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Date: Tue, 6 Nov 2012 21:52:12 +0000 (UTC)
From: "Timothy Bish (JIRA)" <jira@apache.org>
To: dev@activemq.apache.org
Message-ID: <592909703.77082.1352238733058.JavaMail.jiratomcat@arcas>
Subject: [jira] [Resolved] (AMQ-3016) Race condition in
 DemandForwardingBridgeSupport can cause remote connection to be leaked.
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     [ https://issues.apache.org/jira/browse/AMQ-3016?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Timothy Bish resolved AMQ-3016.
-------------------------------

       Resolution: Fixed
    Fix Version/s: 5.8.0

Noticed this also while doing some work in the bridging code.  
                
> Race condition in DemandForwardingBridgeSupport can cause remote connection to be leaked.
> -----------------------------------------------------------------------------------------
>
>                 Key: AMQ-3016
>                 URL: https://issues.apache.org/jira/browse/AMQ-3016
>             Project: ActiveMQ
>          Issue Type: Bug
>          Components: Connector, Transport
>    Affects Versions: 5.4.1
>            Reporter: Stirling Chow
>             Fix For: 5.8.0
>
>         Attachments: ConnectionLeakTest.java, patch.txt
>
>
> Symptom
> ========
> I set up two Brokers and a network bridge from Broker A to Broker B over HTTP.  When the bridge is established, each Broker has a single transport connection (VM on broker A and HTTP on broker B) as recorded in RegionBroker.connections
> I noticed that when Broker A was stopped (normally), periodically the HTTP connection would remain in Broker B's RegionBroker.connections until the InactivityMonitor on the connection timed out.  If the InactivityMonitor was disbled, then the connection would remain indefinitely.  
> If Broker A was restarted, the bridge would be restarted and a second connection would be recorded in Broker B's RegionBroker.connections.  Repeating restarts of Broker A would cause an accumulation of "dead" connections, which would eventually lead to an OOM.
> Cause
> =====
> When Broker A is stopped, DemandForwardingBridgeSupport.stop() is called and sends a ShutdownInfo command to the local and remote transports.  When the transports receive the ShutdownInfo, they remove the connection from their associated RegionBroker.connections as part of  TransportConnection.processRemoveConnection(ConnectionId, long):
>     public synchronized Response processRemoveConnection(ConnectionId id, long lastDeliveredSequenceId)
>             throws InterruptedException {
> ...
>             try {
>                 broker.removeConnection(cs.getContext(), cs.getInfo(), null);
>             } catch (Throwable e) {
>                 SERVICELOG.warn("Failed to remove connection " + cs.getInfo(), e);
>             }
> In the cases were Broker B would not clean up its connection, I traced the code and discovered that the ShutdownInfo message was not being sent because the remote transport (HttpClientTransport) had already had its "stopped" flag set to true as part of HttpClientTransport.oneway(Object command):
>     public void oneway(Object command) throws IOException {
>         if (isStopped()) {
>             throw new IOException("stopped.");
>         }
> ...
> DemandForwardingBridgeSupport's stop() method has the following code:
>     public void stop() throws Exception {
> ...
>                     ASYNC_TASKS.execute(new Runnable() {
>                         public void run() {
>                             try {
>                                 localBroker.oneway(new ShutdownInfo());
>                                 sendShutdown.countDown();
>                                 remoteBroker.oneway(new ShutdownInfo());
>                             } catch (Throwable e) {
>                                 LOG.debug("Caught exception sending shutdown", e);
>                             } finally {
>                                 sendShutdown.countDown();
>                             }
>                         }
>                     });
>                     if (!sendShutdown.await(10, TimeUnit.SECONDS)) {
>                         LOG.info("Network Could not shutdown in a timely manner");
>                     }
>                 } finally {
>                     ServiceStopper ss = new ServiceStopper();
>                     ss.stop(remoteBroker);
>                     ss.stop(localBroker);
>                     // Release the started Latch since another thread could be
>                     // stuck waiting for it to start up.
>                     startedLatch.countDown();
>                     startedLatch.countDown();
>                     localStartedLatch.countDown();
>                     ss.throwFirstException();
>                 }
>             }
> ShutdownInfo is sent asynchronously to the local and remote transports by a slave thread:
>                                 localBroker.oneway(new ShutdownInfo());
>                                 sendShutdown.countDown();
>                                 remoteBroker.oneway(new ShutdownInfo());
> The sendShutdown  latch is used by the master thread to prevent running the finally clause until the ShutdownInfo has been sent:
>                     if (!sendShutdown.await(10, TimeUnit.SECONDS)) {
>                         LOG.info("Network Could not shutdown in a timely manner");
>                     }
>                 } finally {
>                     ServiceStopper ss = new ServiceStopper();
>                     ss.stop(remoteBroker);
>                     ss.stop(localBroker);
> ...
>                 }
>             }
> However, because the latch is countdown *before* remoteTransport.oneway(new ShutdownInfo()) there is a race condition and in most cases the main thread calls ss.stop(remoteBroker) before the slave thread has completed its call to remoteTransport.oneway(new ShutdownInfo()).  As a result, the remoteTransport appears already stopped and the ShutdownInfo is discarded.  This leaves the connection dangling on the remote broker until the InactivityMonitor closes it.
> Solution
> ======
> The sendShutdown latch should be countdown *after* remoteTransport.oneway(new ShutdownInfo()).

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