[jira] [Created] (CURATOR-644) CLONE - Race conditions in LeaderLatch after reconnecting to ensemble

["Ken Huang (Jira)" <ji...@apache.org>]

Ken Huang created CURATOR-644:
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             Summary: CLONE - Race conditions in LeaderLatch after reconnecting to ensemble
                 Key: CURATOR-644
                 URL: https://issues.apache.org/jira/browse/CURATOR-644
             Project: Apache Curator
          Issue Type: Bug
    Affects Versions: 4.1.0
            Reporter: Ken Huang
            Assignee: Jordan Zimmerman


We use LeaderLatch in a lot of places in our system and when ZooKeeper ensemble is unstable and clients are reconnecting to logs are full of messages like the following:

{{[2017-08-31 19:18:34,562][ERROR][org.apache.curator.framework.recipes.leader.LeaderLatch] Can't find our node. Resetting. Index: -1 {}}}

According to the [implementation|https://github.com/apache/curator/blob/4251fe328908e5fca37af034fabc190aa452c73f/curator-recipes/src/main/java/org/apache/curator/framework/recipes/leader/LeaderLatch.java#L529-L536], this can happen in two cases:
 * When internal state `ourPath` is null
 * When the list of latches does not have the expected one.

I believe we hit the first condition because of races that occur after client reconnects to ZooKeeper.
 * Client reconnects to ZooKeeper and LeaderLatch gets the event and calls reset method which set the internal state (`ourPath`) to null, removes old latch and creates a new one. This happens in thread "Curator-ConnectionStateManager-0".
 * Almost simultaneously, LeaderLatch gets another even NodeDeleted ([here|https://github.com/apache/curator/blob/4251fe328908e5fca37af034fabc190aa452c73f/curator-recipes/src/main/java/org/apache/curator/framework/recipes/leader/LeaderLatch.java#L543-L554]) and tries to re-read the list of latches and check leadership. This happens in the thread "main-EventThread".

Therefore, sometimes there is a situation when method `checkLeadership` is called when `ourPath` is null.

Below is an approximate diagram of what happens:

!51868597-65791000-231c-11e9-9bfa-1def62bc3ea1.png|width=1261,height=150!



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