Add merge mutex

Former-commit-id: 3e65e9a3195ec1eff635b17997b599dbca1a0457

cpp/src/scheduler/task/SearchTask.cpp

@@ -269,6 +269,7 @@ Status XSearchTask::MergeResult(SearchContext::Id2DistanceMap &distance_src,
         return Status::OK();
     }
 
+    merge_mutex_.lock();
     if (distance_target.empty()) {
         distance_target.swap(distance_src);
         return Status::OK();
@@ -328,6 +329,7 @@ Status XSearchTask::MergeResult(SearchContext::Id2DistanceMap &distance_src,
     }
 
     distance_target.swap(distance_merged);
+    merge_mutex_.unlock();
 
     return Status::OK();
 }

cpp/src/scheduler/task/SearchTask.h

@@ -48,6 +48,8 @@ public:
     int index_type_ = 0;
     ExecutionEnginePtr index_engine_ = nullptr;
     bool metric_l2 = true;
+
+    static std::mutex merge_mutex_;
 };
 
 }