[jboss-jira] [JBoss JIRA] Closed: (JBAS-5228) Race condition maintaining acknowledgements when pushing messages to the client

[Adrian Brock (JIRA)]

     [ http://jira.jboss.com/jira/browse/JBAS-5228?page=all ]

Adrian Brock closed JBAS-5228.
------------------------------

    Resolution: Done

> Race condition maintaining acknowledgements when pushing messages to the client
> -------------------------------------------------------------------------------
>
>                 Key: JBAS-5228
>                 URL: http://jira.jboss.com/jira/browse/JBAS-5228
>             Project: JBoss Application Server
>          Issue Type: Bug
>      Security Level: Public(Everyone can see) 
>          Components: JMS service
>    Affects Versions: JBossAS-4.2.2.GA
>            Reporter: Adrian Brock
>         Assigned To: Adrian Brock
>             Fix For: JBossAS-5.0.0.CR1, JBossAS-4.2.3.GA
>
>
> There is a race condition when maintaining/checking message acknowledgements
> for a subscription when messages are pushed to the client.
> i.e. when the client asks for a message and there are none, the client is registered
> as wanting a message and when a message arrives, the message is sent to the client.
> This involves maintaining a "list" of unacknowledged messages for that client/subscription
> which must be checked before we allow removal of the the subscription during close.
> However, for pushing the maintenance of the unacknowledged messages can
> race with the close of the subscription because they are not maintained in the same synchronized block
> as deciding the client wants the message.
> BasicQueue::internalAddMessage()
>       try
>       {
>          Subscription found = null;
> // In this synchronized block we decide that a client wants the message
>          synchronized (receivers)
>          {
>             if (receivers.size() != 0)
>             {
>                for (Iterator it = receivers.iterator(); it.hasNext();)
>                {
>                   Subscription sub = (Subscription) it.next();
>                   if (sub.accepts(message.getHeaders()))
>                   {
>                      it.remove();
>                      found = sub;
>                      break;
>                   }
>                }
>             }
> ...
> But only later outside this block do we maintain the unacknowledged messages
>          
>          // Queue to the receiver
>          if (found != null)
>             queueMessageForSending(found, message);
> This means that during the ... above a close of the subscription which leads to an invocation
> of hasUnackedMessages() could decide there are no unacknoweldged messages.

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