[jira] [Reopened] (ZOOKEEPER-1361) Leader.lead iterates over 'learners' set without proper synchronisation

["Camille Fournier (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ZOOKEEPER-1361?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Camille Fournier reopened ZOOKEEPER-1361:
-----------------------------------------


Reopening this to propose a 3.4.X branch patch
                
> 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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