Fixed ALF-10665: Immutable caches do not respond well to null (=> @@VALUE_NOT_FOUND@@)

- Immutable caches assume that values are correct and write through
 - Null value markers will get overridden


git-svn-id: https://svn.alfresco.com/repos/alfresco-enterprise/alfresco/HEAD/root@31144 c4b6b30b-aa2e-2d43-bbcb-ca4b014f7261

source/java/org/alfresco/repo/cache/CacheTest.java

@@ -774,6 +774,33 @@ public class CacheTest extends TestCase
         executeAndCheck(callback, false, COMMON_KEY, null);
         executeAndCheck(callback, true, COMMON_KEY, null);
     }
+    /**
+     * <ul>
+     *   <li>Remove from the backing cache</li>
+     *   <li>Add to the transactional cache</li>
+     *   <li>Add to the backing cache</li>
+     *   <li>Commit</li>
+     * </ul>
+     */
+    public void testConcurrentAddAgainstAdd_NoPreExisting()throws Throwable
+    {
+        RetryingTransactionCallback<Object> callback = new RetryingTransactionCallback<Object>()
+        {
+            public Object execute() throws Throwable
+            {
+                backingCache.remove(COMMON_KEY);
+                transactionalCache.put(COMMON_KEY, "aaa2-x");
+                backingCache.put(COMMON_KEY, "aaa2");
+                return null;
+            }
+        };
+        transactionalCache.setMutable(true);
+        executeAndCheck(callback, false, COMMON_KEY, null);
+        executeAndCheck(callback, true, COMMON_KEY, "aaa2");    // Doesn't write through
+        transactionalCache.setMutable(false);
+        executeAndCheck(callback, false, COMMON_KEY, "aaa2-x"); // Always overwrites
+        executeAndCheck(callback, true, COMMON_KEY, "aaa2-x");  // Always overwrites
+    }
     /**
      * <ul>
      *   <li>Add to the backing cache</li>

source/java/org/alfresco/repo/cache/TransactionalCache.java

@@ -838,13 +838,10 @@ public class TransactionalCache<K extends Serializable, V extends Object>
             Object sharedObj = sharedCache.get(key);
             if (!mutable)
             {
-                // Value can't change
+                // The value can't change so we can write through on the assumption
-                if (sharedObj == null)
+                // that the value is always correct
-                {
-                    // Still nothing in the cache
                 sharedCache.put(key, value);
             }
-            }
             else if (readOnly)
             {
                 // Only add if nothing else has been added in the interim