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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CURATOR-157) Avoid stack traces closing PathChildrenCache followed by closing CuratorFramework
Date Thu, 30 Oct 2014 15:15:34 GMT

    [ https://issues.apache.org/jira/browse/CURATOR-157?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14190195#comment-14190195
] 

ASF GitHub Bot commented on CURATOR-157:
----------------------------------------

Github user bdumon commented on a diff in the pull request:

    https://github.com/apache/curator/pull/51#discussion_r19611104
  
    --- Diff: curator-recipes/src/main/java/org/apache/curator/framework/recipes/cache/PathChildrenCache.java
---
    @@ -382,6 +383,11 @@ public void close() throws IOException
                 connectionStateListener = null;
                 childrenWatcher = null;
                 dataWatcher = null;
    +
    +            synchronized (backgroundTaskMonitor) {
    --- End diff --
    
    An empty synchronized block is not like an empty if statement or such, since "getting
the lock" is a meaningful operation by itself. This being said, I could rewrite it to use
an java.util.concurrent Lock object, or maybe something else?
    
    At a high level, my change will make the close() method wait until a potentially running
background refresh operation has finished (and it also assures that no new refresh can start
after the close method has returned). I guess this should be fine, though it could cause some
delay on the close() in case of very large caches. This could be avoided by extra is-closed
checks in the for-loop in processChildren(). I didn't consider this earlier, I can add it
to my patch.


> Avoid stack traces closing PathChildrenCache followed by closing CuratorFramework
> ---------------------------------------------------------------------------------
>
>                 Key: CURATOR-157
>                 URL: https://issues.apache.org/jira/browse/CURATOR-157
>             Project: Apache Curator
>          Issue Type: Improvement
>            Reporter: Bruno Dumon
>            Assignee: Jordan Zimmerman
>         Attachments: LogProblemIllustration.java
>
>
> When closing PathChildrenCache, and immediately afterwards closing CuratorFramework,
some ERROR-level stack traces are logged.
> This was previously reported on the mailing list: http://curator.markmail.org/thread/bmfr62ekx5p2vv7f
> The cause is that the BackgroundCallback defined in PathChildrenCache.refresh() will,
when triggered, perform some more ZooKeeper operations.
> Thus one can get in sequences such as:
>  * operation with BackgroundCallback is submitted
>  * processResult of the BackgroundCallback is called
>  * PathChildrenCache is closed
>  * CuratorFramework is closed
>  * processResult, which is running on another thread, comes to the point it does operations
on ZooKeeper, which fail because ZooKeeper is closed.
> There is no real impact on the application, it is just for log-esthetical reasons that
I'd like to avoid it.
> In the more common case, the processResult will receive an IllegalStateException, which
could be easily catched and ignored in PathChildrenCache if the PathChildrenCache is closed:
> {noformat}
> 14/10/30 11:24:51 ERROR org.apache.curator.framework.imps.CuratorFrameworkImpl: Background
exception was not retry-able or retry gave up
> java.lang.IllegalStateException: instance must be started before calling this method
> 	at com.google.common.base.Preconditions.checkState(Preconditions.java:149)
> 	at org.apache.curator.framework.imps.CuratorFrameworkImpl.getData(CuratorFrameworkImpl.java:360)
> 	at org.apache.curator.framework.recipes.cache.PathChildrenCache.getDataAndStat(PathChildrenCache.java:545)
> 	at org.apache.curator.framework.recipes.cache.PathChildrenCache.processChildren(PathChildrenCache.java:668)
> 	at org.apache.curator.framework.recipes.cache.PathChildrenCache.access$200(PathChildrenCache.java:68)
> 	at org.apache.curator.framework.recipes.cache.PathChildrenCache$4.processResult(PathChildrenCache.java:490)
> 	at org.apache.curator.framework.imps.CuratorFrameworkImpl.sendToBackgroundCallback(CuratorFrameworkImpl.java:715)
> 	at org.apache.curator.framework.imps.CuratorFrameworkImpl.processBackgroundOperation(CuratorFrameworkImpl.java:502)
> 	at org.apache.curator.framework.imps.GetChildrenBuilderImpl$2.processResult(GetChildrenBuilderImpl.java:166)
> 	at org.apache.zookeeper.ClientCnxn$EventThread.processEvent(ClientCnxn.java:590)
> 	at org.apache.zookeeper.ClientCnxn$EventThread.run(ClientCnxn.java:498)
> {noformat}
> But sometimes it also fails with other async operations deeper down:
> {noformat}
> 14/10/30 11:24:51 ERROR org.apache.curator.framework.imps.CuratorFrameworkImpl: Background
exception was not retry-able or retry gave up
> java.lang.IllegalStateException: Client is not started
> 	at com.google.common.base.Preconditions.checkState(Preconditions.java:149)
> 	at org.apache.curator.CuratorZookeeperClient.getZooKeeper(CuratorZookeeperClient.java:113)
> 	at org.apache.curator.framework.imps.CuratorFrameworkImpl.getZooKeeper(CuratorFrameworkImpl.java:474)
> 	at org.apache.curator.framework.imps.GetDataBuilderImpl.performBackgroundOperation(GetDataBuilderImpl.java:263)
> 	at org.apache.curator.framework.imps.OperationAndData.callPerformBackgroundOperation(OperationAndData.java:65)
> 	at org.apache.curator.framework.imps.CuratorFrameworkImpl.performBackgroundOperation(CuratorFrameworkImpl.java:789)
> 	at org.apache.curator.framework.imps.CuratorFrameworkImpl.processBackgroundOperation(CuratorFrameworkImpl.java:487)
> 	at org.apache.curator.framework.imps.GetDataBuilderImpl.forPath(GetDataBuilderImpl.java:275)
> 	at org.apache.curator.framework.imps.GetDataBuilderImpl.forPath(GetDataBuilderImpl.java:41)
> 	at org.apache.curator.framework.recipes.cache.PathChildrenCache.getDataAndStat(PathChildrenCache.java:545)
> 	at org.apache.curator.framework.recipes.cache.PathChildrenCache.processChildren(PathChildrenCache.java:668)
> 	at org.apache.curator.framework.recipes.cache.PathChildrenCache.access$200(PathChildrenCache.java:68)
> 	at org.apache.curator.framework.recipes.cache.PathChildrenCache$4.processResult(PathChildrenCache.java:490)
> 	at org.apache.curator.framework.imps.CuratorFrameworkImpl.sendToBackgroundCallback(CuratorFrameworkImpl.java:715)
> 	at org.apache.curator.framework.imps.CuratorFrameworkImpl.processBackgroundOperation(CuratorFrameworkImpl.java:502)
> 	at org.apache.curator.framework.imps.GetChildrenBuilderImpl$2.processResult(GetChildrenBuilderImpl.java:166)
> 	at org.apache.zookeeper.ClientCnxn$EventThread.processEvent(ClientCnxn.java:590)
> 	at org.apache.zookeeper.ClientCnxn$EventThread.run(ClientCnxn.java:498)
> {noformat}
> Therefore I have created a patch where PathChildrenCache.close() will wait until the
possibly running BackgroundCallback is finished.
> I will also attach a small class that illustrates the problem.



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