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

src/profiler/profiler-listener.h

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_PROFILER_PROFILER_LISTENER_H_
 #define V8_PROFILER_PROFILER_LISTENER_H_
 
 #include <vector>
 
 #include "src/code-events.h"
@@ skipping 61 matching lines @@
   }
   const char* GetFunctionName(const char* name) {
     return function_and_resource_names_.GetFunctionName(name);
   }
 
  private:
   void RecordInliningInfo(CodeEntry* entry, AbstractCode* abstract_code);
   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);
     }
   }
 
   StringsStorage function_and_resource_names_;
   std::vector<CodeEntry*> code_entries_;
   std::vector<CodeEventObserver*> observers_;
+  base::Mutex mutex_;
 
   DISALLOW_COPY_AND_ASSIGN(ProfilerListener);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_PROFILER_PROFILER_LISTENER_H_