2.x: Add Completable.takeUntil(Completable) operator

[akarnokd]

This PR adds the missing dedicated takeUntil operator to Completable.

Previously, the same effect could be achieved via the ambWith but there are two benefits of a dedicated operator:

• easier to discover based on the takeUntil operator in other types
• more direct implementation unlike ambWith which is built upon an N-ary amb operator with additional overhead.

There was a feature request in #3708 some time ago but apparently the issue got closed off after Single.takeUntil was implemented.

[codecov]

Codecov Report

Merging #6079 into 2.x will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6079      +/-   ##
============================================
+ Coverage     98.23%   98.26%   +0.02%     
- Complexity     6189     6194       +5     
============================================
  Files           666      667       +1     
  Lines         44806    44856      +50     
  Branches       6206     6211       +5     
============================================
+ Hits          44014    44076      +62     
+ Misses          249      238      -11     
+ Partials        543      542       -1

Impacted Files	Coverage Δ	Complexity Δ
...s/completable/CompletableTakeUntilCompletable.java	100% <100%> (ø)	2 <2> (?)
src/main/java/io/reactivex/Completable.java	100% <100%> (ø)	116 <1> (+1)	⬆️
...l/operators/observable/ObservableFlatMapMaybe.java	88.88% <0%> (-4.58%)	2% <0%> (ø)
.../operators/mixed/FlowableConcatMapCompletable.java	97.43% <0%> (-2.57%)	2% <0%> (ø)
...perators/mixed/ObservableConcatMapCompletable.java	97.74% <0%> (-2.26%)	3% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	94.56% <0%> (-2.18%)	2% <0%> (ø)
...internal/operators/flowable/FlowableSwitchMap.java	95.75% <0%> (-1.42%)	3% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	95.18% <0%> (-1.07%)	5% <0%> (ø)
...perators/observable/ObservableMergeWithSingle.java	99.06% <0%> (-0.94%)	2% <0%> (ø)
...operators/observable/ObservableMergeWithMaybe.java	99.1% <0%> (-0.9%)	2% <0%> (ø)
... and 16 more

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[artem-zinnatullin]

Emit error if called more than once?

[akarnokd]

No. It could legitimately lose a race against onComplete and we don't do it for any other operator either.

[davidmoten]

I guess the bird has already flown with this one but the operator take refers to onNext events and we are more or less using it for all event types (take till terminates or the other thing terminates). I'm not suggesting a change perhaps a naming review for 3.x.

[akarnokd]

I find it more discoverable this way and besides, the main behavior of takeUntil is to stop the main source and complete the downstream, regardless of there could be items from the main source or not.

[kirk-l]

Shouldn't there be a Completable.takeUntil(Publisher) operator, too? Single has overloads for both Publisher and CompletableSource.

[akarnokd]

@sailorpsy PR welcome.