https://issues.apache.org/jira/browse/AMQ-5668

This commit fixes a race condition in AbstractStoreCursor setLastCacheId that could
cause a null pointer exception in certain cases.

activemq-broker/src/main/java/org/apache/activemq/broker/region/cursors/AbstractStoreCursor.java

@@ -314,10 +314,18 @@ public abstract class AbstractStoreCursor extends AbstractPendingMessageCursor i
     }
 
     private void setLastCachedId(final int index, MessageId candidate) {
-        if (lastCachedIds[index] == null || lastCachedIds[index].getFutureOrSequenceLong() == null) {  // possibly null for topics
+        MessageId lastCacheId = lastCachedIds[index];
+        if (lastCacheId == null) {
             lastCachedIds[index] = candidate;
-        } else if (Long.compare(((Long) candidate.getFutureOrSequenceLong()), ((Long) lastCachedIds[index].getFutureOrSequenceLong())) > 0) {
+        } else {
+            Object lastCacheFutureOrSequenceLong = lastCacheId.getFutureOrSequenceLong();
+            Object candidateOrSequenceLong = candidate.getFutureOrSequenceLong();
+            if (lastCacheFutureOrSequenceLong == null) { // possibly null for topics
                 lastCachedIds[index] = candidate;
+            } else if (candidateOrSequenceLong != null &&
+                    Long.compare(((Long) candidateOrSequenceLong), ((Long) lastCacheFutureOrSequenceLong)) > 0) {
+                lastCachedIds[index] = candidate;
+            }
         }
     }
 

activemq-unit-tests/src/test/java/org/apache/activemq/broker/region/cursors/AbstractStoreCursorNpeTest.java

@@ -0,0 +1,93 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.activemq.broker.region.cursors;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import javax.jms.Connection;
+import javax.jms.DeliveryMode;
+import javax.jms.MessageConsumer;
+import javax.jms.MessageProducer;
+import javax.jms.Session;
+import javax.jms.TextMessage;
+import javax.jms.Topic;
+
+import org.apache.activemq.command.ActiveMQTextMessage;
+import org.apache.activemq.test.TestSupport;
+
+/**
+ * This test shows that a null pointer exception will not occur when unsubscribing from a
+ * subscription while a producer is sending messages rapidly to the topic.  A null pointer
+ * exception was occurring in the setLastCachedId method of AbstractMessageCursor due to
+ * a race condition.  If this test is run before the patch that is applied in this commit
+ * on AbstractStoreCusor, it will consistently fail with a NPE.
+ *
+ */
+public class AbstractStoreCursorNpeTest extends TestSupport {
+
+    protected Connection connection;
+    protected Session session;
+    protected MessageConsumer consumer;
+    protected MessageProducer producer;
+    protected Topic destination;
+
+
+    public void testSetLastCachedIdNPE() throws Exception {
+        connection = createConnection();
+        connection.setClientID("clientId");
+        connection.start();
+        session = connection.createSession(false, Session.AUTO_ACKNOWLEDGE);
+        destination = session.createTopic("test.topic");
+        producer = session.createProducer(destination);
+        producer.setDeliveryMode(DeliveryMode.PERSISTENT);
+
+
+        Connection durableCon = createConnection();
+        durableCon.setClientID("testCons");
+        durableCon.start();
+        final Session durSession = durableCon.createSession(false, Session.AUTO_ACKNOWLEDGE);
+
+        //In a new thread rapidly subscribe and unsubscribe to a durable
+        ExecutorService executorService = Executors.newCachedThreadPool();
+        executorService.execute(new Runnable() {
+            @Override
+            public void run() {
+                try{
+                    //Repeatedly create a durable subscription and then unsubscribe which used to
+                    //cause a NPE while messages were sending
+                    while(true) {
+                        MessageConsumer cons = durSession.createDurableSubscriber(durSession.createTopic("test.topic"), "sub1");
+                        Thread.sleep(100);
+                        cons.close();
+                        durSession.unsubscribe("sub1");
+                    }
+                } catch (Exception ignored) {
+                    ignored.printStackTrace();
+                }
+            }
+        });
+
+        TextMessage myMessage = new ActiveMQTextMessage();
+        myMessage.setText("test");
+        //Make sure that we can send a bunch of messages without a NPE
+        //This would fail if the patch is not applied
+        for (int i = 0; i < 10000; i++) {
+            producer.send(myMessage);
+        }
+    }
+}