[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/timer/timer.h"
 #include "base/trace_event/trace_event_impl.h"
 
 // Fwd.
 template <typename Type>
 struct StaticMemorySingletonTraits;
 
 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();
 
   // Enables/disables exporting of events to ETW. If disabled,
   // AddEvent and AddCustomEvent will simply return when called.
   static void EnableETWExport();
   static void DisableETWExport();
 
-  static bool IsETWExportEnabled() {
-    return (GetInstance() && GetInstance()->etw_match_any_keyword_);
-  }
+  // Returns true if ETW is enabled. For now, this is true if the command line
+  // flag is specified.
+  static bool IsETWExportEnabled();
 
   // Exports an event to ETW. This is mainly used in
   // TraceLog::AddTraceEventWithThreadIdAndTimestamp to export internal events.
   static void AddEvent(
       char phase,
       const unsigned char* category_group_enabled,
       const char* name,
       unsigned long long id,
       int num_args,
       const char** arg_names,
       const unsigned char* arg_types,
       const unsigned long long* arg_values,
       const scoped_refptr<ConvertableToTraceFormat>* convertable_values);
 
   // Exports an event to ETW. This should be used when exporting an event only
   // to ETW. Supports three arguments to be passed to ETW.
   // TODO(georgesak): Allow different providers.
   static void AddCustomEvent(const char* name,
                              const char* phase,
                              const char* arg_name_1,
                              const char* arg_value_1,
                              const char* arg_name_2,
                              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);
 
+  // Starts the timer that keeps polls the keyword for changes.
+  static void StartKeywordUpdateTimer();
+
+  // Exposed for unittesting only, allows resurrecting our
+  // singleton instance post-AtExit processing
   void Resurrect();
 
  private:
   // Ensure only the provider can construct us.
   friend struct StaticMemorySingletonTraits<TraceEventETWExport>;
   TraceEventETWExport();
 
   // Updates the list of enabled categories by consulting the ETW keyword.
-  void UpdateEnabledCategories();
+  // Returns true if there was a change, false if unchanged.
+  bool UpdateEnabledCategories();
 
   // Returns true if the category is enabled.
   bool IsCategoryEnabled(const char* category_name) const;
 
+  // Called back by the timer to check for potential changes to the keyword.
+  static void KeywordUpdateTimerCallback();
+
   // 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_;
 
+  // Repeating timer that is responsible for polling the keyword for changes.
+  base::RepeatingTimer<TraceEventETWExport> keyword_update_timer_;
+
   DISALLOW_COPY_AND_ASSIGN(TraceEventETWExport);
 };
 
 }  // namespace trace_event
 }  // namespace base
 
 #endif  // BASE_TRACE_EVENT_TRACE_EVENT_ETW_EXPORT_WIN_H_