canvas.getContext with alpha:undefined should behave like alpha:true

Source/bindings/v8/custom/V8HTMLCanvasElementCustom.cpp

 /*
  * Copyright (C) 2007-2009 Google Inc. All rights reserved.
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 29 matching lines @@
 #include "core/html/HTMLCanvasElement.h"
 #include "core/html/canvas/Canvas2DContextAttributes.h"
 #include "core/html/canvas/CanvasRenderingContext.h"
 #include "core/html/canvas/WebGLContextAttributes.h"
 #include "core/inspector/InspectorCanvasInstrumentation.h"
 #include "wtf/MathExtras.h"
 #include "wtf/text/WTFString.h"
 
 namespace WebCore {
 
+static bool determineAlpha(v8::Handle<v8::Value> alphaValue)

[Justin Novosad, 2014/05/20 19:28:39]

We are going to need a more generic version of this function to handle all the
other attributes that suffer from the same bug with WebGL.  Suggestion:
template<T> static T readDictionaryValue(v8::Handle<v8::Value>, const T&
default)

+{
+    // WebIDL dictionary spec says that setting a dictionary value to undefined
+    // should result in the attribute retaining its default value.
+    if (isUndefinedOrNull(alphaValue))
+        return true;
+
+    return alphaValue->BooleanValue();
+}
+
 void V8HTMLCanvasElement::getContextMethodCustom(const v8::FunctionCallbackInfo<v8::Value>& info)
 {
     v8::Handle<v8::Object> holder = info.Holder();
     v8::Isolate* isolate = info.GetIsolate();
     HTMLCanvasElement* impl = V8HTMLCanvasElement::toNative(holder);
     TOSTRING_VOID(V8StringResource<>, contextIdResource, info[0]);
     String contextId = contextIdResource;
     RefPtr<CanvasContextAttributes> attributes;
     if (contextId == "webgl" || contextId == "experimental-webgl") {
         RefPtr<WebGLContextAttributes> webGLAttributes = WebGLContextAttributes::create();
         if (info.Length() > 1 && info[1]->IsObject()) {
             v8::Handle<v8::Object> jsAttributes = info[1]->ToObject();
             v8::Handle<v8::String> alpha = v8AtomicString(isolate, "alpha");
             if (jsAttributes->Has(alpha))
-                webGLAttributes->setAlpha(jsAttributes->Get(alpha)->BooleanValue());
+                webGLAttributes->setAlpha(determineAlpha(jsAttributes->Get(alpha)));

[Justin Novosad, 2014/05/20 19:28:39]

Theroriginal bug was primarily about 2D canvas. If you are going to fix the
webGL case as well, you should add a test for it, and you should also fix the
other attributes that have the same problem.

             v8::Handle<v8::String> depth = v8AtomicString(isolate, "depth");
             if (jsAttributes->Has(depth))
                 webGLAttributes->setDepth(jsAttributes->Get(depth)->BooleanValue());
             v8::Handle<v8::String> stencil = v8AtomicString(isolate, "stencil");
             if (jsAttributes->Has(stencil))
                 webGLAttributes->setStencil(jsAttributes->Get(stencil)->BooleanValue());
             v8::Handle<v8::String> antialias = v8AtomicString(isolate, "antialias");
             if (jsAttributes->Has(antialias))
                 webGLAttributes->setAntialias(jsAttributes->Get(antialias)->BooleanValue());
             v8::Handle<v8::String> premultipliedAlpha = v8AtomicString(isolate, "premultipliedAlpha");
             if (jsAttributes->Has(premultipliedAlpha))
                 webGLAttributes->setPremultipliedAlpha(jsAttributes->Get(premultipliedAlpha)->BooleanValue());
             v8::Handle<v8::String> preserveDrawingBuffer = v8AtomicString(isolate, "preserveDrawingBuffer");
             if (jsAttributes->Has(preserveDrawingBuffer))
                 webGLAttributes->setPreserveDrawingBuffer(jsAttributes->Get(preserveDrawingBuffer)->BooleanValue());
             v8::Handle<v8::String> failIfMajorPerformanceCaveat = v8AtomicString(isolate, "failIfMajorPerformanceCaveat");
             if (jsAttributes->Has(failIfMajorPerformanceCaveat))
                 webGLAttributes->setFailIfMajorPerformanceCaveat(jsAttributes->Get(failIfMajorPerformanceCaveat)->BooleanValue());
         }
         attributes = webGLAttributes;
     } else {
         RefPtr<Canvas2DContextAttributes> canvas2DAttributes = Canvas2DContextAttributes::create();
         if (info.Length() > 1 && info[1]->IsObject()) {
             v8::Handle<v8::Object> jsAttributes = info[1]->ToObject();
             v8::Handle<v8::String> alpha = v8AtomicString(isolate, "alpha");
             if (jsAttributes->Has(alpha))
-                canvas2DAttributes->setAlpha(jsAttributes->Get(alpha)->BooleanValue());
+                canvas2DAttributes->setAlpha(determineAlpha(jsAttributes->Get(alpha)));
         }
         attributes = canvas2DAttributes;
     }
     CanvasRenderingContext* result = impl->getContext(contextId, attributes.get());
     if (!result) {
         v8SetReturnValueNull(info);
         return;
     }
     if (result->is2d()) {
         v8::Handle<v8::Value> v8Result = toV8(toCanvasRenderingContext2D(result), info.Holder(), info.GetIsolate());
@@ skipping 40 matching lines @@
         quality = info[1]->NumberValue();
         qualityPtr = &quality;
     }
 
     String result = canvas->toDataURL(type, qualityPtr, exceptionState);
     exceptionState.throwIfNeeded();
     v8SetReturnValueStringOrUndefined(info, result, info.GetIsolate());
 }
 
 } // namespace WebCore