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https://issues.apache.org/jira/browse/ZOOKEEPER-1361?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13217012#comment-13217012
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Hudson commented on ZOOKEEPER-1361:
-----------------------------------
Integrated in ZooKeeper-trunk #1472 (See
[https://builds.apache.org/job/ZooKeeper-trunk/1472/])
ZOOKEEPER-1361. Leader.lead iterates over 'learners' set without proper
synchronisation (henryr via camille) (Revision 1294000)
Result = SUCCESS
camille :
http://svn.apache.org/viewcvs.cgi/?root=Apache-SVN&view=rev&rev=1294000
Files :
* /zookeeper/trunk/CHANGES.txt
* /zookeeper/trunk/src/java/main/org/apache/zookeeper/server/NIOServerCnxn.java
*
/zookeeper/trunk/src/java/main/org/apache/zookeeper/server/NettyServerCnxn.java
* /zookeeper/trunk/src/java/main/org/apache/zookeeper/server/quorum/Leader.java
*
/zookeeper/trunk/src/java/main/org/apache/zookeeper/server/quorum/LeaderBean.java
*
/zookeeper/trunk/src/java/main/org/apache/zookeeper/server/quorum/QuorumPeer.java
* /zookeeper/trunk/src/java/test/org/apache/zookeeper/test/QuorumTest.java
> Leader.lead iterates over 'learners' set without proper synchronisation
> -----------------------------------------------------------------------
>
> Key: ZOOKEEPER-1361
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1361
> Project: ZooKeeper
> Issue Type: Bug
> Affects Versions: 3.4.2
> Reporter: Henry Robinson
> Assignee: Henry Robinson
> Fix For: 3.5.0
>
> Attachments: ZOOKEEPER-1361-no-whitespace.patch, ZOOKEEPER-1361.patch
>
>
> This block:
> {code}
> HashSet<Long> followerSet = new HashSet<Long>();
> for(LearnerHandler f : learners)
> followerSet.add(f.getSid());
> {code}
> is executed without holding the lock on learners, so if there were ever a
> condition where a new learner was added during the initial sync phase, I'm
> pretty sure we'd see a concurrent modification exception. Certainly other
> parts of the code are very careful to lock on learners when iterating.
> It would be nice to use a {{ConcurrentHashMap}} to hold the learners instead,
> but I can't convince myself that this wouldn't introduce some correctness
> bugs. For example the following:
> Learners contains A, B, C, D
> Thread 1 iterates over learners, and gets as far as B.
> Thread 2 removes A, and adds E.
> Thread 1 continues iterating and sees a learner view of A, B, C, D, E
> This may be a bug if Thread 1 is counting the number of synced followers for
> a quorum count, since at no point was A, B, C, D, E a correct view of the
> quorum.
> In practice, I think this is actually ok, because I don't think ZK makes any
> strong ordering guarantees on learners joining or leaving (so we don't need a
> strong serialisability guarantee on learners) but I don't think I'll make
> that change for this patch. Instead I want to clean up the locking protocols
> on the follower / learner sets - to avoid another easy deadlock like the one
> we saw in ZOOKEEPER-1294 - and to do less with the lock held; i.e. to copy
> and then iterate over the copy rather than iterate over a locked set.
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