[ETW Export] Add polling for ETW keyword.

base/trace_event/trace_event_etw_export_win.h

 // Copyright 2015 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.
 
 // This file contains the Windows-specific exporting to ETW.
 #ifndef BASE_TRACE_EVENT_TRACE_EVENT_ETW_EXPORT_WIN_H_
 #define BASE_TRACE_EVENT_TRACE_EVENT_ETW_EXPORT_WIN_H_
 
 #include <map>
 
 #include "base/base_export.h"
 #include "base/strings/string_piece.h"
 #include "base/trace_event/trace_event_impl.h"
 
 // Fwd.
 template <typename Type>
 struct StaticMemorySingletonTraits;
 
+namespace {
+class ETWKeywordUpdateThread;
+}
+
 namespace base {
 namespace trace_event {
 
 class BASE_EXPORT TraceEventETWExport {
  public:
   ~TraceEventETWExport();
 
   // Retrieves the singleton.
   // Note that this may return NULL post-AtExit processing.
   static TraceEventETWExport* GetInstance();
@@ skipping 31 matching lines @@
                              const char* arg_value_2,
                              const char* arg_name_3,
                              const char* arg_value_3);
 
   // Returns true if any category in the group is enabled.
   static bool IsCategoryGroupEnabled(const char* category_group_name);
 
  private:
   // Ensure only the provider can construct us.
   friend struct StaticMemorySingletonTraits<TraceEventETWExport>;
+  // To have access to UpdateKeyword().
+  friend class ETWKeywordUpdateThread;
   TraceEventETWExport();
 
   // Updates the list of enabled categories by consulting the ETW keyword.
   // Returns true if there was a change, false otherwise.
   bool UpdateEnabledCategories();
 
   // Returns true if the category is enabled.
   bool IsCategoryEnabled(const char* category_name) const;
 
+  // Called back by the update thread to check for potential changes to the
+  // keyword.
+  static void UpdateETWKeyword();
+
   // True if ETW is enabled. Allows hiding the exporting behind a flag.
   bool etw_export_enabled_;
 
   // Maps category names to their status (enabled/disabled).
   std::map<base::StringPiece, bool> categories_status_;
 
   // Local copy of the ETW keyword.
   uint64 etw_match_any_keyword_;
 
+  // Background thread that monitors changes to the ETW keyword and updates
+  // the enabled categories when a change occurs.
+  scoped_ptr<ETWKeywordUpdateThread> keyword_update_thread_;
+  PlatformThreadHandle keyword_update_thread_handle_;
+
   DISALLOW_COPY_AND_ASSIGN(TraceEventETWExport);
 };
 
 }  // namespace trace_event
 }  // namespace base
 
 #endif  // BASE_TRACE_EVENT_TRACE_EVENT_ETW_EXPORT_WIN_H_