renamed a member variable

test/core/object/ObjectCacheTest.cpp

@@ -38,7 +38,7 @@ public:
   : participants_(participants) {}
 
   void arrive_and_wait() {
-    std::unique_lock<std::mutex> lock(m_);
+    std::unique_lock lock(mutex_);
     ++arrived_;
     if (arrived_ >= participants_) {
       open_ = true;
@@ -49,7 +49,7 @@ public:
   }
 
 private:
-  std::mutex m_;
+  std::mutex mutex_;
   std::condition_variable cv_;
   int participants_{0};
   int arrived_{0};