[profiler] Allow thread-safe access to add/remove code event observers.

src/profiler/profiler-listener.h

Index: src/profiler/profiler-listener.h
diff --git a/src/profiler/profiler-listener.h b/src/profiler/profiler-listener.h
index 7e24ceaa8629d4e9bd34446f6237c62008ed43fe..72feb02ff5829069f197e0f3c83ad0d9967c810a 100644
--- a/src/profiler/profiler-listener.h
+++ b/src/profiler/profiler-listener.h
@@ -79,6 +79,8 @@ class ProfilerListener : public CodeEventListener {
   void RecordDeoptInlinedFrames(CodeEntry* entry, AbstractCode* abstract_code);
   Name* InferScriptName(Name* name, SharedFunctionInfo* info);
   V8_INLINE void DispatchCodeEvent(const CodeEventsContainer& evt_rec) {
+    if (observers_.empty()) return;
+    base::LockGuard<base::Mutex> guard(&mutex_);

[jochen (gone - plz use gerrit), 2016/09/09 13:21:09]

shouldn't you lock before checking observers_?

[alph, 2016/09/09 21:03:30]

I'd like to have a fast path if there're no observers (the common case).

It's ok to have a race here if another thread is adding/removing an observer at
this point.

According to http://www.cplusplus.com/reference/vector/vector/empty/
"No contained elements are accessed: concurrently accessing or modifying them is
safe."

     for (auto observer : observers_) {
       observer->CodeEventHandler(evt_rec);
     }
@@ -87,6 +89,7 @@ class ProfilerListener : public CodeEventListener {
   StringsStorage function_and_resource_names_;
   std::vector<CodeEntry*> code_entries_;
   std::vector<CodeEventObserver*> observers_;
+  base::Mutex mutex_;
 
   DISALLOW_COPY_AND_ASSIGN(ProfilerListener);
 };