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