mesos/spark — pull request #311 by woggling (2012-11-28 00:22:31+00:00)
Title: Fix NullPointerException when map output unregistered from MapOutputTracker twice

Apparently, if two fetch failed messages are in flight (for the same map output), DAGScheduler will call unregisterMapOutput twice for that output. MapOutputTracker will currently give a NullPointerException in that case, killing the DAGScheduler thread and causing spark to hang. This patch fixes the NPE and adds tests for MapOutputTracker (incl. one which triggers this bug).