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From "Patrick Hunt (JIRA)" <j...@apache.org>
Subject [jira] Commented: (ZOOKEEPER-251) NullPointerException stopping and starting Zookeeper servers
Date Mon, 08 Dec 2008 21:36:44 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-251?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12654583#action_12654583
] 

Patrick Hunt commented on ZOOKEEPER-251:
----------------------------------------

Looks to me like the issue is in the while loop:

            goToNextLog();
            if (!next())
                return;
            while (hdr.getZxid() < zxid) {
                next();  // ISSUE IS HERE
            }

we aren't checking the next return value, we will call next again, which will fail if next
previously returned false (ia will not be null but inputstream might be)

The question in my mind is what Thomas asked - why is hdr zxid < zxid and next returning
false? Is this ok, or signalling a problem elsewhere?


> NullPointerException stopping and starting Zookeeper servers
> ------------------------------------------------------------
>
>                 Key: ZOOKEEPER-251
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-251
>             Project: Zookeeper
>          Issue Type: Bug
>          Components: server
>    Affects Versions: 3.0.0, 3.0.1
>         Environment: Tested with JDK 1.5, Solaris, but I suspect it is not relevant in
this case.
>            Reporter: Thomas Vinod Johnson
>            Assignee: Mahadev konar
>             Fix For: 3.1.0
>
>
> See the following thread for the original report:
> http://mail-archives.apache.org/mod_mbox/hadoop-zookeeper-user/200812.mbox/browser
> Steps to reproduce:
> 1) Start a replicated zookeeper service consisting of 3 zookeeper (3.0.1) servers all
running on the same host (of course, all using their own ports and log directories)
> 2) Create one znode in this ensemble (using the zookeeper client console, I issued 'create
/node1 node1data').
> 3) Stop, then restart a single zookeeper server; moving onto the next one a few seconds
later. 
> 4) Go back to 3. After 4-5 iterations, the following should occur, with the failing server
exiting:
> java.lang.NullPointerException
>         at 
> org.apache.zookeeper.server.persistence.FileTxnLog$FileTxnIterator.next(FileTxnLog.java:447)
>         at 
> org.apache.zookeeper.server.persistence.FileTxnLog$FileTxnIterator.init(FileTxnLog.java:358)
>         at 
> org.apache.zookeeper.server.persistence.FileTxnLog$FileTxnIterator.<init>(FileTxnLog.java:333)
>         at 
> org.apache.zookeeper.server.persistence.FileTxnLog.read(FileTxnLog.java:250)
>         at 
> org.apache.zookeeper.server.persistence.FileTxnSnapLog.restore(FileTxnSnapLog.java:102)
>         at 
> org.apache.zookeeper.server.ZooKeeperServer.loadData(ZooKeeperServer.java:183)
>         at org.apache.zookeeper.server.quorum.Leader.lead(Leader.java:245)
>         at 
> org.apache.zookeeper.server.quorum.QuorumPeer.run(QuorumPeer.java:421)
> 2008-12-08 14:14:24,880 - INFO  
> [QuorumPeer:/0:0:0:0:0:0:0:0:2183:Leader@336] - Shutdown called
> java.lang.Exception: shutdown Leader! reason: Forcing shutdown
>         at 
> org.apache.zookeeper.server.quorum.Leader.shutdown(Leader.java:336)
>         at 
> org.apache.zookeeper.server.quorum.QuorumPeer.run(QuorumPeer.java:427)
> Exception in thread "QuorumPeer:/0:0:0:0:0:0:0:0:2183" 
> java.lang.NullPointerException
>         at 
> org.apache.zookeeper.server.quorum.Leader.shutdown(Leader.java:339)
>         at 
> org.apache.zookeeper.server.quorum.QuorumPeer.run(QuorumPeer.java:427)
> The inputStream field is null, apparently because next is being called 
> at line 358 even after next returns false. Having very little knowledge 
> about the implementation, I don't know if the existence of hdr.getZxid() 
>  >= zxid is supposed to be an invariant across all invocations of the 
> server; however the following change to FileTxnLog.java seems to make 
> the problem go away.
> diff FileTxnLog.java /tmp/FileTxnLog.java
> 358c358,359
> <                 next();
> ---
>  >               if (!next())
>  >                   return;
> 447c448,450
> <                 inputStream.close();
> ---
>  >               if (inputStream != null) {
>  >                   inputStream.close();
>  >               }

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