SupportsUserData is not thread safe

base/supports_user_data.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_SUPPORTS_USER_DATA_H_
 #define BASE_SUPPORTS_USER_DATA_H_
 
 #include <map>
 
 #include "base/base_export.h"
 #include "base/memory/linked_ptr.h"
 #include "base/memory/ref_counted.h"
+#include "base/threading/thread_checker.h"
 
 namespace base {
 
 // This is a helper for classes that want to allow users to stash random data by
 // key. At destruction all the objects will be destructed.
 class BASE_EXPORT SupportsUserData {
  public:
   SupportsUserData();
 
   // Derive from this class and add your own data members to associate extra
   // information with this object. Alternatively, add this as a public base
   // class to any class with a virtual destructor.
   class BASE_EXPORT Data {
    public:
     virtual ~Data() {}
   };
 
   // The user data allows the clients to associate data with this object.
   // Multiple user data values can be stored under different keys.
   // This object will TAKE OWNERSHIP of the given data pointer, and will
   // delete the object if it is changed or the object is destroyed.
   Data* GetUserData(const void* key) const;
   void SetUserData(const void* key, Data* data);
   void RemoveUserData(const void* key);
 
+  // SupportsUserData is not thread-safe, and on debug build will assert it is
+  // only used on one thread. Calling this method allows the caller to hand
+  // the SupportsUserData instance across threads. Use only if you are taking
+  // full control of the synchronization of that hand over.
+  void DetachUserDataThread();
+
  protected:
   virtual ~SupportsUserData();
 
  private:
   typedef std::map<const void*, linked_ptr<Data> > DataMap;
 
   // Externally-defined data accessible by key.
   DataMap user_data_;
+  // Guards usage of |user_data_|
+  ThreadChecker thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(SupportsUserData);
 };
 
 // Adapter class that releases a refcounted object when the
 // SupportsUserData::Data object is deleted.
 template <typename T>
 class UserDataAdapter : public base::SupportsUserData::Data {
  public:
   static T* Get(SupportsUserData* supports_user_data, const char* key) {
    UserDataAdapter* data =
       static_cast<UserDataAdapter*>(supports_user_data->GetUserData(key));
     return static_cast<T*>(data->object_.get());
   }
 
   UserDataAdapter(T* object) : object_(object) {}
   T* release() { return object_.release(); }
 
  private:
   scoped_refptr<T> object_;
 
   DISALLOW_COPY_AND_ASSIGN(UserDataAdapter);
 };
 
 }  // namespace base
 
 #endif  // BASE_SUPPORTS_USER_DATA_H_