Bug 626962. Implement empty transactions for GL. r=bas,a=blocking

gfx/layers/Layers.h

@@ -301,10 +301,8 @@ public:
    * returns false, and the caller must proceed with a normal layer tree
    * update and EndTransaction.
    */
-  virtual bool EndEmptyTransaction()
+  virtual bool EndEmptyTransaction() = 0;
-  {
+
-    return false;
-  }
   /**
    * Function called to draw the contents of each ThebesLayer.
    * aRegionToDraw contains the region that needs to be drawn.

gfx/layers/opengl/LayerManagerOGL.cpp

@@ -392,6 +392,16 @@ LayerManagerOGL::BeginTransactionWithTarget(gfxContext *aTarget)
   mTarget = aTarget;
 }
 
+bool
+LayerManagerOGL::EndEmptyTransaction()
+{
+  if (!mRoot)
+    return false;
+
+  EndTransaction(nsnull, nsnull);
+  return true;
+}
+
 void
 LayerManagerOGL::EndTransaction(DrawThebesLayerCallback aCallback,
                                 void* aCallbackData)
@@ -413,10 +423,7 @@ LayerManagerOGL::EndTransaction(DrawThebesLayerCallback aCallback,
   mThebesLayerCallback = aCallback;
   mThebesLayerCallbackData = aCallbackData;
 
-  // NULL callback means "non-painting transaction"
-  if (aCallback) {
   Render();
-  }
 
   mThebesLayerCallback = nsnull;
   mThebesLayerCallbackData = nsnull;

gfx/layers/opengl/LayerManagerOGL.h

@@ -136,6 +136,7 @@ public:
 
   void EndConstruction();
 
+  virtual bool EndEmptyTransaction();
   virtual void EndTransaction(DrawThebesLayerCallback aCallback,
                               void* aCallbackData);
 

gfx/layers/opengl/ThebesLayerOGL.cpp

@@ -693,6 +693,9 @@ ThebesLayerOGL::RenderLayer(int aPreviousFrameBuffer,
 
     LayerManager::DrawThebesLayerCallback callback =
       mOGLManager->GetThebesLayerCallback();
+    if (!callback) {
+      NS_ERROR("GL should never need to update ThebesLayers in an empty transaction");
+    } else {
       void* callbackData = mOGLManager->GetThebesLayerCallbackData();
       callback(this, state.mContext, state.mRegionToDraw,
                state.mRegionToInvalidate, callbackData);
@@ -702,6 +705,7 @@ ThebesLayerOGL::RenderLayer(int aPreviousFrameBuffer,
       // representation of a region.)
       mValidRegion.Or(mValidRegion, mVisibleRegion);
     }
+  }
 
   DEBUG_GL_ERROR_CHECK(gl());