pool.query fails after calling pool.end

Sometimes, I need to close the old pool each 5 mins and create a new one. So I can’t make sure pool.end is called after all queries end. Since it is said

The end method takes an optional callback that you can use to know once all the connections have ended. The connections end gracefully, so all pending queries will still complete and the time to end the pool will vary.

in the doc, I think I can just call pool.end any time I want, but I will occasionally get an error:

Error: Pool is closed.

So, What does pending queries mean in the doc? Is a query which have been called pool.query a pending query?

For example,

var mysql = require('mysql');
var pool  = mysql.createPool({
  connectionLimit : 10,
  host            : 'example.org',
  user            : 'bob',
  password        : 'secret',
  database        : 'my_db'
});

pool.query('SELECT 1 + 1 AS solution', function (error, results, fields) {
  if (error) throw error;
  console.log('The solution is: ', results[0].solution);
});
pool.end();  // will be called somewhere else, but will raise the same error here

I’ve found this commit 76de66e2c36a6da71dcd010b3afe0029e5eb68bb fixed pool.getConnection race conditions, but I don’t quite understand what are the race conditions here. In my opinion, after I have called pool.query, this query is a pending query no matter what phase the operation is under the hood such as ping.

[HQidea]

example:

const mysql = require('mysql');

class Driver {
  constructor() {
    this.pool = null;
  }

  getPool() {
    if (!this.pool) {
      const pool = this.pool = mysql.createPool({
        host: '',
        port: 3306,
        user: '',
        password: '',
        database: ''
      });
      setTimeout(() => {
        this.pool = null;
        pool.end();
      }, 1000);
    }
    return this.pool;
  }

  query(cb) {
    this.getPool().query('show tables', cb);
  }
}

const driver = new Driver();

function call() {
  driver.query((err, data) => {
    if (err) {
      console.log(err);
      return;
    }
    setTimeout(() => {
      call();
    }, Math.random() * 1000);
  });
}
call();

error:

{ Error: Pool is closed.
    at /path/to/code/node_modules/_mysql@2.14.1@mysql/lib/Pool.js:149:17
    at Array.forEach (native)
    at Pool.releaseConnection (/path/to/code/node_modules/_mysql@2.14.1@mysql/lib/Pool.js:148:37)
    at Pool._removeConnection (/path/to/code/node_modules/_mysql@2.14.1@mysql/lib/Pool.js:279:8)
    at Pool._purgeConnection (/path/to/code/node_modules/_mysql@2.14.1@mysql/lib/Pool.js:260:8)
    at Ping.onOperationComplete [as _callback] (/path/to/code/node_modules/_mysql@2.14.1@mysql/lib/Pool.js:101:12)
    at Ping.Sequence.end (/path/to/code/node_modules/_mysql@2.14.1@mysql/lib/protocol/sequences/Sequence.js:88:24)
    at Ping.Sequence.OkPacket (/path/to/code/node_modules/_mysql@2.14.1@mysql/lib/protocol/sequences/Sequence.js:97:8)
    at Protocol._parsePacket (/path/to/code/node_modules/_mysql@2.14.1@mysql/lib/protocol/Protocol.js:279:23)
    at Parser.write (/path/to/code/node_modules/_mysql@2.14.1@mysql/lib/protocol/Parser.js:76:12)
    --------------------
    at Pool.query (/path/to/code/node_modules/_mysql@2.14.1@mysql/lib/Pool.js:199:23)
    at Driver.query (/path/to/code/test.js:26:20)
    at call (/path/to/code/test.js:33:10)
    at Timeout.setTimeout (/path/to/code/test.js:39:7)
    at ontimeout (timers.js:365:14)
    at tryOnTimeout (timers.js:237:5)
    at Timer.listOnTimeout (timers.js:207:5) code: 'POOL_CLOSED' }

It may not raise an error the first time you run, but it will eventually if you run several times.

[sidorares]

Thanks @HQidea , I can reproduce it.

So what’s happening is

1. you do pool.query() first.
2. pool is in the state when number of active connections already maxed so inside pool.query() it has to wait for idle connection
3. pool.end() marks pool to a ‘closing’ state
4. one of connections finishes previous query and becomes idle, pool.query() from 1) gets connection tries to do connectcion.query() and fails because pool is in closed state