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

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

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 177 matching lines @@
         groupChildren.insert(persistentForWrappable(node));
       }
       m_groups.push_back(std::make_pair(new RetainedDOMInfo(groupPair.first),
                                         std::move(groupChildren)));
     }
   }
 
   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));
     m_foundV8Wrappers.insert(value);
   }
 
   void dispatchTraceWrappers(const TraceWrapperBase* traceable) const override {
     if (!m_onlyTraceSingleLevel || !traceable->isScriptWrappable() ||
         !reinterpret_cast<const ScriptWrappable*>(traceable)
              ->containsWrapper() ||
@@ skipping 329 matching lines @@
   double startTime = WTF::currentTimeMS();
   v8::HandleScope scope(isolate);
   PendingActivityVisitor visitor(isolate, executionContext);
   toIsolate(executionContext)->VisitHandlesWithClassIds(&visitor);
   scanPendingActivityHistogram.count(
       static_cast<int>(WTF::currentTimeMS() - startTime));
   return visitor.pendingActivityFound();
 }
 
 }  // namespace blink