Merge pull request #106 from ajohnstonTE/fix-cache-group-race-condition

Fix cache group race condition

gemini/src/main/java/com/techempower/cache/CacheGroup.java

@@ -179,23 +179,38 @@ public void setObjects(Collection<T> objects)
   /**
    * Resets this group, removing all the objects, and setting the initialized
    * flag to false.  The group will be rebuilt from the database on next use.
+   * Can lead to temporary stale data in certain edge cases. See
+   * <a href="https://github.com/TechEmpower/gemini/issues/105">this issue</a>
+   * for details.
    */
   @Override
   public void reset()
   {
     synchronized (this)
     {
       setInitialized(false);
-      // Instantiate a new object so that any threads currently working with
-      // an old reference outside of a synchronized block will not be
-      // affected.
-      this.objects = new ConcurrentHashMap<>();
-      this.objectsInOrder = new CopyOnWriteArrayList<>();
       setErrorOnInitialize(false);
       resetHighLowIdentities();
     }
   }
 
+  /**
+   * Synchronously resets and re-initializes this group, removing all the
+   * objects, and setting the initialized flag to false.  The group will
+   * is then rebuilt from the database before the synchronization block
+   * ends. To avoids stale data in certain edge cases that {@link #reset()}
+   * can lead to.
+   */
+  @Override
+  public void resetSynchronous()
+  {
+    synchronized (this)
+    {
+      reset();
+      initializeIfNecessary();
+    }
+  }
+
   @Override
   public T get(long id)
   {
@@ -1073,4 +1088,4 @@ public Builder<T> constructorArgs(Object... arguments)
 
   } // End Builder.
 
-}   // End CacheGroup.
+}   // End CacheGroup.

gemini/src/main/java/com/techempower/data/EntityGroup.java

@@ -369,6 +369,17 @@ public void reset()
     // Does nothing here.
   }
 
+  /**
+   * Synchronously resets and re-initializes this group of entities.  In
+   * the base class, this doesn't do anything, but subclasses such as
+   * CacheGroup act differently.
+   */
+  public void resetSynchronous()
+  {
+    // Call reset() at least, for subclasses besides CacheGroup.
+    reset();
+  }
+
   /**
    * Returns the comparator to use when sorting entities of this type.
    */