Determine if OffscreenCanvas is paintable in different rendering contexts

third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/offscreencanvas/OffscreenCanvas.h"
 
 #include "core/dom/ExceptionCode.h"
 #include "core/html/canvas/CanvasContextCreationAttributes.h"
 #include "core/html/canvas/CanvasRenderingContext.h"
 #include "core/html/canvas/CanvasRenderingContextFactory.h"
+#include "platform/graphics/ImageBuffer.h"
 #include "wtf/MathExtras.h"
 #include <memory>
 
 namespace blink {
 
 OffscreenCanvas::OffscreenCanvas(const IntSize& size)
     : m_size(size)
     , m_originClean(true)
 {
 }
@@ skipping 81 matching lines @@
     ASSERT(type < CanvasRenderingContext::ContextTypeCount);
     ASSERT(!renderingContextFactories()[type]);
     renderingContextFactories()[type] = std::move(renderingContextFactory);
 }
 
 bool OffscreenCanvas::originClean() const
 {
     return m_originClean && !m_disableReadingFromCanvas;
 }
 
+bool OffscreenCanvas::isPaintable() const
+{
+    if (!m_context)
+        return ImageBuffer::canCreateImageBuffer(m_size);

[Ken Russell (switch to Gerrit), 2016/08/19 20:31:01]

How reliable is this query, and how much does that matter?

This might return true but fail to allocate the image buffer for several
reasons:

 - It tries to allocate an OpenGL texture but the canvas's width or height is
over the OpenGL maximum texture limit (I'm not sure whether this can happen or
whether e.g. Skia would fall back to software in this case)
 - It might run out of CPU-side memory if the ImageBuffer's backing store is
allocated there
 - More?

+    return m_context->isPaintable();
+}
+
 DEFINE_TRACE(OffscreenCanvas)
 {
     visitor->trace(m_context);
 }
 
 } // namespace blink