Ensure Origin Thread Affinity for BlameContext

base/trace_event/blame_context.h

Index: base/trace_event/blame_context.h
diff --git a/base/trace_event/blame_context.h b/base/trace_event/blame_context.h
index 81c35afc8b932e0533874dbf43beae418817e6d2..9ec62c5078984d434a5b9e321ea9261f65bb951c 100644
--- a/base/trace_event/blame_context.h
+++ b/base/trace_event/blame_context.h
@@ -9,6 +9,7 @@
 
 #include "base/base_export.h"
 #include "base/macros.h"
+#include "base/threading/thread_checker.h"
 #include "base/trace_event/trace_log.h"
 
 namespace base {
@@ -32,6 +33,19 @@ namespace trace_event {
 // page instead looking at each <iframe> individually. In this case the work
 // should be attributed to a blame context which is the parent of all <iframe>
 // blame contexts.
+//
+//
+// *** THREAD SAFETY ***
+//
+// All member functions of BlameContext, except those marked with "ThreadSafe",
+// must be called from the same thread where the instance is constructed.
+//
+// Any subclass overriding OnTraceLogEnabled()/Disabled() must perform thread
+// checking and make sure that any call to a other non-ThreadSafe function from
+// a different thread is done as a task posted to task_runner().
+//
+// Any subclass must call UnregisterFromTraceLog() at the beginning of its dtor
+// to prevent access from the TraceLog when the instance is being destructed.
 class BASE_EXPORT BlameContext
     : public trace_event::TraceLog::EnabledStateObserver {
  public:
@@ -94,6 +108,7 @@ class BASE_EXPORT BlameContext
   const char* scope() const { return scope_; }
   int64_t id() const { return id_; }
 
+  // ThreadSafe
   // trace_event::TraceLog::EnabledStateObserver implementation:
   void OnTraceLogEnabled() override;
   void OnTraceLogDisabled() override;
@@ -105,6 +120,16 @@ class BASE_EXPORT BlameContext
   // call this base method.
   virtual void AsValueInto(trace_event::TracedValue* state);
 
+  // NOTE: Call this function at the beginning of any subclass's destructor to
+  // prevent access from the TraceLog when the instance is being destructed.
+  void UnregisterFromTraceLog();
+
+  // ThreadSafe
+  // Allow subclasses to access task runner and weak pointers so that they can
+  // override OnTraceLogEnabled()/Disabled() without breaking thread safety.
+  SingleThreadTaskRunner* task_runner() { return task_runner_.get(); }
+  WeakPtr<BlameContext> get_weak() { return weak_factory_.GetWeakPtr(); }
+
  private:
   bool WasInitialized() const;
 
@@ -120,6 +145,13 @@ class BASE_EXPORT BlameContext
 
   const unsigned char* category_group_enabled_;
 
+  // Indicator of whether the instance is registered at the TraceLog.
+  bool registered_;
+
+  ThreadChecker thread_checker_;
+  scoped_refptr<SingleThreadTaskRunner> task_runner_;
+  WeakPtrFactory<BlameContext> weak_factory_;
+
   DISALLOW_COPY_AND_ASSIGN(BlameContext);
 };