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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-2789) Race condition in ipc.Server prevents responce being written back to client.
Date Thu, 07 Feb 2008 02:13:11 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-2789?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12566433#action_12566433
] 

Raghu Angadi commented on HADOOP-2789:
--------------------------------------

Thanks Clint. We should include your test in any apatch committed for this issue.

Both cancelling and and registering the channel could be done by responder  or synchronized
differently. Also we could fix the potential problem with doPurge().

> Race condition in ipc.Server prevents responce being written back to client.
> ----------------------------------------------------------------------------
>
>                 Key: HADOOP-2789
>                 URL: https://issues.apache.org/jira/browse/HADOOP-2789
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: ipc
>    Affects Versions: 0.16.0
>            Reporter: Clint Morgan
>            Priority: Critical
>         Attachments: failure-with-patch.log, failure.log, HADOOP-2789.patch, success.log
>
>
> I encountered a race condition in ipc.Server when writing the response
> back to the socket. Sometimes the write SelectKey is being canceled
> when it should not be, and thus the full response never gets
> written. This results in clients timing out on the socket while waiting for the response.
> I am attaching a unit test that demonstrates the problem. It follows
> closely the TestIPC test, however the socket output buffer is set
> smaller than the result being sent back, so that partial writes
> occur. I also put random sleep in the client to help provoke the race
> condition.
> On my machine this fails over half of the time.
> Looking at the code in ipc.Server.java. The problem is manifested in
> Responder.doAsyncWrite(). If I comment out the key.cancel() line, then
> everything works fine. 
> So we need to identify when to safely cancel the key.
> I tried the following:
> {noformat}
>     private void doAsyncWrite(SelectionKey key) throws IOException {
>       Call call = (Call)key.attachment();
>       if (call == null) {
>         return;
>       }
>       if (key.channel() != call.connection.channel) {
>         throw new IOException("doAsyncWrite: bad channel");
>       }
>       if (processResponse(call.connection.responseQueue)) {
>           synchronized(call.connection.responseQueue) {
>               if (call.connection.responseQueue.size() == 0) {
>                   LOG.info("Cancelling key for call "+call.toString()+ " key: "+ key.toString());
>                   key.cancel();          // remove item from selector.
>               } else {
>                   LOG.warn("NOT REALLY DONE: "+call.toString()+ " key: "+ key.toString());
>               }
>           }
>       }
>     }
> {noformat}
> And this does catch some of the cases (EG, the LOG.warn message gets hit), but i still
hit the race condition.

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