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https://issues.apache.org/jira/browse/SOLR-6631?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14188554#comment-14188554
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Jessica Cheng Mallet commented on SOLR-6631:
--------------------------------------------
I originally thought NodeChildrenChanged would be enough too, but it made the
tests hang forever. That's when I realized that the zk.exist() call in offer()
also uses this watcher, so it's not enough to just watch for
NodeChildrenChanged.
We can either make the watcher set all not None events (None events don't
remove watches, so they need to be excluded), or use a different kind of watch
in the zk.exist() call.
> 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
> Attachments: 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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