upd: ready to fix read in half way of write

backend/src/bs-utility.cpp

@@ -48,4 +48,7 @@ void debugPrint() {
   BookStore_ZYM::debug_Print_Mutex.lock();
   std::cerr << std::endl;
   BookStore_ZYM::debug_Print_Mutex.unlock();
-}
+}
+
+void BlockingStringStream::lock() { custom_mutex.lock(); }
+void BlockingStringStream::unlock() { custom_mutex.unlock(); }