Speculative fix for OnSwapCompositorFrame crasher.

content/browser/frame_host/render_widget_host_view_guest.cc

Index: content/browser/frame_host/render_widget_host_view_guest.cc
diff --git a/content/browser/frame_host/render_widget_host_view_guest.cc b/content/browser/frame_host/render_widget_host_view_guest.cc
index cdb2d1799beccd2d50f0f35f1589df85cae22850..3fd814852f9e1445ecc8af62fe82e213d3628bd5 100644
--- a/content/browser/frame_host/render_widget_host_view_guest.cc
+++ b/content/browser/frame_host/render_widget_host_view_guest.cc
@@ -283,7 +283,7 @@ void RenderWidgetHostViewGuest::OnSwapCompositorFrame(
   if (output_surface_id != last_output_surface_id_ ||
       frame_size != current_surface_size_ ||
       scale_factor != current_surface_scale_factor_ ||
-      guest_->has_attached_since_surface_set()) {
+      (guest_ && guest_->has_attached_since_surface_set())) {
     ClearCompositorSurfaceIfNecessary();
     last_output_surface_id_ = output_surface_id;
     current_surface_size_ = frame_size;
@@ -305,8 +305,14 @@ void RenderWidgetHostViewGuest::OnSwapCompositorFrame(
     // SurfaceLayer.
     cc::SurfaceManager* manager = GetSurfaceManager();
     manager->GetSurfaceForId(surface_id_)->AddDestructionDependency(sequence);
-    guest_->SetChildFrameSurface(surface_id_, frame_size, scale_factor,
-                                 sequence);
+    // TODO(wjmaclean): I'm not sure what it means to create a surface id
+    // without setting it on the child, though since we will in this case be
+    // guaranteed to call ClearCompositorSurfaceIfNecessary() below, I suspect
+    // skipping SetChildFrameSurface() here is irrelevant.
+    if (guest_ && !guest_->is_in_destruction()) {

[Charlie Reis, 2016/09/19 17:29:21]

If the crash is actually a UaF on guest_, then neither a null check nor a method
call on guest_ will avoid it.  We'll just get the UaF on this line instead, when
calling is_in_destruction.

An actual UaF on guest_ might suggest that |this| has been deleted, but we
probably would have crashed much earlier in this method if so.  Thus, I'm not
sure what to recommend.

+      guest_->SetChildFrameSurface(surface_id_, frame_size, scale_factor,
+                                   sequence);
+    }
   }
 
   cc::SurfaceFactory::DrawCallback ack_callback = base::Bind(