Fix FlowableCache GC issue

[DragonFSKY]

Summary

• Split the source-facing state of FlowableCache into a static Multicaster.
• Keep the cache head on the outer FlowableCache, while the upstream retains only the shared multicaster state and current tail.
• Clear the multicaster tail on termination and guard against post-terminal signals.
• Add a focused regression test verifying that consumed cached values become reclaimable.

Rationale

Previously, the upstream subscribed directly to the outer FlowableCache. This allowed the upstream connection to retain the outer cache and its head, keeping all cached nodes reachable even after application code could no longer create another subscription to that cache instance.

With the ownership split, the upstream retains only the static Multicaster. Once the outer FlowableCache becomes unreachable, consumed node prefixes can be reclaimed, while active subscribers continue to retain the suffix they still need.

The established behavior remains unchanged for valid upstream signals: the source is subscribed to at most once, the upstream is requested with Long.MAX_VALUE, each downstream keeps independent request accounting, replay remains serialized per downstream, and cancellation removes only that downstream without cancelling the shared upstream.

Validation

Local validation performed by the contributor:

• Full FlowableCacheTest passed.
• valuesAreReclaimable passed in five independent runs.
• The same regression test failed against the unmodified 4.x implementation, with local heap usage increasing from approximately 8.2 MiB to 408.3 MiB.
• checkstyleMain and checkstyleTest passed.
• Validator tests passed.
• ./gradlew build passed on Temurin 26.
• Javadoc and git diff --check passed.

AI disclosure

I used an AI assistant to help analyze the FlowableCache retention graph, adapt the ownership-split approach to RxJava 4.x, and review the concurrency and regression-test implications.

I reviewed and decided the final implementation, performed the validation listed above, and am responsible for the code and submission.

References

• Addresses 4.x: Cache/Replay operators to be more GC friendly #7912.
• Supersedes [Problem] Fix FlowableCache GC issue #8247.

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.465 bits/char
Verdict: ✅ Mid-4 beauty detected (thoughtful human code!)

Changed files entropy:

src/main/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableCache.java: 4.226
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableCacheTest.java: 4.703

✅ 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 98.63%. Comparing base (47b6795) to head (1d277c9).

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8249      +/-   ##
============================================
- Coverage     98.64%   98.63%   -0.02%     
+ Complexity     7160     7125      -35     
============================================
  Files           822      822              
  Lines         50683    50690       +7     
  Branches       6889     6892       +3     
============================================
- Hits          49998    49996       -2     
- Misses          472      478       +6     
- Partials        213      216       +3     

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