Github user aarondav commented on the pull request:
https://github.com/apache/spark/pull/1461#issuecomment-49405145
This PR also changed the behavior with respect to mutating CoGroups -- now
we mutate combiner1 in place rather than returning a new ArrayBuffer. This is
_probably_ not an issue given our current usage of these combiners, but we
should be careful when making such seemingly innocuous changes, as they are
certainly not worth introducing a bug.
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