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https://issues.apache.org/jira/browse/ZOOKEEPER-1194?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Mahadev konar updated ZOOKEEPER-1194:
-------------------------------------
Fix Version/s: 3.4.0
> Two possible race conditions during leader establishment
> --------------------------------------------------------
>
> Key: ZOOKEEPER-1194
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1194
> Project: ZooKeeper
> Issue Type: Bug
> Components: server
> Reporter: Alexander Shraer
> Assignee: Alexander Shraer
> Fix For: 3.4.0, 3.5.0
>
> Attachments: zookeeper-1194-ver1.patch, zookeeper-1194-ver2.patch,
> zookeeper-1194.patch
>
>
> Leader.getEpochToPropose() and Leader.waitForNewEpoch() act as barriers -
> they make sure that a leader/follower can return from calling the method only
> once connectingFollowers (or electingFollowers) contain a quorum. But these
> methods don't make sure that the leader itself is in
> connectingFollowers/electingFollowers. So the leader didn't necessarily reach
> the barrier when followers pass it. This can cause the following problems:
> 1. If the leader is not in connectingFollowers when a LearnerHandler returns
> from getEpochToPropose(), then the epoch sent by the leader to the follower
> might be smaller than the leader's own last accepted epoch.
> 2. If the leader is not in electingFollowers when LearnerHandler returns from
> waitForNewEpoch() then the leader will send a NEWLEADER message to followers,
> and the followers will respond, but it is possible that the NEWLEADER message
> is not in outstandingProposals when these NEWLEADER acks arrive, which will
> cause the NEWLEADER acks to be dropped.
> To fix this I propose to explicitly check that the leader is in
> connectingFollowers/electingFollowers before anyone can pass these barriers.
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