https://issues.apache.org/jira/browse/AMQ-6572

On journal start use lastFileLength instead of maxFileLength when
checking for unused space in the last journal file as it is possible the
length of the last journal file is not the same length as maxFileLength

activemq-kahadb-store/src/main/java/org/apache/activemq/store/kahadb/disk/journal/Journal.java

@@ -319,8 +319,9 @@ public class Journal {
         }
 
         // ensure we don't report unused space of last journal file in size metric
-        if (totalLength.get() > maxFileLength && lastAppendLocation.get().getOffset() > 0) {
-            totalLength.addAndGet(lastAppendLocation.get().getOffset() - maxFileLength);
+        int lastFileLength = dataFiles.getTail().getLength();
+        if (totalLength.get() > lastFileLength && lastAppendLocation.get().getOffset() > 0) {
+            totalLength.addAndGet(lastAppendLocation.get().getOffset() - lastFileLength);
         }
 
         cleanupTask = scheduler.scheduleAtFixedRate(new Runnable() {

activemq-kahadb-store/src/test/java/org/apache/activemq/store/kahadb/disk/journal/JournalMaxFileLengthChangeTest.java

@@ -0,0 +1,143 @@
+/**
+ * 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.store.kahadb.disk.journal;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
+
+import java.io.File;
+import java.util.concurrent.TimeUnit;
+
+import org.apache.activemq.broker.ConnectionContext;
+import org.apache.activemq.command.ActiveMQQueue;
+import org.apache.activemq.command.ActiveMQTextMessage;
+import org.apache.activemq.command.MessageId;
+import org.apache.activemq.store.MessageStore;
+import org.apache.activemq.store.kahadb.KahaDBStore;
+import org.junit.After;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
+import org.junit.rules.Timeout;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class JournalMaxFileLengthChangeTest {
+
+    private static final Logger LOG = LoggerFactory.getLogger(JournalMaxFileLengthChangeTest.class);
+
+    private static int ONE_MB = 1024*1024;
+
+    @Rule
+    public TemporaryFolder dataDir = new TemporaryFolder();
+
+    @Rule
+    public Timeout globalTimeout= new Timeout(20, TimeUnit.SECONDS);
+
+    private KahaDBStore store;
+
+    @After
+    public void after() throws Exception {
+        if (store != null) {
+            store.stop();
+        }
+    }
+
+    /**
+     * Test that reported size is correct if the maxFileLength grows
+     * in between journal restarts.  Verify all messages still received.
+     */
+    @Test
+    public void testMaxFileLengthGrow() throws Exception {
+        MessageStore messageStore = createStore(8 * ONE_MB);
+        addMessages(messageStore, 4);
+
+        long sizeBeforeChange = store.getJournal().getDiskSize();
+        LOG.info("Journal size before: " + sizeBeforeChange);
+
+        store.stop();
+        messageStore = createStore(6 * ONE_MB);
+        verifyMessages(messageStore, 4);
+
+        long sizeAfterChange = store.getJournal().getDiskSize();
+        LOG.info("Journal size after: " + sizeAfterChange);
+
+        //verify the size is the same - will be slightly different as checkpoint journal
+        //commands are written but should be close
+        assertEquals(sizeBeforeChange, sizeAfterChange, 4096);
+    }
+
+    /**
+     * Test that reported size is correct if the maxFileLength shrinks
+     * in between journal restarts.  Verify all messages still received.
+     */
+    @Test
+    public void testMaxFileLengthShrink() throws Exception {
+        MessageStore messageStore = createStore(8 * ONE_MB);
+        addMessages(messageStore, 4);
+
+        long sizeBeforeChange = store.getJournal().getDiskSize();
+        LOG.info("Journal size before: " + sizeBeforeChange);
+
+        store.stop();
+        messageStore = createStore(2 * ONE_MB);
+        verifyMessages(messageStore, 4);
+
+        long sizeAfterChange = store.getJournal().getDiskSize();
+        LOG.info("Journal size after: " + sizeAfterChange);
+
+        //verify the size is the same - will be slightly different as checkpoint journal
+        //commands are written but should be close
+        assertEquals(sizeBeforeChange, sizeAfterChange, 4096);
+    }
+
+    private void addMessages(MessageStore messageStore, int num) throws Exception {
+        String text = getString(ONE_MB);
+
+        for (int i=0; i < num; i++) {
+            ActiveMQTextMessage textMessage = new ActiveMQTextMessage();
+            textMessage.setMessageId(new MessageId("1:2:3:" + i));
+            textMessage.setText(text);
+            messageStore.addMessage(new ConnectionContext(), textMessage);
+        }
+    }
+
+    private void verifyMessages(MessageStore messageStore, int num) throws Exception {
+        for (int i=0; i < num; i++) {
+            assertNotNull(messageStore.getMessage(new MessageId("1:2:3:" + i)));
+        }
+    }
+
+    private String getString(int size) {
+        StringBuilder builder = new StringBuilder();
+
+        for (int i = 0; i < size; i++) {
+            builder.append("a");
+        }
+        return builder.toString();
+    }
+
+    private MessageStore createStore(int length) throws Exception {
+        File dataDirectory = dataDir.getRoot();
+        store = new KahaDBStore();
+        store.setJournalMaxFileLength(length);
+        store.setDirectory(dataDirectory);
+        store.setForceRecoverIndex(true);
+        store.start();
+        return store.createQueueMessageStore(new ActiveMQQueue("test"));
+    }
+}