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Date: Sun, 20 Feb 2011 19:22:38 +0000 (UTC)
From: "Stirling Chow (JIRA)" <jira@apache.org>
To: dev@activemq.apache.org
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Subject: [jira] Commented: (AMQ-3185) All VM transports on a broker can be
 prematurely closed.
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    [ https://issues.apache.org/jira/browse/AMQ-3185?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12997197#comment-12997197 ] 

Stirling Chow commented on AMQ-3185:
------------------------------------

It should be noted that we originally encountered this error on AMQ 5.3.1.  While the bug was in VMTransport/VMTransportServer, it was exposed by the existence of broker-to-broker bridges that frequently failed (i.e., in our network of brokers, several brokers are for failover and are not started during normal operations).  A number of changes in AMQ 5.4.2, specifically the modification of VMTransport's stop() method to send a ShutdownInfo to the peer rather than triggering its exception handler, prevented the bug from appearing in our runtime environments.  However, further analysis of the AMQ trunk source revealed that the root cause, even though no longer triggered in our runtime environment, was still present --- specifically the ability of VMTransportServer to mistakenly decrement connectionCount multiple times.

Given the reported deadlock with bridge creation, this turned out to be a significant issue with the stability of our system.  We would like the root cause addressed to prevent possible reintroduction in subsequent versions.

> All VM transports on a broker can be prematurely closed.
> --------------------------------------------------------
>
>                 Key: AMQ-3185
>                 URL: https://issues.apache.org/jira/browse/AMQ-3185
>             Project: ActiveMQ
>          Issue Type: Bug
>          Components: Transport
>    Affects Versions: 5.4.2
>            Reporter: Stirling Chow
>            Priority: Critical
>         Attachments: VMTransportClosureTest.java, patch.diff
>
>
> Symptom
> =======
> We have eight servers running AMQ 5.3.1 connected in a network-of-brokers over HTTP.  Each broker maintains local connections to internal consumers using the VM transport.  We were noticing that about once every day, all the local VM connections on a broker would fail with the following error:
> 2010-12-10 04:29:11,663 [processBroker-process-pool-thread-4] ERROR - The worker encountered an exception and will pause for 5 seconds before continuing.
> javax.jms.JMSException: Peer (vm://broker-mbus-200005#1052452) disposed.
> 	at org.apache.activemq.util.JMSExceptionSupport.create(JMSExceptionSupport.java:62)
> 	at org.apache.activemq.ActiveMQMessageConsumer.dequeue(ActiveMQMessageConsumer.java:453)
> 	at org.apache.activemq.ActiveMQMessageConsumer.receive(ActiveMQMessageConsumer.java:570)
> 	at com.invoqsystems.foundation.component.communication.jms.source.JMSMessageSource.getMessage(JMSMessageSource.java:33)
> 	at com.invoqsystems.foundation.component.communication.jms.source.JMSMessageSource.getMessage(JMSMessageSource.java:95)
> 	at com.invoqsystems.foundation.component.communication.jms.worker.MessageProcessingWorker.getTask(MessageProcessingWorker.java:9)
> 	at com.invoqsystems.foundation.component.communication.jms.worker.AbstractWorker.iterate(AbstractWorker.java:14)
> 	at com.invoqsystems.foundation.component.communication.jms.worker.AbstractWorker.runUntilStop(AbstractWorker.java:17)
> 	at com.invoqsystems.foundation.component.communication.jms.worker.AbstractWorker.run(AbstractWorker.java:41)
> 	at java.lang.Thread.run(Unknown Source)
> Caused by: org.apache.activemq.transport.TransportDisposedIOException: Peer (vm://broker-mbus-200005#1052452) disposed.
> 	at org.apache.activemq.transport.vm.VMTransport.stop(VMTransport.java:70)
> 	at org.apache.activemq.transport.TransportFilter.stop(TransportFilter.java:64)
> 	at org.apache.activemq.transport.TransportFilter.stop(TransportFilter.java:64)
> 	at org.apache.activemq.transport.ResponseCorrelator.stop(ResponseCorrelator.java:132)
> 	at org.apache.activemq.broker.TransportConnection.doStop(TransportConnection.java:956)
> 	at org.apache.activemq.broker.TransportConnection$3.run(TransportConnection.java:918)
> 	at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
> 	at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
> 	... 1 more
> This was quite unexpected since we create the VM transport connection at broker startup and maintain it (the single connection) throughout the life of the application.  The connection is only closed when the process terminates and the broker is stopped.
> Since we wrote our consumers against the JMS spec, we handle periodic connection failures by creating a new connection.  This is works fine sometimes; however, because of AMQ-3127, the sudden reregistration of our consumers simultaneously occurring with bridge creation causes frequent deadlock that can only be resolved by restarting the systems.
> Cause
> =====
> We were unable to reliably recreate the failure, so it became clear that a timing issue was involved.  Eventually, we determined the cause of the VM transport failure was due to the following code in VMTransportServer:
>     public VMTransport connect() throws IOException {
>         TransportAcceptListener al;
>         synchronized (this) {
>             if (disposed) {
>                 throw new IOException("Server has been disposed.");
>             }
>             al = acceptListener;
>         }
>         if (al == null) {
>             throw new IOException("Server TransportAcceptListener is null.");
>         }
>         connectionCount.incrementAndGet();
>         VMTransport client = new VMTransport(location) {
>             public void stop() throws Exception {
>                 if (disposed) {
>                     return;
>                 }
>                 super.stop();
>                 if (connectionCount.decrementAndGet() == 0 && disposeOnDisconnect) {
>                     VMTransportServer.this.stop();
>                 }
>             };
>         };
>         VMTransport server = new VMTransport(location);
>         client.setPeer(server);
>         server.setPeer(client);
>         al.onAccept(configure(server));
>         return client;
>     }
> At issue is the override VMTransport.stop() method:
>             public void stop() throws Exception {
>                 if (disposed) {
>                     return;
>                 }
>                 super.stop();
>                 if (connectionCount.decrementAndGet() == 0 && disposeOnDisconnect) {
>                     VMTransportServer.this.stop();
>                 }
>             };
> Note that VMTransport.disposed is used to protect against multiple calls and subsequently multiple decrements of connectionCount.  However, in the implementation of super.stop(), the disposed flag is only set after the peer transport is informed of the stop:
>     public void stop() throws Exception {
>         stopping.set(true);
>         
>         // If stop() is called while being start()ed.. then we can't stop until we return to the start() method.
>         if( enqueueValve.isOn() ) {
>         	
>             // let the peer know that we are disconnecting..
>             try {
> >>>                peer.transportListener.onCommand(new ShutdownInfo());
>             } catch (Exception ignore) {
>             }
>         	
>         	
>             TaskRunner tr = null;
>             try {
>                 enqueueValve.turnOff();
>                 if (!disposed) {
>                     started = false;
> >>>                    disposed = true;
>                     if (taskRunner != null) {
>                         tr = taskRunner;
>                         taskRunner = null;
>                     }
>                 }
>             } finally {
>                 stopping.set(false);
>                 enqueueValve.turnOn();
>             }
>             if (tr != null) {
>                 tr.shutdown(1000);
>             }
>             
>         }
>         
>     }
> TransportConnection implements of peer.transportListener.onCommand(new ShutdownInfo()) by launching a asynchronous task that eventually calls back to the same transport that initiated the closure.  If the timing is right, VMTransportServer's VMTransport.stop() method is called a second time before the disposed flag is set to true.  As a result, the connectionCount is decremented *TWICE* instead of just once.
> In other words, the diposed check and decrement as implemented by VMTransport's anonymous VMTransport subclass are not thread-safe.  If VMTransportServer miscounts the connections, it can end up stopping itself while there are still live connections.  The result is that the live connections see their peer (the server part of the VMTransport) unexpectedly closed.
> Solution
> ========
> The attached patch prevents multiple decrements of the connectionCount by preventing reentrant calls to VMTransportServer's VMTransport stop() method.
> A patch is included which demonstrates the problem with the existing AMQ trunk code.

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