[jira] [Resolved] (SOLR-6631) DistributedQueue spinning on calling zookeeper getChildren()

["Shalin Shekhar Mangar (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/SOLR-6631?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Shalin Shekhar Mangar resolved SOLR-6631.
-----------------------------------------
       Resolution: Fixed
    Fix Version/s: 4.10.4

> DistributedQueue spinning on calling zookeeper getChildren()
> ------------------------------------------------------------
>
>                 Key: SOLR-6631
>                 URL: https://issues.apache.org/jira/browse/SOLR-6631
>             Project: Solr
>          Issue Type: Bug
>          Components: SolrCloud
>            Reporter: Jessica Cheng Mallet
>            Assignee: Timothy Potter
>              Labels: solrcloud
>             Fix For: 4.10.4, 5.0
>
>         Attachments: SOLR-6631.patch, SOLR-6631.patch
>
>
> The change from SOLR-6336 introduced a bug where now I'm stuck in a loop making getChildren() request to zookeeper with this thread dump:
> {quote}
> Thread-51 [WAITING] CPU time: 1d 15h 0m 57s
> java.lang.Object.wait()
> org.apache.zookeeper.ClientCnxn.submitRequest(RequestHeader, Record, Record, ZooKeeper$WatchRegistration)
> org.apache.zookeeper.ZooKeeper.getChildren(String, Watcher)
> org.apache.solr.common.cloud.SolrZkClient$6.execute()<2 recursive calls>
> org.apache.solr.common.cloud.ZkCmdExecutor.retryOperation(ZkOperation)
> org.apache.solr.common.cloud.SolrZkClient.getChildren(String, Watcher, boolean)
> org.apache.solr.cloud.DistributedQueue.orderedChildren(Watcher)
> org.apache.solr.cloud.DistributedQueue.getChildren(long)
> org.apache.solr.cloud.DistributedQueue.peek(long)
> org.apache.solr.cloud.DistributedQueue.peek(boolean)
> org.apache.solr.cloud.Overseer$ClusterStateUpdater.run()
> java.lang.Thread.run()
> {quote}
> Looking at the code, I think the issue is that LatchChildWatcher#process always sets the event to its member variable event, regardless of its type, but the problem is that once the member event is set, the await no longer waits. In this state, the while loop in getChildren(long), when called with wait being Integer.MAX_VALUE will loop back, NOT wait at await because event != null, but then it still will not get any children.
> {quote}
> while (true) \{
>   if (!children.isEmpty()) break;
>   watcher.await(wait == Long.MAX_VALUE ? DEFAULT_TIMEOUT : wait);
>   if (watcher.getWatchedEvent() != null)
>     \{ children = orderedChildren(null); \}
>   if (wait != Long.MAX_VALUE) break;
> \}
> {quote}
> I think the fix would be to only set the event in the watcher if the type is not None.



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