added mutex

src/users/get_all_users.cpp

@@ -33,6 +33,7 @@ void get_all_users(const httplib::Request& request, httplib::Response& response)
         syslog(LOG_ERR, "access reject in get all user request");
         return;
     }
+    std::lock_guard<std::mutex> lock(usersfilemutex);
     std::ifstream usersfile(USERS_FILE);
     nlohmann::json all_users = nlohmann::json::parse(usersfile);
     usersfile.close();