cc: Make async readback path use async glRreadPixels.

cc/trees/thread_proxy.cc

Index: cc/trees/thread_proxy.cc
diff --git a/cc/trees/thread_proxy.cc b/cc/trees/thread_proxy.cc
index 8e44bb4fe999c6b8254f901471d54ff686f54cf5..6fffabbe1fadd7686d9ec1ef4bc1e6dff04b85a1 100644
--- a/cc/trees/thread_proxy.cc
+++ b/cc/trees/thread_proxy.cc
@@ -535,12 +535,21 @@ void ThreadProxy::Stop() {
   DCHECK(IsMainThread());
   DCHECK(started_);
 
-  // Synchronously deletes the impl.
+  // Synchronously finishes pending GL operations and deletes the impl.
   {
     DebugScopedSetMainThreadBlocked main_thread_blocked(this);
 
     CompletionEvent completion;
     Proxy::ImplThread()->PostTask(
+        base::Bind(&ThreadProxy::FinishGLOnImplThread,
+                   impl_thread_weak_ptr_,
+                   &completion));
+    completion.Wait();
+  } {

[piman, 2013/04/27 00:42:25]

nit: we usually put those on separate lines.

Also, why do they need to be 2 separate tasks?

[danakj, 2013/04/29 14:59:05]

So, my thinking was that Finish will cause callbacks to be queued, but not run.
And we want them to run before we destroy the GLRenderer, or we'll lose those
readback requests.

[piman, 2013/04/29 17:29:13]

Ok, I see. I didn't think it would generally work without race, but thinking
about it some more, it probably does:
- on gpu process, the sync point retire would be passed before the Finish is
done, so the signal IPC would be sent before the FlushSync reply
- on the IO thread, the signal task would be posted before the SyncMessageFilter
gets signalled to continue
- so by the time FinishGLOnImplThread signals the main thread, the task is
posted to finish the readback on the compositor thread, before the task is
posted for the LayerTreeHostClosedOnImplThread so we're good.

Definitely worth a comment though.

[danakj, 2013/04/30 01:30:52]

Done.

+    DebugScopedSetMainThreadBlocked main_thread_blocked(this);
+
+    CompletionEvent completion;
+    Proxy::ImplThread()->PostTask(
         base::Bind(&ThreadProxy::LayerTreeHostClosedOnImplThread,
                    impl_thread_weak_ptr_,
                    &completion));
@@ -1172,6 +1181,14 @@ void ThreadProxy::InitializeOutputSurfaceOnImplThread(
   completion->Signal();
 }
 
+void ThreadProxy::FinishGLOnImplThread(CompletionEvent* completion) {
+  TRACE_EVENT0("cc", "ThreadProxy::LayerTreeHostClosedOnImplThread");

[piman, 2013/04/27 00:42:25]

nit: copy & paste of trace, I saw that.

[danakj, 2013/04/29 14:59:05]

Done. *^_^*

+  DCHECK(IsImplThread());
+  if (layer_tree_host_impl_->resource_provider())
+    layer_tree_host_impl_->resource_provider()->Finish();
+  completion->Signal();
+}
+
 void ThreadProxy::LayerTreeHostClosedOnImplThread(CompletionEvent* completion) {
   TRACE_EVENT0("cc", "ThreadProxy::LayerTreeHostClosedOnImplThread");
   DCHECK(IsImplThread());