问题描述：
最近在玩node.js连接mysql数据库，也连接成功，可以增删改了。但是有一个问题，每当抛出异常时，我的程序自动停止？？？并且报以下错误：

  events.js:160
      throw er; // Unhandled 'error' event
      ^

Error: Connection lost: The server closed the connection.
    at Protocol.end (D:\SoftWare\ZBJWORKSPACE\WebStormWK\node-log-reg\node_modules\mysql\lib\protocol\Protocol.js:109:13)
    at Socket.<anonymous> (D:\SoftWare\ZBJWORKSPACE\WebStormWK\node-log-reg\node_modules\mysql\lib\Connection.js:109:28)
    at emitNone (events.js:91:20)
    at Socket.emit (events.js:185:7)
    at endReadableNT (_stream_readable.js:974:12)
    at _combinedTickCallback (internal/process/next_tick.js:74:11)
    at process._tickCallback (internal/process/next_tick.js:98:9)

很奇怪为什么自动停止程序？
我的连接数据库程序如下：

var mysql = require('mysql');

var db_config = {
    host: '127.0.0.1',
    port:'3306',
    user: 'root',
    password: 'root',
    database: 'test'
};

function connectServer() {
    var client = mysql.createConnection(db_config);
    return client;
}


function selectFun(client, username, callback) {
    //console.log("查询参数："+username);
    //client为一个mysql连接对象
    client.query('select * from user where name="' + username + '"', function (err, results, fields) {
      // console.log("查询结果："+results);
        if (err) throw err;

        callback(results);
    });
}

function insertFun(client, username, password, callback) {
    var usr = {name:username,pwd:password};
    client.query('insert into user set ?', usr, function (err, result) {
        if (err) {
            console.log("error:" + err.message);
            return err;
        }
        callback(result);
    });
}
exports.connect = connectServer;
exports.selectFun = selectFun;
exports.insertFun = insertFun;