Update tracing framework to optionally use a ringbuffer.

base/debug/trace_event_impl.h

Index: base/debug/trace_event_impl.h
diff --git a/base/debug/trace_event_impl.h b/base/debug/trace_event_impl.h
index f405e00116463b52bb53e00946c40cfd58ebb1a6..8e044d3a6661f4b96fb06e84fc09f6c2fd360ab4 100644
--- a/base/debug/trace_event_impl.h
+++ b/base/debug/trace_event_impl.h
@@ -208,6 +208,11 @@ class BASE_EXPORT TraceLog {
   void SetEnabled(bool enabled);
   bool IsEnabled() { return !!enable_count_; }
 
+  void SetContinuousTracing(bool continuous_tracing) {

[nduca, 2013/02/12 19:42:22]

I like it, but can we make it a mode and a required argument on the enable call?
That way we can correctly fail when two different callers enable the system but
in two different modes [which can happen]. And, correclty handle the case where
someone tries to mutate the logging mode when tracing is active.

[dsinclair, 2013/02/25 19:04:34]

Done.

+    continuous_tracing_ = continuous_tracing;
+  }
+  bool GetContinuousTracing() { return continuous_tracing_; }
+
 #if defined(OS_ANDROID)
   static void InitATrace();
 #endif
@@ -364,6 +369,8 @@ class BASE_EXPORT TraceLog {
   static void ApplyATraceEnabledFlag(unsigned char* category_enabled);
 #endif
 
+  void AddEvent(const TraceEvent& event);
+
   // TODO(nduca): switch to per-thread trace buffers to reduce thread
   // synchronization.
   // This lock protects TraceLog member accesses from arbitrary threads.
@@ -371,6 +378,9 @@ class BASE_EXPORT TraceLog {
   int enable_count_;
   NotificationCallback notification_callback_;
   std::vector<TraceEvent> logged_events_;
+  bool continuous_tracing_;

[nduca, 2013/02/12 19:42:22]

Hm... can you factor out logged_events and logged_events_newest_/oldest_ to be a
class that is the Log object? I think it'd benefit us architecturally to have
this just be a log->AddEvent and then just have two log implementations.

[dsinclair, 2013/02/15 19:38:55]

Done.

+  uint32 logged_events_newest_;
+  uint32 logged_events_oldest_;
   std::vector<std::string> included_categories_;
   std::vector<std::string> excluded_categories_;
   bool dispatching_to_observer_list_;