Surface Synchronization: Distinguish between dependencies and references

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 62654865f4d51e641bd820b3652b2b945f2c1adc..344eb45d30a9fc8c238c528fb221c88687111196 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -1624,13 +1624,14 @@ CompositorFrameMetadata LayerTreeHostImpl::MakeCompositorFrameMetadata() const {
   for (LayerImpl* surface_layer : active_tree_->SurfaceLayers()) {
     SurfaceLayerImpl* surface_layer_impl =
         static_cast<SurfaceLayerImpl*>(surface_layer);
-    metadata.referenced_surfaces.push_back(
-        surface_layer_impl->primary_surface_info().id());
-    // We need to retain a reference to the fallback surface too so that it's
-    // guaranteed to be available when needed.
-    if (surface_layer_impl->fallback_surface_info().is_valid()) {
+    if (settings_.enable_surface_synchronization) {
+      if (surface_layer_impl->fallback_surface_info().is_valid()) {
+        metadata.referenced_surfaces.push_back(
+            surface_layer_impl->fallback_surface_info().id());
+      }

[piman, 2017/04/12 00:09:34]

Should we add a reference to the primary when we don't have a fallback (because
we don't need one, we're embedding an already known surface)?

[Fady Samuel, 2017/04/12 00:19:44]

Currently we have both primary and fallback SurfaceInfos but we'll only emit a
single SurfaceDrawQuad:

https://cs.chromium.org/chromium/src/cc/layers/surface_layer_impl.cc?q=Surfac...

I don't want to add only the primary if a fallback doesn't exist because the
display compositor may not know about that primary yet, and I don't want to add
a reference to a thing that doesn't yet exist.

In Aura-Mus, we generate a big gutter that fills the window on the first frame
if it arrives late. Other OSes like Windows 10, I noticed does something
similar. 

+    } else {
       metadata.referenced_surfaces.push_back(
-          surface_layer_impl->fallback_surface_info().id());
+          surface_layer_impl->primary_surface_info().id());
     }
   }
   if (!InnerViewportScrollLayer())