cc: Fix resource provider fences and add DCHECKs.

cc/gl_renderer.cc

 // Copyright 2010 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 "cc/gl_renderer.h"
 
 #include <set>
 #include <string>
 #include <vector>
 
@@ skipping 1291 matching lines @@
 }
 
 bool GLRenderer::swapBuffers()
 {
     DCHECK(m_visible);
     DCHECK(!m_isBackbufferDiscarded);
 
     TRACE_EVENT0("cc", "GLRenderer::swapBuffers");
     // We're done! Time to swapbuffers!
 
-    scoped_refptr<ResourceProvider::Fence> lastSwapFence = m_resourceProvider->getReadLockFence();
-    if (lastSwapFence)
-        static_cast<SimpleSwapFence*>(lastSwapFence.get())->setHasPassed();
-    m_resourceProvider->setReadLockFence(new SimpleSwapFence());
-
     if (m_capabilities.usingPartialSwap) {
         // If supported, we can save significant bandwidth by only swapping the damaged/scissored region (clamped to the viewport)
         m_swapBufferRect.Intersect(gfx::Rect(gfx::Point(), viewportSize()));
         int flippedYPosOfRectBottom = viewportHeight() - m_swapBufferRect.y() - m_swapBufferRect.height();
         m_context->postSubBufferCHROMIUM(m_swapBufferRect.x(), flippedYPosOfRectBottom, m_swapBufferRect.width(), m_swapBufferRect.height());
     } else {
         // Note that currently this has the same effect as swapBuffers; we should
         // consider exposing a different entry point on WebGraphicsContext3D.
         m_context->prepareTexture();
     }
 
     m_swapBufferRect = gfx::Rect();
 
+    // We don't have real fences, so we mark read fences as passed
+    // assuming a double-buffered GPU pipeline. A texture can be
+    // written to after one full frame has past since it was last read.
+    if (m_lastSwapFence)
+        static_cast<SimpleSwapFence*>(m_lastSwapFence.get())->setHasPassed();
+    m_lastSwapFence = m_resourceProvider->getReadLockFence();
+    m_resourceProvider->setReadLockFence(new SimpleSwapFence());
+
     return true;
 }
 
 void GLRenderer::onSwapBuffersComplete()
 {
     m_client->onSwapBuffersComplete();
 }
 
 void GLRenderer::onMemoryAllocationChanged(WebGraphicsMemoryAllocation allocation)
 {
@@ skipping 473 matching lines @@
 
     releaseRenderPassTextures();
 }
 
 bool GLRenderer::isContextLost()
 {
     return (m_context->getGraphicsResetStatusARB() != GL_NO_ERROR);
 }
 
 }  // namespace cc