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https://issues.apache.org/jira/browse/ZOOKEEPER-1361?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13186558#comment-13186558
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Hadoop QA commented on ZOOKEEPER-1361:
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+1 overall. Here are the results of testing the latest attachment
http://issues.apache.org/jira/secure/attachment/12510631/ZOOKEEPER-1361.patch
against trunk revision 1231389.
+1 @author. The patch does not contain any @author tags.
+1 tests included. The patch appears to include 3 new or modified tests.
+1 javadoc. The javadoc tool did not generate any warning messages.
+1 javac. The applied patch does not increase the total number of javac
compiler warnings.
+1 findbugs. The patch does not introduce any new Findbugs (version 1.3.9)
warnings.
+1 release audit. The applied patch does not increase the total number of
release audit warnings.
+1 core tests. The patch passed core unit tests.
+1 contrib tests. The patch passed contrib unit tests.
Test results:
https://builds.apache.org/job/PreCommit-ZOOKEEPER-Build/906//testReport/
Findbugs warnings:
https://builds.apache.org/job/PreCommit-ZOOKEEPER-Build/906//artifact/trunk/build/test/findbugs/newPatchFindbugsWarnings.html
Console output:
https://builds.apache.org/job/PreCommit-ZOOKEEPER-Build/906//console
This message is automatically generated.
> 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.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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