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

["Jessica Cheng Mallet (JIRA)" <ji...@apache.org>]

Jessica Cheng Mallet created SOLR-6631:
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             Summary: 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


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 a NodeChildrenChanged.



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