Bug 1073219 - Use a simple RAII struct instead of nsRefPtr to manage mLayerCount for MaskLayerImageKey. r=dholbert

layout/base/FrameLayerBuilder.cpp

@@ -1380,7 +1380,8 @@ struct MaskLayerUserData : public LayerUserData
            mAppUnitsPerDevPixel == aOther.mAppUnitsPerDevPixel;
   }
 
-  nsRefPtr<const MaskLayerImageCache::MaskLayerImageKey> mImageKey;
+  // Keeps a MaskLayerImageKey alive by managing its mLayerCount member-var
+  MaskLayerImageCache::MaskLayerImageKeyRef mImageKey;
   // properties of the mask layer; the mask layer may be re-used if these
   // remain unchanged.
   nsTArray<DisplayItemClip::RoundedRect> mRoundedClipRects;
@@ -5914,7 +5915,7 @@ ContainerState::CreateMaskLayer(Layer *aLayer,
   userData->mOffset = newData.mOffset;
   userData->mAppUnitsPerDevPixel = newData.mAppUnitsPerDevPixel;
   userData->mRoundedClipRects.SwapElements(newData.mRoundedClipRects);
-  userData->mImageKey = lookupKey;
+  userData->mImageKey.Reset(lookupKey);
 
   return maskLayer.forget();
 }

layout/base/MaskLayerImageCache.cpp

@@ -57,14 +57,6 @@ MaskLayerImageCache::PutImage(const MaskLayerImageKey* aKey, ImageContainer* aCo
   entry->mContainer = aContainer;
 }
 
-// This case is particularly 'clever' because it uses AddRef/Release to track the use
-// not to release the object.
-template<>
-struct HasDangerousPublicDestructor<MaskLayerImageCache::MaskLayerImageKey>
-{
-  static const bool value = true;
-};
-
 MaskLayerImageCache::MaskLayerImageKey::MaskLayerImageKey()
   : mLayerCount(0)
   , mRoundedClipRects()

layout/base/MaskLayerImageCache.h

@@ -136,8 +136,8 @@ public:
 
     ~MaskLayerImageKey();
 
-    void AddRef() const { ++mLayerCount; }
-    void Release() const
+    void IncLayerCount() const { ++mLayerCount; }
+    void DecLayerCount() const
     {
       NS_ASSERTION(mLayerCount > 0, "Inconsistent layer count");
       --mLayerCount;
@@ -163,7 +163,43 @@ public:
     nsTArray<PixelRoundedRect> mRoundedClipRects;
   };
 
-  
+  /**
+   * This struct maintains a reference to a MaskLayerImageKey, via a variant on
+   * refcounting. When a key is passed in via Reset(), we increment the
+   * passed-in key's mLayerCount, and we decrement its mLayerCount when we're
+   * destructed (or when the key is replaced via a second Reset() call).
+   *
+   * However, unlike standard refcounting smart-pointers, this object does
+   * *not* delete the tracked MaskLayerImageKey -- instead, deletion happens
+   * in MaskLayerImageCache::Sweep(), for any keys whose mLayerCount is 0.
+   */
+  struct MaskLayerImageKeyRef
+  {
+    ~MaskLayerImageKeyRef()
+    {
+      if (mRawPtr) {
+        mRawPtr->DecLayerCount();
+      }
+    }
+
+    MaskLayerImageKeyRef() : mRawPtr(nullptr) {}
+    MaskLayerImageKeyRef(const MaskLayerImageKeyRef&) = delete;
+    void operator=(const MaskLayerImageKeyRef&) = delete;
+
+    void Reset(const MaskLayerImageKey* aPtr)
+    {
+      MOZ_ASSERT(aPtr, "Cannot initialize a MaskLayerImageKeyRef with a null pointer");
+      aPtr->IncLayerCount();
+      if (mRawPtr) {
+        mRawPtr->DecLayerCount();
+      }
+      mRawPtr = aPtr;
+    }
+
+  private:
+    const MaskLayerImageKey* mRawPtr;
+  };
+
   // Find an image container for aKey, returns nullptr if there is no suitable
   // cached image. If there is an image, then aKey is set to point at the stored
   // key for the image.