CC: Force push properties for all layers when tracing is started

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index 426927aaaba8b18b75a68fdc0c77979e39d026fd..68bfe251020c0e08331fbb75baa6118c14cfabd8 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -215,6 +215,10 @@ static void LayerTreeHostOnOutputSurfaceCreatedCallback(Layer* layer) {
   layer->OnOutputSurfaceCreated();
 }
 
+static void SetNeedsPushPropertiesForLayer(Layer* layer) {
+  layer->SetNeedsPushProperties();
+}
+
 void LayerTreeHost::DeleteContentsTexturesOnImplThread(
     ResourceProvider* resource_provider) {
   DCHECK(proxy_->IsImplThread());
@@ -298,6 +302,23 @@ void LayerTreeHost::FinishCommitOnImplThread(LayerTreeHostImpl* host_impl) {
 
   sync_tree->set_source_frame_number(source_frame_number());
 
+  bool is_new_trace;
+  TRACE_EVENT_IS_NEW_TRACE(&is_new_trace);
+  if (is_new_trace) {
+    // We're about to start tracing snaphsots of layer tree, so force
+    // push properties for all layers to propagate LayerDebugInfo to

[danakj, 2015/02/20 17:13:18]

AFAICT LayerDebugInfo is not an actual type name, so we should not make it look
like one in a comment

+    // the impl side.

[danakj, 2015/02/20 17:13:18]

Can you help fill me in on how this DebugInfo works?

Is the debug info not up to date until a trace starts? Why didn't we just push
it when the layer was created? If not, what generates all the DebugInfo later?
Is there code looking for TRACE_EVENT stuff somewhere else to generate it for
each layer?

+    bool will_snapshot_layer_tree;
+    TRACE_EVENT_CATEGORY_GROUP_ENABLED(
+        TRACE_DISABLED_BY_DEFAULT("cc.debug") "," TRACE_DISABLED_BY_DEFAULT(
+            "devtools.timeline.layers"),

[danakj, 2015/02/20 17:13:18]

If it's reasonable, I would much prefer we expose this as a method on LTH like
SetNextCommitRequiresAllLayersPushProperties() or something. Then we can write
unit tests in cc for it. Then the TRACE_EVENT snooping code can call that
function..

+        &will_snapshot_layer_tree);
+    if (will_snapshot_layer_tree) {
+      LayerTreeHostCommon::CallFunctionForSubtree(
+          root_layer(), base::Bind(SetNeedsPushPropertiesForLayer));

[danakj, 2015/02/20 17:13:18]

I think that walking all the layers to make us walk all the layers is not the
most performant way to go about this. It seems wrong that LayerTreeHost needs to
know this secret knowledge about how Layers trace debug info too. But I have
some other questions above to help tease out our options here..

+    }
+  }
+
   if (needs_full_tree_sync_) {
     sync_tree->SetRootLayer(TreeSynchronizer::SynchronizeTrees(
         root_layer(), sync_tree->DetachLayerTree(), sync_tree));