4.x: Fix consumer retention in cache() upon subscribe-cancel race

[akarnokd]

When the incoming Subscriber or Observer cancels while in their respective onSubscribe or race it afterwards, the operator could retain it in its consumer tracking array indefinitely, depending on the cached source.

The fix is, like many other place is to after add, check if the consumer cancelled and then manually remove them for sure.

In addition, apparently the test cancelledUpFrontConnectAnyway was missing from ObservableCacheTest which is important because the cache operator is expected to connect, even with dead consumers.

Privately reported as an ??? exploit ???

@August829

RxJava-09: ObservableCache/FlowableCache Register Consumers Before Checking Disposed State — Self-Disposing Subscribers Permanently Retained

🛑 Do not report bugs as security exploits. 🛑

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.46 bits/char
Verdict: ✅ Mid-4 beauty detected (thoughtful human code!)

Changed files entropy:

src/main/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableCache.java: 4.229
src/main/java/io/reactivex/rxjava4/internal/operators/observable/ObservableCache.java: 4.191
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableCacheTest.java: 4.696
src/test/java/io/reactivex/rxjava4/internal/operators/observable/ObservableCacheTest.java: 4.724

✅ No secrets or suspicious high-entropy strings found.

Mid-4 beauty heuristic in action — powered by our entropy chats! 😊

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.492 bits/char
Verdict: ✅ Mid-4 beauty detected (thoughtful human code!)

Changed files entropy:

src/main/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableCache.java: 4.229
src/main/java/io/reactivex/rxjava4/internal/operators/observable/ObservableCache.java: 4.191
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableCacheTest.java: 4.696
src/test/java/io/reactivex/rxjava4/internal/operators/observable/ObservableCacheTest.java: 4.724
src/test/java/io/reactivex/rxjava4/internal/operators/observable/ObservableFlatMapTest.java: 4.622

✅ No secrets or suspicious high-entropy strings found.

Mid-4 beauty heuristic in action — powered by our entropy chats! 😊

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.61%. Comparing base (dde738d) to head (4351a47).
⚠️ Report is 1 commits behind head on 4.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8283      +/-   ##
============================================
- Coverage     98.65%   98.61%   -0.04%     
- Complexity     7147     7148       +1     
============================================
  Files           824      824              
  Lines         50784    50789       +5     
  Branches       6907     6910       +3     
============================================
- Hits          50101    50087      -14     
- Misses          473      481       +8     
- Partials        210      221      +11     

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