apache/incubator-spark — pull request #623 by aarondav (2014-02-20 23:45:50+00:00)
Title: Super minor: Add require for mergeCombiners in combineByKey

We changed the behavior in 0.9.0 from requiring that mergeCombiners be null when mapSideCombine was false to requiring that mergeCombiners *never* be null, for external sorting. This patch adds a require() to make this behavior change explicitly messaged rather than resulting in a NPE.