node-mysql是一个node.js下的mysql驱动，前段时间在处理连接池的问题上遇到了连接不释放的疑难杂症，虽已解决，但仍需总结经验避免下次重蹈覆辙。下面是node-mysql中的连接池的部分代码，我加入了详细日志，以作备忘之用。
/*** 在连接池中获取Connection* @param cb* @returns {*}*/Pool.prototype.getConnection = function (cb) {//本地加的日志console.log("getConnection _allConnections.length: %j, _freeConnections.length: %j", this._allConnections.length, this._freeConnections.length);//池关闭检查if (this._closed) {return process.nextTick(function () {return cb(new Error('Pool is closed.'));});}var connection;//检查可用Connection，大于0直接从池中出栈并返回if (this._freeConnections.length > 0) {connection = this._freeConnections.shift();return process.nextTick(function () {return cb(null, connection);});}//检查连接数是否超过上限（默认10），没有超过则创建（真正的CreateConnection）if (this.config.connectionLimit === 0 || this._allConnections.length < this.config.connectionLimit) {connection = new PoolConnection(this, { config: this.config.connectionConfig });//新创建的连接入栈池中this._allConnections.push(connection);//验证新创建的Connection是否可用return connection.connect(function (err) {if (this._closed) {return cb(new Error('Pool is closed.'));}if (err) {return cb(err);}this.emit('connection', connection);return cb(null, connection);}.bind(this));}//检查是否允许排队等待（默认True），False时直接抛错if (!this.config.waitForConnections) {return process.nextTick(function () {return cb(new Error('No connections available.'));});}//检查排队人数是否超过上限（默认0，无限）if (this.config.queueLimit && this._connectionQueue.length >= this.config.queueLimit) {return cb(new Error('Queue limit reached.'));}//开始排队this._connectionQueue.push(cb);};/*** 释放Connection* @param connection*/Pool.prototype.releaseConnection = function (connection) {var cb;//非连接池模式处理if (!connection._pool) {//如果有人排队if (this._connectionQueue.length) {//出栈一个排队回调cb = this._connectionQueue.shift();//调用getConnection返回Connection给予回调使用process.nextTick(this.getConnection.bind(this, cb));}}/***连接池模式处理 有人排队*/else if (this._connectionQueue.length) {cb = this._connectionQueue.shift();//将释放的connection直接给予排队列表的第一个人使用process.nextTick(cb.bind(null, null, connection));}//连接池模式处理 无人排队else {//将释放的connection加入可用连接数组，待使用this._freeConnections.push(connection);//我本地加的日志console.log("releaseConnection _allConnections.length: %j, _freeConnections.length: %j", this._allConnections.length, this._freeConnections.length);}};
