committedLog in ZKDatabase is not properly synchronized
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                 Key: ZOOKEEPER-783
                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-783
             Project: Zookeeper
          Issue Type: Bug
          Components: server
    Affects Versions: 3.3.1
            Reporter: Henry Robinson
            Priority: Critical


ZKDatabase.getCommittedLog() returns a reference to the LinkedList<Proposal> 
committedLog in ZKDatabase. This is then iterated over by at least one caller. 

I have seen a bug that causes a NPE in LinkedList.clear on committedLog, which 
I am pretty sure is due to the lack of synchronization. This bug has not been 
apparent in normal ZK operation, but in code that I have that starts and stops 
a ZK server in process repeatedly (clear() is called from 
ZooKeeperServerMain.shutdown()). 

It's better style to defensively copy the list in getCommittedLog, and to 
synchronize on the list in ZKDatabase.clear.



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