2.x: BehaviorProcessor & Subject terminate-subscribe race

[akarnokd]

This PR fixes the race condition in BehaviorProcessor and BehaviorSubject when onComplete() or onError() is called concurrently with subscribe and the consumer throws a NullPointerException instead of relaying the terminal event.

The fix involves having a separate terminalEvent atomic field, CAS-ing in the actual or marker Throwable and reading that field in subscribe.

[artem-zinnatullin]

assertFailure()? it also checks that no completion events were emitted

[codecov]

Codecov Report

Merging #5281 into 2.x will decrease coverage by 0.1%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5281      +/-   ##
============================================
- Coverage     96.12%   96.01%   -0.11%     
+ Complexity     5755     5745      -10     
============================================
  Files           628      628              
  Lines         41079    41077       -2     
  Branches       5699     5699              
============================================
- Hits          39486    39441      -45     
- Misses          631      653      +22     
- Partials        962      983      +21

Impacted Files	Coverage Δ	Complexity Δ
...in/java/io/reactivex/subjects/BehaviorSubject.java	84.89% <100%> (+0.95%)	56 <5> (+1)	⬆️
...ava/io/reactivex/processors/BehaviorProcessor.java	87.61% <100%> (-0.06%)	61 <5> (ø)
...al/operators/observable/ObservableSampleTimed.java	88.33% <0%> (-10%)	3% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	58.86% <0%> (-5.68%)	31% <0%> (-4%)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
...internal/operators/completable/CompletableAmb.java	94.91% <0%> (-3.39%)	10% <0%> (-1%)
...ernal/operators/flowable/FlowableFromIterable.java	91.97% <0%> (-3.21%)	5% <0%> (ø)
...x/internal/operators/maybe/MaybeSwitchIfEmpty.java	97.22% <0%> (-2.78%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableTakeLastTimed.java	96.29% <0%> (-2.78%)	2% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	61.53% <0%> (-2.57%)	12% <0%> (-1%)
... and 36 more

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[vanniktech]

UnicastSubject as well as SerializedSubject still have the boolean done. Should those be changed for consistency too?

[akarnokd]

No need for that.