CollectorSink hangs due to ConcurrentModificationException in RollSink

Apparently, there can be a race on the collector ack manager's. It seems that the collector hangs after this happens.

10/12/15 20:08:43 INFO endtoend.AckChecksumChecker: initial checksum is 12ce79b3c59
10/12/15 20:08:43 INFO endtoend.AckChecksumChecker: Finishing checksum group called 'log.00001136.20101215-114127844-0500.6785369377806000.seq'
10/12/15 20:08:43 INFO endtoend.AckChecksumChecker: Checksum succeeded 12ceae86624
Exception in thread "Roll-TriggerThread-1" java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
at java.util.HashMap$KeyIterator.next(HashMap.java:828)
at com.google.common.collect.StandardMultimap$WrappedCollection$WrappedIterator.next(StandardMultimap.java:491)
at com.cloudera.flume.collector.CollectorSink$RollDetectDeco.close(CollectorSink.java:154)
at com.cloudera.flume.handlers.rolling.RollSink.rotate(RollSink.java:181)
at com.cloudera.flume.handlers.rolling.RollSink$TriggerThread.run(RollSink.java:122)
10/12/15 20:08:43 INFO endtoend.AckChecksumChecker: moved from partial to complete log.00001136.20101215-114127844-0500.6785369377806000.seq