Bug 1331925 - blob returns null if canvas width or height is zero. sr=smaug

dom/canvas/CanvasRenderingContextHelper.cpp

@@ -53,7 +53,7 @@ CanvasRenderingContextHelper::ToBlob(JSContext* aCx,
 
       RefPtr<Blob> newBlob = Blob::Create(mGlobal, blob->Impl());
 
-      mBlobCallback->Call(*newBlob, rv);
+      mBlobCallback->Call(newBlob, rv);
 
       mGlobal = nullptr;
       mBlobCallback = nullptr;
@@ -65,6 +65,16 @@ CanvasRenderingContextHelper::ToBlob(JSContext* aCx,
     RefPtr<BlobCallback> mBlobCallback;
   };
 
+   nsIntSize elemSize = GetWidthHeight();
+   if (elemSize.width == 0 || elemSize.height == 0) {
+     // According to spec, blob should return null if either its horizontal
+     // dimension or its vertical dimension is zero. See link below.
+     // https://html.spec.whatwg.org/multipage/scripting.html#dom-canvas-toblob
+     BlobCallback* blobCallback = &aCallback;
+     blobCallback->Call(nullptr, aRv);
+     return;
+   }
+
   RefPtr<EncodeCompleteCallback> callback =
     new EncodeCallback(aGlobal, &aCallback);
 

dom/webidl/HTMLCanvasElement.webidl

@@ -68,4 +68,4 @@ interface MozCanvasPrintState
 
 callback PrintCallback = void(MozCanvasPrintState ctx);
 
-callback BlobCallback = void(Blob blob);
+callback BlobCallback = void(Blob? blob);