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https://issues.apache.org/jira/browse/SOLR-16046?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17532981#comment-17532981
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Michael Gibney commented on SOLR-16046:
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I think the core of the issue is that {{ZooKeeper.close()}} is
asynchronous/[does not wait for connection threads to
die|https://github.com/apache/zookeeper/blob/c2ffb55dce01300250d7a4fcb81916966faaeccf/zookeeper-server/src/main/java/org/apache/zookeeper/ZooKeeper.java#L1655-L1656].
This method is called all over the place, basically any time anything
"zookeeper" is closed -- e.g., ZkController, SolrZkClient, ZkTestServer, etc....
It seems possibly connected to a [bunch of
failures|https://lists.apache.org/list?bui...@solr.apache.org:dfr=2022-4-1|dto=2022-5-31:app//org.apache.zookeeper.ClientCnxn$EventThread.run],
and even at that I think this may underestimate the number of failures because
some of these have been BadAppled ...
I think the failures are essentially harmless (i.e. I think the threads are in
the process of quickly dying, they just sometimes get detected before they
exit. [apache/solr#842|https://github.com/apache/solr/pull/842] seeks to
address this issue.
> Thread leak in TestLeaderElectionZkExpiry
> -----------------------------------------
>
> Key: SOLR-16046
> URL: https://issues.apache.org/jira/browse/SOLR-16046
> Project: Solr
> Issue Type: Bug
> Security Level: Public(Default Security Level. Issues are Public)
> Reporter: Houston Putman
> Priority: Major
> Time Spent: 40m
> Remaining Estimate: 0h
>
> Currently the {{TestLeaderElectionZkExpiry.testLeaderElectionWithZkExpiry}}
> test fails roughly 11% of the time due to zombie threads.
> These are {{zkConnectionManagerCallback}} threads used by the SolrZkClient.
> In {{ConnectionManager.process()}}, the method being called by the thread, it
> is currently swallowing up an interrupted exception and always looping until
> a valid connection can be made to zookeeper.
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