Pool connections are not gracefully closed once idle connection cleanup occurs.

[dtimofeev]

Hi,
When Pool._removeIdleTimeoutConnections() is executed and an idle connection is selected for removal (regardless of the conditions) the following code is executed:
https://github.com/sidorares/node-mysql2/blob/master/lib/pool.js#L208
This in turn removes the connection from the pool and directly (non-gracefully) closes it by calling PoolConnection.destroy() instead of PoolConnection.end().
https://github.com/sidorares/node-mysql2/blob/master/lib/pool_connection.js#L50
As a result the connection is closed without notifying the server in any way.

Because of the above the SQL(MariaDB) server is flooded with warnings like:
Aborted connection 123 to db: 'dbname' user: 'dbuser' host: '127.0.0.1' (Got an error reading communication packets)

A possible solution might be to change the current PoolConnection.end() implementation with:

end() {
    this._removeFromPool();
    super.end();
}

but I'm not sure if this solves the issue correctly.

Looking forward to your thoughts on this.

[dtimofeev]

Reproduce script:

const mysql = require("mysql2/promise");

(function () {
  const pool = mysql.createPool({
    host: "localhost",
    user: "root",
    password: "random",
    database: "tests",
    idleTimeout: 1000,
    connectionLimit: 5,
    // this has to be lower than connectionLimit in order for cleanup process to start at all
    maxIdle: 4,
  });

  setInterval(() => {
    pool.query("SELECT 1");
  }, 1000);
})();

[dtimofeev]

The issue also occurs in latest MySQL:

docker run  --env=MYSQL_ROOT_PASSWORD=random -p 3306:3306 -d mysql:latest --log_error_verbosity=3
docker logs -f <ID from the above command>

[Note] [MY-010914] [Server] Aborted connection 8 to db: 'unconnected' user: 'root' host: '172.17.0.1' (Got an error reading communication packets).

[dstankovd]

Hello,

I have submitted a new pull request (#3180) that addresses this issue with minimal changes.

In my view, an improved long-term solution would involve removing the deprecation warning, as it has been in place for some time, and implementing proper end functionality in pool_connection without relying on _realEnd. If you would like me to work on this enhancement, I would be happy to update the pull request accordingly.

Best regards,
Dimitar

[dstankovd]

Upon further consideration, it seems more appropriate to implement proper end functionality directly within pool_connection, as the current fix could be susceptible to unexpected edge cases. For example, when using the _realEnd function, the connection is removed from the pool upon the "end" event, which can create a brief window during which a closed connection might still be allocated.