[wrapper-tracing] Don't call into V8 during heap snapshot creation

third_party/WebKit/Source/bindings/core/v8/V8GCController.cpp

Index: third_party/WebKit/Source/bindings/core/v8/V8GCController.cpp
diff --git a/third_party/WebKit/Source/bindings/core/v8/V8GCController.cpp b/third_party/WebKit/Source/bindings/core/v8/V8GCController.cpp
index ad0f11b1d9bf46d062554580886b5ca12c7cc158..ff0d71f4f8e33edfea4f877c2dd5c906611e6d2e 100644
--- a/third_party/WebKit/Source/bindings/core/v8/V8GCController.cpp
+++ b/third_party/WebKit/Source/bindings/core/v8/V8GCController.cpp
@@ -195,6 +195,13 @@ class HeapSnaphotWrapperVisitor : public ScriptWrappableVisitor,
   v8::HeapProfiler::RetainerEdges edges() { return std::move(m_edges); }
   v8::HeapProfiler::RetainerGroups groups() { return std::move(m_groups); }
 
+  void markWrappersInAllWorlds(
+      const ScriptWrappable* traceable) const override {

[haraken, 2017/02/03 03:55:45]

What happens if this is called on a worker thread? Won't it end up with tracing
edges on the main thread?

[Michael Lippautz, 2017/02/03 04:49:36]

Snapshots can only be created from the main thread.

+    // Only mark the main thread wrapper as we cannot properly intercept
+    // DOMWrapperMap::markWrapper.

[haraken, 2017/02/03 03:55:45]

Also explain that this means that we won't be able to explain edges from
wrappers in isolated worlds.
 

[Michael Lippautz, 2017/02/03 04:49:36]

Done.

+    traceable->markWrapper(this);
+  }
+
   void markWrapper(const v8::PersistentBase<v8::Value>* value) const override {
     if (m_currentParent && m_currentParent != value)
       m_edges.push_back(std::make_pair(m_currentParent, value));