6.74%)	55% <0%> (-1%)
...ex/internal/operators/maybe/MaybeTimeoutMaybe.java	94.11% <0%> (-4.42%)	2% <0%> (ø)
...erators/completable/CompletableConcatIterable.java	95.91% <0%> (-4.09%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	94.11% <0%> (-3.75%)	5% <0%> (ø)
...internal/operators/completable/CompletableAmb.java	94.91% <0%> (-3.39%)	10% <0%> (-1%)
...ternal/operators/completable/CompletableCache.java	96.96% <0%> (-3.04%)	23% <0%> (-1%)
.../operators/observable/ObservableFlatMapSingle.java	94.02% <0%> (-2.99%)	2% <0%> (ø)
... and 35 more

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[akarnokd]

flatMap is one of the few operators where it is extremely difficult to make sure certain lines get executed and this probabilistic nature may rarely manifest itself on the slow CI.

[JadenChoi]

@akarnokd thanks for the fix for FlatMap, but is there a possibility that we have a similar issue with FlatMapCompletable?

On the same StackOverflow thread, Nantoka commented:

Had the strange exception with Rx Java 2.2.8 as well. I had a flatmapComplete operator wrapped into a flatmapComplete operator. After replacing this with a flatmap followed by a flatmapComplete the error seems to have disappeared. Is say "seems" as the error happened sporadically.

And we are seeing very similar exception for FlatMapCompletable on rxjava:2.2.19 but we aren't getting any useful stacktrace to share though

io.reactivex.internal.util.ExceptionHelper$Termination: No further exceptions
at unknown method(unknown file)

[akarnokd]

It could be. Note though that 2.x is end-of-life so can't do much about it.