3.x: Add RxJavaPlugins.createExecutorScheduler

[akarnokd]

This PR adds the RxJavaPlugins.createExecutorScheduler that instantiates the already existing ExecutorScheduler independent of the Schedulers class. The from methods in the Schedulers class are now pointing to this new method.

Resolves #7305
Related #7300

[codecov]

Codecov Report

Merging #7306 (aff7834) into 3.x (e24e725) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                3.x    #7306      +/-   ##
============================================
- Coverage     99.53%   99.52%   -0.02%     
- Complexity     6783     6786       +3     
============================================
  Files           751      751              
  Lines         47489    47490       +1     
  Branches       6378     6378              
============================================
- Hits          47268    47263       -5     
- Misses          100      103       +3     
- Partials        121      124       +3     

Impacted Files	Coverage Δ
...va/io/reactivex/rxjava3/plugins/RxJavaPlugins.java	100.00% <100.00%> (ø)
...va/io/reactivex/rxjava3/schedulers/Schedulers.java	100.00% <100.00%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	89.43% <0.00%> (-8.46%)	⬇️
...nternal/operators/observable/ObservableCreate.java	96.58% <0.00%> (-3.42%)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	94.44% <0.00%> (-1.59%)	⬇️
...ators/observable/ObservableFlatMapCompletable.java	98.52% <0.00%> (-1.48%)	⬇️
...va3/internal/operators/parallel/ParallelRunOn.java	98.53% <0.00%> (-1.47%)	⬇️
...java3/internal/operators/flowable/FlowableZip.java	98.97% <0.00%> (-1.03%)	⬇️
.../operators/maybe/MaybeFlatMapIterableFlowable.java	99.18% <0.00%> (-0.82%)	⬇️
...ternal/operators/observable/ObservableFlatMap.java	97.87% <0.00%> (ø)
... and 9 more

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[matthew-swift]

Hi there. I'm not sure this change is working as expected. The following code creates the standard default schedulers:

ExecutorService executor = Executors.newSingleThreadExecutor();
Scheduler scheduler = RxJavaPlugins.createExecutorScheduler(executor, false, false);
Flowable.intervalRange(1, 10, 1, 1, TimeUnit.SECONDS, scheduler)
        .blockingForEach(System.out::println);
executor.shutdown();

But according to the new Javadoc it should not trigger the creation of the default schedulers:

It is possible to wrap an Executor into a Scheduler without triggering the initialization of all the standard schedulers by using the RxJavaPlugins.createExecutorScheduler(Executor, boolean, boolean) method before the Schedulers class itself is accessed.

It clearly will though because the ExecutorScheduler class contains a static reference HELPER which references Schedulers.single() which then initializes all of the default schedulers as observed by putting a breakpoint in the static initializer block on line 80 of Schedulers.

Apologies if I'm misunderstanding the intent of this change. If I am then that may indicate that the new Javadoc is a bit misleading.

[akarnokd]

Indeed the method should not trigger that initialization. I'll work out how to break that cycle and post a fix shortly.

[matthew-swift]

Thanks for your quick response David. 👍