Clarify GraphicsContext::beginLayer()/endLayer() have unexpected behaviors.

Source/core/html/canvas/CanvasRenderingContext2D.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008 Dirk Schulze <krit@webkit.org>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  * Copyright (C) 2012, 2013 Intel Corporation. All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
  *
@@ skipping 1546 matching lines @@
         FloatRect dirtyRect;
         if (computeDirtyRect(dstRect, clipBounds, &dirtyRect)) {
             if (!imageSource->isVideoElement()) {
                 c->drawImage(image.get(), dstRect, srcRect, op, blendMode);
             } else {
                 drawVideo(static_cast<HTMLVideoElement*>(imageSource), srcRect, dstRect);
             }
             didDraw(dirtyRect);
         }
     }
+    validateStateStack();

[dshwang, 2014/10/14 19:00:28]

I move it to here, because drawVideo is used as follows:
c->beginLayer(1, op);
drawVideo();
c->endLayer();

c->beginLayer(1, op) slightly increase state stack while
CanvasRenderingContext2D doesn't know it.

It's a bug. I'll fix it.

 
     if (sourceImageStatus == ExternalSourceImageStatus && isAccelerated() && canvas()->buffer())
         canvas()->buffer()->flush();
 
     if (canvas()->originClean() && wouldTaintOrigin(imageSource))
         canvas()->setOriginTainted();
 }
 
 void CanvasRenderingContext2D::drawVideo(HTMLVideoElement* video, FloatRect srcRect, FloatRect dstRect)
 {
     GraphicsContext* c = drawingContext();
     GraphicsContextStateSaver stateSaver(*c);
     c->clip(dstRect);
     c->translate(dstRect.x(), dstRect.y());
     c->scale(dstRect.width() / srcRect.width(), dstRect.height() / srcRect.height());
     c->translate(-srcRect.x(), -srcRect.y());
     video->paintCurrentFrameInContext(c, IntRect(IntPoint(), IntSize(video->videoWidth(), video->videoHeight())));
     stateSaver.restore();
-    validateStateStack();

[Justin Novosad, 2014/10/14 17:50:25]

why are you removing this?

 }
 
 void CanvasRenderingContext2D::drawImageFromRect(HTMLImageElement* image,
     float sx, float sy, float sw, float sh,
     float dx, float dy, float dw, float dh,
     const String& compositeOperation)
 {
     if (!image)
         return;
     CompositeOperator op;
@@ skipping 44 matching lines @@
     context->fillPath(path);
 }
 
 template<class T> void CanvasRenderingContext2D::fullCanvasCompositedFill(const T& area)
 {
     ASSERT(isFullCanvasCompositeMode(state().m_globalComposite));
 
     GraphicsContext* c = drawingContext();
     ASSERT(c);
     c->beginLayer(1, state().m_globalComposite);
-    CompositeOperator previousOperator = c->compositeOperation();
     c->setCompositeOperation(CompositeSourceOver);
     fillPrimitive(area, c);
-    c->setCompositeOperation(previousOperator);
     c->endLayer();
 }
 
 static void strokePrimitive(const FloatRect& rect, GraphicsContext* context)
 {
     context->strokeRect(rect);
 }
 
 static void strokePrimitive(const Path& path, GraphicsContext* context)
 {
     context->strokePath(path);
 }
 
 template<class T> void CanvasRenderingContext2D::fullCanvasCompositedStroke(const T& area)
 {
     ASSERT(isFullCanvasCompositeMode(state().m_globalComposite));
 
     GraphicsContext* c = drawingContext();
     ASSERT(c);
     c->beginLayer(1, state().m_globalComposite);
-    CompositeOperator previousOperator = c->compositeOperation();
     c->setCompositeOperation(CompositeSourceOver);
     strokePrimitive(area, c);
-    c->setCompositeOperation(previousOperator);
     c->endLayer();
 }
 
 PassRefPtrWillBeRawPtr<CanvasGradient> CanvasRenderingContext2D::createLinearGradient(float x0, float y0, float x1, float y1)
 {
     RefPtrWillBeRawPtr<CanvasGradient> gradient = CanvasGradient::create(FloatPoint(x0, y0), FloatPoint(x1, y1));
     return gradient.release();
 }
 
 PassRefPtrWillBeRawPtr<CanvasGradient> CanvasRenderingContext2D::createRadialGradient(float x0, float y0, float r0, float x1, float y1, float r1, ExceptionState& exceptionState)
@@ skipping 541 matching lines @@
         location = FloatPoint();
     }
 
     FloatRect clipBounds;
     if (!c->getTransformedClipBounds(&clipBounds)) {
         return;
     }
 
     if (isFullCanvasCompositeMode(state().m_globalComposite)) {
         c->beginLayer(1, state().m_globalComposite);
-        CompositeOperator previousOperator = c->compositeOperation();
         c->setCompositeOperation(CompositeSourceOver);
         c->drawBidiText(font, textRunPaintInfo, location, Font::UseFallbackIfFontNotReady);
-        c->setCompositeOperation(previousOperator);
         c->endLayer();
         didDraw(clipBounds);
     } else if (state().m_globalComposite == CompositeCopy) {
         clearCanvas();
         c->drawBidiText(font, textRunPaintInfo, location, Font::UseFallbackIfFontNotReady);
         didDraw(clipBounds);
     } else {
         FloatRect dirtyRect;
         if (computeDirtyRect(textRunPaintInfo.bounds, clipBounds, &dirtyRect)) {
             c->drawBidiText(font, textRunPaintInfo, location, Font::UseFallbackIfFontNotReady);
@@ skipping 210 matching lines @@
 
 unsigned CanvasRenderingContext2D::hitRegionsCount() const
 {
     if (m_hitRegionManager)
         return m_hitRegionManager->getHitRegionsCount();
 
     return 0;
 }
 
 } // namespace blink