/admin/reassign_partitions deleted before reassignment completes

https://github.com/apache/kafka/blob/0.8.1.1/core/src/main/scala/kafka/controller/KafkaController.scala#L477-L517 describes the process of reassigning partitions. Specifically,by the time /admin/reassign_partitions is updated, the newly assigned replicas (RAR) should be in sync, and the assigned replicas (AR) in ZooKeeper should be updated:

4. Wait until all replicas in RAR are in sync with the leader.
...
10. Update AR in ZK with RAR.
11. Update the /admin/reassign_partitions path in ZK to remove this partition.

This worked in 0.8.1, but in 0.8.1.1 we observe /admin/reassign_partitions being removed before step 4 has completed.

For example, if we have AR [1,2] and then put [3,4] in /admin/reassign_partitions, the cluster will end up with AR [1,2,3,4] and ISR [1,2] when the key is removed. Eventually, the AR will be updated to [3,4].

This means that the kafka-reassign-partitions.sh tool will accept a new batch of reassignments before the current reassignments have finished, and our own tool that feeds in reassignments in small batches (see KAFKA-1677) can't rely on this key to detect active reassignments.

Although we haven't observed this, it seems likely that if a controller resignation happens, the new controller won't know that a reassignment is in progress, and the AR will never be updated to the RAR.