WebGL: Restore atomically both WebGraphicsContext3D and DrawingBuffer together.

Source/core/html/canvas/WebGLRenderingContextBase.cpp

Index: Source/core/html/canvas/WebGLRenderingContextBase.cpp
diff --git a/Source/core/html/canvas/WebGLRenderingContextBase.cpp b/Source/core/html/canvas/WebGLRenderingContextBase.cpp
index 71e6f1022aec0114bced0f9d3fee4b4e4a32b73b..700485f902ca552a852dbe746d514871d89ea99d 100644
--- a/Source/core/html/canvas/WebGLRenderingContextBase.cpp
+++ b/Source/core/html/canvas/WebGLRenderingContextBase.cpp
@@ -5339,6 +5339,21 @@ void WebGLRenderingContextBase::maybeRestoreContext(Timer<WebGLRenderingContextB
 
     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);
@@ -5349,16 +5364,7 @@ void WebGLRenderingContextBase::maybeRestoreContext(Timer<WebGLRenderingContextB
         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();