canvas: fix bugs on HTMLCanvasElement::copiedImage()

Source/core/html/HTMLCanvasElement.cpp

 /*
  * Copyright (C) 2004, 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 688 matching lines @@
     m_didFailToCreateImageBuffer = !m_imageBuffer;
 }
 
 PassRefPtr<Image> HTMLCanvasElement::copiedImage(SourceDrawingBuffer sourceBuffer) const
 {
     if (!canCreateImageBuffer(size()))
         return nullptr;
     if (!m_context)
         return createTransparentImage(size());
 
-    if (!m_copiedImage && buffer()) {
-        if (m_context && m_context->is3d())
-            m_context->paintRenderingResultsToCanvas(sourceBuffer);
+    // The concept of BackBuffer is valid on only WebGL.
+    if (m_context->is2d())

[aandrey, 2014/12/08 20:36:46]

Seems like this code has no effect (unless a context can be both 2d and 3d).

[dshwang, 2014/12/08 20:46:25]

That's right. It has no effect, because
CanvasRenderingContext2D::paintRenderingResultsToCanvas does nothing. I add it
for documentation purpose.

+        sourceBuffer = FrontBuffer;
+
+    bool needToUpdate = !m_copiedImage;
+    if (m_context->is3d())
+        needToUpdate |= m_context->paintRenderingResultsToCanvas(sourceBuffer);

[dshwang, 2014/12/08 20:17:08]

That's good question! After re-thinking, vector is not needed because this line
lets HTMLCanvasElement::copiedImage know if it's needed to update m_copiedImage.

One thing to note is that m_copiedImage exists to avoid repeated image creation
in one frame, and code without Vector must create image every time if clients
interleave to call BackBuffer and FrontBuffer although it's very hard on current
code base. 

+    if (needToUpdate && buffer()) {
         m_copiedImage = buffer()->copyImage(CopyBackingStore, Unscaled);
         updateExternallyAllocatedMemory();
     }
     return m_copiedImage;
 }
 
 void HTMLCanvasElement::discardImageBuffer()
 {
     m_contextStateSaver.clear(); // uses context owned by m_imageBuffer
     m_imageBuffer.clear();
@@ skipping 79 matching lines @@
 {
     return !originClean();
 }
 
 FloatSize HTMLCanvasElement::sourceSize() const
 {
     return FloatSize(width(), height());
 }
 
 }