2.x: Fix window(Observable|Callable) upstream handling

[akarnokd]

This PR fixes the upstream handling in Observable.window(ObservableSource) and Observable.window(Callable<ObservableSource>) operators to make sure if both the main output and the inner windows have been disposed, the upstream is also disposed.

Fixes #5881.

Remark:

There are a couple of w != null checks showing up as partially covered. In theory, with the right interleaving, w can become null, but unfortunately the usual TestHelper.race() testing is unlikely to trigger that case. There would be a higher chance with 3 threads (one disposing, one completing the main and one completing the inner window) but the current CI is effectively 1.5 - 2 cores/threads that tend to not produce a thorough interleaving.

[codecov]

Codecov Report

Merging #5887 into 2.x will increase coverage by 0.07%.
The diff coverage is 97.98%.

@@             Coverage Diff              @@
##                2.x    #5887      +/-   ##
============================================
+ Coverage     97.43%   97.51%   +0.07%     
- Complexity     5946     5948       +2     
============================================
  Files           655      655              
  Lines         43877    43900      +23     
  Branches       6109     6108       -1     
============================================
+ Hits          42752    42807      +55     
+ Misses          348      340       -8     
+ Partials        777      753      -24

Impacted Files	Coverage Δ	Complexity Δ
...s/observable/ObservableWindowBoundarySupplier.java	97.95% <97.22%> (+8.32%)	2 <1> (ø)	⬇️
...operators/observable/ObservableWindowBoundary.java	99.21% <98.9%> (+11.18%)	2 <1> (ø)	⬇️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	88.88% <0%> (-5.32%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-5.23%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.17% <0%> (-3.93%)	2% <0%> (ø)
...java/io/reactivex/processors/PublishProcessor.java	98.23% <0%> (-1.77%)	43% <0%> (-1%)
...ernal/operators/flowable/FlowableFromIterable.java	95.18% <0%> (-1.07%)	5% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	95.96% <0%> (-0.9%)	59% <0%> (-1%)
...perators/single/SingleFlatMapIterableFlowable.java	96.66% <0%> (-0.84%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableTimeoutTimed.java	98.37% <0%> (-0.82%)	3% <0%> (ø)
... and 15 more

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[vanniktech]

The issue will get closed once this PR is merged but I don't see the Flowable implementation here.

[akarnokd]

The issue refers to window(Observable) thus the Observable variants. I'll post a separate PR once I checked the Flowable variants.

[akarnokd]

Yes, Flowable.window(Publisher) is also incorrect.