2.x: Fix refCount() connect/subscribe/cancel deadlock

[akarnokd]

This PR fixes a deadlock issue with the refCount operator when a subscription leads to a blocking execution while the lock is being held, preventing other subscription or cancellation from executing on other threads.

The bug was discovered as the cause of a reported hang on the Google groups.

The code has been developed in the extensions project where the operator has more features. The overloads supporting these features can be added in a separate enhancement PR.

[vanniktech]

should be f

[codecov]

Codecov Report

Merging #5975 into 2.x will decrease coverage by 0.06%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##                2.x   #5975      +/-   ##
===========================================
- Coverage     98.27%   98.2%   -0.07%     
- Complexity     6019    6049      +30     
===========================================
  Files           656     656              
  Lines         44037   44074      +37     
  Branches       6100    6118      +18     
===========================================
+ Hits          43278   43285       +7     
- Misses          224     246      +22     
- Partials        535     543       +8

Impacted Files	Coverage Δ	Complexity Δ
.../internal/operators/flowable/FlowableRefCount.java	100% <100%> (ø)	22 <22> (+15)	⬆️
...ernal/operators/observable/ObservableRefCount.java	100% <100%> (ø)	22 <22> (+15)	⬆️
...l/operators/observable/ObservableFlatMapMaybe.java	89.54% <0%> (-4.58%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-3.81%)	2% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	95.83% <0%> (-2.78%)	55% <0%> (-2%)
...tivex/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	95.83% <0%> (-2.5%)	2% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	96.86% <0%> (-2.25%)	60% <0%> (ø)
.../io/reactivex/internal/schedulers/IoScheduler.java	89.24% <0%> (-2.16%)	9% <0%> (ø)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
... and 23 more

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[akarnokd]

/cc @davidmoten as you devised the previous refCount algorithm.

[davidmoten]

👍 Great to see that bit of locking removed. I actually had launched into a non-blocking rewrite of the refCount operator a couple of months ago and had it working with current tests but started bringing in tests from 1.x and got a failure or two with some quite unusual tests, then jumped to something else leaving it on my to-do queue. I'll cast another look at the rewrite soon.

[artem-zinnatullin]

looks good, but comment :)

[artem-zinnatullin]

looks like this condition is only actual for resubscribeBeforeTimeout test

[artem-zinnatullin]

It's strange to see logic and tests for something that user cannot use in RxJava

As you mentioned in PR description, let's maybe expose additional time related refCount api or try to shrink unused code?

[akarnokd]

Done: #5986.