2.x: fix concatMapEager should accept 0 for prefetch

[nmorioka]

Fixes a bug that Flowable.concatMapEager(mapper , maxConcurrency , prefetch) and Observable.concatMapEager(mapper , maxConcurrency , prefetch) operators will not accept 0 and negative numbers.

Reported in #5185.

[akarnokd]

I believe a 0 breaks the internals of the operator and -1 has the completely opposite effect, taking more memory.

[akarnokd]

The verifyPositive should be restored.
Optionally, you could add the ", positive" to the javadoc changes.

[akarnokd]

No, the validations were right, you shouldn't call with non-positive value.

[akarnokd]

No, the validations were right, you shouldn't call with non-positive value.

[akarnokd]

I think this should be reinforced:

hints about the number of expected values from each inner Publisher, must be positive

[nmorioka]

I am sorry with the wrong fix.
Is it only necessary to modify the document?

[akarnokd]

Is it only necessary to modify the document?

Yes. Please modify only the javadoc.

[nmorioka]

Thank you!

Also, I think that the test case name is wrong, so I modifed it.

[codecov]

Codecov Report

Merging #5189 into 2.x will decrease coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5189      +/-   ##
============================================
- Coverage     96.02%   95.97%   -0.06%     
  Complexity     5670     5670              
============================================
  Files           621      621              
  Lines         40581    40581              
  Branches       5620     5620              
============================================
- Hits          38969    38946      -23     
- Misses          640      661      +21     
- Partials        972      974       +2

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	506 <0> (ø)	❌
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	522 <0> (ø)	❌
...al/operators/observable/ObservableSampleTimed.java	88.33% <0%> (-10%)	3% <0%> (ø)
...vex/internal/operators/single/SingleTakeUntil.java	86.88% <0%> (-8.2%)	2% <0%> (ø)
.../operators/completable/CompletableConcatArray.java	93.75% <0%> (-6.25%)	2% <0%> (ø)
...rnal/subscribers/SinglePostCompleteSubscriber.java	94.87% <0%> (-5.13%)	14% <0%> (-1%)
...in/java/io/reactivex/subjects/BehaviorSubject.java	84.97% <0%> (-4.67%)	56% <0%> (+1%)
...a/io/reactivex/internal/util/QueueDrainHelper.java	60.28% <0%> (-4.26%)	32% <0%> (-3%)
...erators/completable/CompletableConcatIterable.java	95.91% <0%> (-4.09%)	2% <0%> (ø)
...vex/internal/operators/parallel/ParallelRunOn.java	94.41% <0%> (-3.05%)	6% <0%> (ø)
... and 35 more

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[akarnokd]

Thanks for the contribution!