WebGL: Restore atomically both WebGraphicsContext3D and DrawingBuffer together.

Source/core/html/canvas/WebGLRenderingContextBase.cpp

 /*
  * Copyright (C) 2009 Apple Inc. 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.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 5321 matching lines @@
     if (!frame)
         return;
 
     Settings* settings = frame->settings();
 
     if (!frame->loader().client()->allowWebGL(settings && settings->webGLEnabled()))
         return;
 
     blink::WebGraphicsContext3D::Attributes attributes = m_requestedAttributes->attributes(canvas()->document().topDocument().url().string(), settings);
     OwnPtr<blink::WebGraphicsContext3D> context = adoptPtr(blink::Platform::current()->createOffscreenGraphicsContext3D(attributes));
+    RefPtr<DrawingBuffer> drawingBuffer;
+    if (context) {
+        // If the context was lost due to RealLostContext, we need to release FBOs before creating new DrawingBuffer to ensure resource budget enough.
+        m_drawingBuffer->releaseResources();
+        RefPtr<WebGLRenderingContextEvictionManager> contextEvictionManager = adoptRef(new WebGLRenderingContextEvictionManager());
+
+        // Construct a new drawing buffer with the new WebGraphicsContext3D.
+        DrawingBuffer::PreserveDrawingBuffer preserve = m_requestedAttributes->preserveDrawingBuffer() ? DrawingBuffer::Preserve : DrawingBuffer::Discard;
+        drawingBuffer = DrawingBuffer::create(context.get(), clampedCanvasSize(), preserve, contextEvictionManager.release());
+
+        if (drawingBuffer->isZeroSized()) {
+            drawingBuffer.clear();
+            context.clear();
+        }
+    }
     if (!context) {
         if (m_contextLostMode == RealLostContext) {
             m_restoreTimer.startOneShot(secondsBetweenRestoreAttempts, FROM_HERE);
         } else {
             // This likely shouldn't happen but is the best way to report it to the WebGL app.
             synthesizeGLError(GL_INVALID_OPERATION, "", "error restoring context");
         }
         return;
     }
 
-    RefPtr<WebGLRenderingContextEvictionManager> contextEvictionManager = adoptRef(new WebGLRenderingContextEvictionManager());
-
-    // Construct a new drawing buffer with the new WebGraphicsContext3D.
-    m_drawingBuffer->releaseResources();
-    DrawingBuffer::PreserveDrawingBuffer preserve = m_requestedAttributes->preserveDrawingBuffer() ? DrawingBuffer::Preserve : DrawingBuffer::Discard;
-    m_drawingBuffer = DrawingBuffer::create(context.get(), clampedCanvasSize(), preserve, contextEvictionManager.release());
-
-    if (m_drawingBuffer->isZeroSized())
-        return;

[dshwang, 2014/03/14 07:48:34]

Unfortunately, this change is very hard to be covered by test, because
1. after fail here, WebGLRenderingContextBase will never use m_drawingBuffer
again.
2. making m_drawingBuffer->isZeroSized() fail is hard. we seem to need internal
test api to change webgl memory budget.

It's kind of logical bug fix not based on real case.

Currently, when drawingBuffer fails to alloc fbo, return here, without
_m_contextLost = false;_ in 5367 line.
It's very weird state. WebGLRenderingContextBase will never use m_drawingBuffer
because isLostContext() is true, but does not emit GL_INVALID_OPERATION
exception or try to restore again.

This CL's goal is to extract logic change from the refactory CL to re-land it;
https://codereview.chromium.org/163773007/

-
+    m_drawingBuffer = drawingBuffer.release();
     m_drawingBuffer->bind();

[Ken Russell (switch to Gerrit), 2014/03/14 01:49:19]

If the code path "if (drawingBuffer->isZeroSized())" is taken above, then
m_drawingBuffer will be a NULL pointer here and this will still crash. Correct?

[Ken Russell (switch to Gerrit), 2014/03/14 02:56:46]

Never mind, I'm wrong.

 
     m_lostContextErrors.clear();
 
     m_context = context.release();
     m_contextLost = false;
 
     setupFlags();
     initializeNewContext();
     markContextChanged(CanvasContextChanged);
     canvas()->dispatchEvent(WebGLContextEvent::create(EventTypeNames::webglcontextrestored, false, true, ""));
@@ skipping 186 matching lines @@
         if (m_textureUnits[i].m_texture2DBinding
             || m_textureUnits[i].m_textureCubeMapBinding) {
             m_onePlusMaxNonDefaultTextureUnit = i + 1;
             return;
         }
     }
     m_onePlusMaxNonDefaultTextureUnit = 0;
 }
 
 } // namespace WebCore