Yuri Tceretian created CURATOR-504:
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Summary: Race conditions in LeaderLatch after reconnecting to
ensemble
Key: CURATOR-504
URL: https://issues.apache.org/jira/browse/CURATOR-504
Project: Apache Curator
Issue Type: Bug
Affects Versions: 4.1.0
Reporter: Yuri Tceretian
Assignee: Jordan Zimmerman
Attachments: 51868597-65791000-231c-11e9-9bfa-1def62bc3ea1.png
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, this can happen in two cases:
https://github.com/apache/curator/blob/4251fe328908e5fca37af034fabc190aa452c73f/curator-recipes/src/main/java/org/apache/curator/framework/recipes/leader/LeaderLatch.java#L529-L536
* 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.
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