3.x: Fix Flowable.groupBy eviction logic double decrement and hang

[akarnokd]

When a group is evicted, the group is synchronously onCompleted during an onNext. If a cancellation happened during this time (for example, when a publish cancels its upstream because its output completes), the cancellation would unconditionally decrement the group counter even though the groups map had no longer the group (because it was evicted before). Then once the eviction logic finishes, it decremented the group counter again, leading to invalid internal state and hangs.

The fix is to only decrement if there was a group actually removed from the map.

Fixes #6974

[codecov]

Codecov Report

Merging #6975 into 3.x will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@ Coverage Diff @@ ## 3.x #6975 +/- ## ============================================ + Coverage 99.65% 99.66% +0.01%  - Complexity 6670 6671 +1  ============================================ Files 742 742 Lines 47254 47254 Branches 6370 6371 +1 ============================================ + Hits 47090 47098 +8  + Misses 51 47 -4  + Partials 113 109 -4 

Impacted Files	Coverage Δ	Complexity Δ
...3/internal/operators/flowable/FlowableGroupBy.java	99.68% <100.00%> (ø)	3.00 <0.00> (ø)
...xjava3/internal/observers/FutureMultiObserver.java	96.61% <0.00%> (-3.39%)	27.00% <0.00%> (-1.00%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	95.34% <0.00%> (-2.33%)	2.00% <0.00%> (ø%)
...perators/single/SingleFlatMapIterableFlowable.java	98.33% <0.00%> (-1.67%)	2.00% <0.00%> (ø%)
...ternal/operators/completable/CompletableMerge.java	97.29% <0.00%> (-1.36%)	2.00% <0.00%> (ø%)
...ternal/operators/observable/ObservableGroupBy.java	98.90% <0.00%> (-1.10%)	2.00% <0.00%> (ø%)
.../operators/observable/ObservableCombineLatest.java	99.38% <0.00%> (-0.62%)	6.00% <0.00%> (ø%)
...a3/internal/operators/flowable/FlowableCreate.java	99.02% <0.00%> (+0.32%)	6.00% <0.00%> (ø%)
...ternal/operators/observable/ObservableFlatMap.java	98.58% <0.00%> (+0.35%)	3.00% <0.00%> (ø%)
...a/io/reactivex/rxjava3/subjects/ReplaySubject.java	99.58% <0.00%> (+0.41%)	49.00% <0.00%> (ø%)
... and 5 more

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[davidmoten]

LGTM, thanks @akarnokd