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From "Stirling Chow (JIRA)" <j...@apache.org>
Subject [jira] Created: (AMQ-3016) Race condition in DemandForwardingBridgeSupport can cause remote connection to be leaked.
Date Thu, 04 Nov 2010 20:03:00 GMT
Race condition in DemandForwardingBridgeSupport can cause remote connection to be leaked.
-----------------------------------------------------------------------------------------

                 Key: AMQ-3016
                 URL: https://issues.apache.org/activemq/browse/AMQ-3016
             Project: ActiveMQ
          Issue Type: Bug
          Components: Connector, Transport
    Affects Versions: 5.4.1
            Reporter: Stirling Chow


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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