4.x: Move TestHelper.race tests into @Isolated classes (flatMap/mergeWith batch) (#8075)

[oaksprout]

You set the work. #8075, and your go-ahead: move TestHelper.race uses into separate classes so they can run @Isolated, named <Original>IsolatedTest.java, movements and conversions in separate PRs.

It's done. First movements batch: the Flowable flatMap-to-{Maybe,Single,Publisher} / mergeWith-{Completable,Maybe,Single} family — 15 race-test methods moved out of 6 classes into 6 new @Isolated classes, following your naming pattern exactly. Kept deliberately small so you can veto the shape (batch grouping, class layout) before I produce the rest: 179 more files use TestHelper.race and can follow in whatever batch size suits your review.

Here's the evidence.

• Fresh clone at b7946f24, patch applied, dependencies warmed, then the network was disconnected.
• Scoped suite in a clean eclipse-temurin:26-jdk container (pinned digest, linux/arm64), network off: 211 tests across all 12 touched classes, 0 failures, 0 errors — per-class counts match the solve machine's run exactly. Full-tree compileTestJava clean.
• checkstyleTest clean (that check ran with network on — the checkstyle jar wasn't pre-cached; stated for accuracy).

Audit trail — an independently checkable record that these checks ran, in this order, before this PR existed

Base commit	b7946f24fc6ec1f6bc984bcc2810024efbad744e
Container	eclipse-temurin:26-jdk@sha256:939e3577… (linux/arm64), network off during tests
The patch, content-addressed	record
The verification result, signed by a second key	record
Timestamped sequence	work assigned → work delivered → checks passed

What this proves: the checks ran, in that order, on exactly this patch, before this PR was opened — none of it can be backdated or swapped afterwards. What it doesn't prove: that the change is right. The two signing keys are distinct but run by the same project, and the record lives on a test network. Correctness is your judgement, which is the point.

Written by an AI agent; reviewed and sent by a human who answers the review — this one on your invitation in #8075.

---

Everything below is written by Claude

Part of #8075 (movements category only; conversions of manual racing tests will come separately as you specified).

Batch contents — moved TestHelper.race( methods per class:

Original	Moved	New class
FlowableFlatMapMaybeTest	3	FlowableFlatMapMaybeIsolatedTest
FlowableFlatMapSingleTest	2	FlowableFlatMapSingleIsolatedTest
FlowableFlatMapTest	3	FlowableFlatMapIsolatedTest
FlowableMergeWithCompletableTest	1	FlowableMergeWithCompletableIsolatedTest
FlowableMergeWithMaybeTest	3	FlowableMergeWithMaybeIsolatedTest
FlowableMergeWithSingleTest	3	FlowableMergeWithSingleIsolatedTest

Each new class is @Isolated, extends RxJavaTest, keeps the license header and package, and takes only the imports the moved methods need. Private helpers used solely by moved methods went with them; originals are otherwise untouched.

Notes for the reviewer

• @Isolated is currently inert in this build: junit.jupiter.execution.parallel.enabled isn't set, so JUnit's parallel executor never runs and the annotation has no observable effect yet. I read 4.x: Separate TestHelper.race uses so they can run @Isolated #8075 as prep for flipping that config later — if you'd rather this PR also enable parallel execution (or set it per-package), say so and I'll extend it.
• TestHelper.raceOther( call sites (9 files) were excluded as out of scope, since the issue names TestHelper.race specifically — flag if you want them included in the movements category.
• Batch grouping (one operator family per PR) is my choice, not something the issue specifies. Happy to switch to per-class PRs or bigger batches — 179 files remain.

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.745 bits/char
Verdict: ⚠️ Consider review — entropy outside sweet spot

Changed files entropy:

src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableFlatMapIsolatedTest.java: 4.689
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableFlatMapMaybeIsolatedTest.java: 4.852
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableFlatMapMaybeTest.java: 4.764
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableFlatMapSingleIsolatedTest.java: 4.908
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableFlatMapSingleTest.java: 4.770
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableFlatMapTest.java: 4.668
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableMergeWithCompletableIsolatedTest.java: 4.873
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableMergeWithCompletableTest.java: 4.774
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableMergeWithMaybeIsolatedTest.java: 4.752
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableMergeWithMaybeTest.java: 4.584
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableMergeWithSingleIsolatedTest.java: 4.739
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableMergeWithSingleTest.java: 4.571

✅ 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 98.62%. Comparing base (b7946f2) to head (13f4cfb).
⚠️ Report is 1 commits behind head on 4.x.

Additional details and impacted files

@@            Coverage Diff            @@
##                4.x    #8253   +/-   ##
=========================================
  Coverage     98.62%   98.62%           
- Complexity     7125     7128    +3     
=========================================
  Files           822      822           
  Lines         50690    50690           
  Branches       6892     6892           
=========================================
+ Hits          49992    49994    +2     
  Misses          476      476           
+ Partials        222      220    -2     

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[akarnokd]

@isolated is currently inert in this build

build.gradle runs tests in parallel for sure because how it takes 7 minutes instead of 14.

TestHelper.raceOther

We'll look into them after you are done with TestHelper.race.