Node js mysql连接未释放

Question

我有这样的代码

mysqlPool.getConnection((err, conn) => {
                 if(!err) {
                   async.series([
                     function (callback) {
                       conn.query('SELECT * FROM table1 WHERE ?', [{map_name: user_data.map}], (err, res) => {
                          callback(err, res);
                       });
                     },
                 function (callback) {
                  conn.query('SELECT * from `table2` WHERE `map_name` = ?', [user_data.map], (err, res) => {
                     callback(err, res);
                  });
                 },
                 function (callback) {
                  conn.query('SELECT * FROM table3 INNER JOIN npcs_items ON users_items.item_id = npcs_items.id WHERE users_items.user_id = ?;', user_data.id, (err, res) => {
                      if(err) {
                        callback(err);
                      }
                      else {
                        if(res.length) {
                          const data = JSON.parse(JSON.stringify(res));

                          callback(null, items);
                        }
                      }
                  });
                 },
                 function (callback) {
                  conn.query('SELECT * FROM npcs_list WHERE respawnTime <= ? AND map = ?', [new Date().getTime(), user_data.map], (err, res) => {

                      if(err) {
                        callback(err);
                      } else {
                        if(res.length) {
                          const data = JSON.parse(JSON.stringify(res));

                         callback(null, _data);
                        }
                      }
                  });
                 }
                ], (err, result) => {
                  conn.release();
                });

我的问题是，当套接字发送'init'mysqlPool._allConnections.length增加值时，mysql连接没有释放。有人知道代码有什么问题吗？