1.x: fix and deprecate evicting groupBy and add new overload

[davidmoten]

See #5868

There is a problem with the existing 1.x Observable.groupBy overload with an evicting map factory where depending on the eviction behaviour of the supplied map factory, groups may not be completed and may leak memory. The problem extends to the method signature in that the eviction action should report the evicted GroupedObservable not the corresponding key.

This PR

• fixes the existing method at the expense of doubling up the internal map groups ( into groupsCopy) so that we can always successfully lookup the evicted group from the groupsCopy map. Evictions from groups are mirrored in groupsCopy after the lookup.
• deprecates the problematic overload indicating that it uses more memory than the preferred new overload
• adds a new method groupBy that has the corrected signature and offers bufferSize and delayError parameters for a bit more flexibility and to differentiate the erased signature from the deprecated method
• adds tests that were ported from the 2.x tests

[codecov]

Codecov Report

Merging #5917 into 1.x will decrease coverage by 0.18%.
The diff coverage is 91.33%.

@@             Coverage Diff             @@
##               1.x    #5917      +/-   ##
===========================================
- Coverage     84.2%   84.01%   -0.19%     
+ Complexity    2894     2893       -1     
===========================================
  Files          290      291       +1     
  Lines        18265    18562     +297     
  Branches      2495     2554      +59     
===========================================
+ Hits         15380    15595     +215     
- Misses        2010     2053      +43     
- Partials       875      914      +39

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/rx/Observable.java	99.28% <60%> (-0.36%)	450 <3> (+1)
...in/java/rx/internal/operators/OperatorGroupBy.java	76.55% <75%> (-14.97%)	5 <0> (ø)
...rx/internal/operators/OperatorGroupByEvicting.java	92.33% <92.33%> (ø)	5 <5> (?)
.../java/rx/internal/util/unsafe/MpscLinkedQueue.java	75.75% <0%> (-15.16%)	9% <0%> (-2%)
...ternal/operators/OperatorOnBackpressureLatest.java	78.84% <0%> (-1.93%)	3% <0%> (ø)
...x/internal/operators/DeferredScalarSubscriber.java	98.27% <0%> (-1.73%)	24% <0%> (-1%)
...in/java/rx/internal/operators/OnSubscribeRedo.java	80.8% <0%> (-1.61%)	16% <0%> (ø)
...a/rx/internal/operators/BufferUntilSubscriber.java	71.42% <0%> (-1.59%)	11% <0%> (-1%)
...c/main/java/rx/observables/BlockingObservable.java	85.21% <0%> (-1.41%)	37% <0%> (-1%)
... and 7 more

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[davidmoten]

Added a null check because a race with cancel could clear groupsCopy

[davidmoten]

Can we get another 1.x release with this change soonish?

[akarnokd]

Done.

[davidmoten]

Thank you!