Flavio Paiva Junqueira commented on ZOOKEEPER-569:

Thanks, Henry, it looks good. I agree with your comment on the confusion 
between LE between instantiated every time it is used, and FLE behaving 
differently. We should really just have one model.

One comment on the patch is that I don't think you need to instantiate 
QuorumCnxManager in mockServer() on the new test. The conditional block that 
checks the listener can also be removed.

> Failure of elected leader can lead to never-ending leader election
> ------------------------------------------------------------------
>                 Key: ZOOKEEPER-569
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-569
>             Project: Zookeeper
>          Issue Type: Bug
>            Reporter: Henry Robinson
>            Assignee: Henry Robinson
>             Fix For: 3.3.0
>         Attachments: zookeeper-569.patch, zookeeper-569.patch, 
> zookeeper-569.patch
> It is possible for basic LeaderElection to enter a situation where it never 
> terminates. 
> As an example, consider a three node cluster A, B and C.
> 1. In the first round, A votes for A, B votes for B and C votes for C
> 2. Since C > B > A, all nodes resolve to vote for C in the second round as 
> there is no first round winner
> 3. A, B vote for C, but C fails.
> 4. C is not elected because neither A nor B hear from it, and so votes for it 
> are discarded
> 5. A and B never reset their votes, despite not hearing from C, so continue 
> to vote for it ad infinitum. 
> Step 5 is the bug. If A and B reset their votes to themselves in the case 
> where the heard-from vote set is empty, leader election will continue.
> I do not know if this affects running ZK clusters, as it is possible that the 
> out-of-band failure detection protocols may cause leader election to be 
> restarted anyhow, but I've certainly seen this in tests. 
> I have a trivial patch which fixes it, but it needs a test (and tests for 
> race conditions are hard to write!)

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