Reconnect feature cause an EventEmitter memory leak

When used with connections pools, this module can cause an EventEmitter memory leak:

(node:9472) MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 error listeners added to [PoolConnection]. Use emitter.setMaxListeners() to increase limit
MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 error listeners added to [PoolConnection]. Use emitter.setMaxListeners() to increase limit
    at _addListener (events.js:261:17)
    at PoolConnection.addListener (events.js:277:10)
    at PoolConnection.once (events.js:306:8)
    at addReconnectHandler (/.../node_modules/promise-mysql/lib/connection.js:138:16)
    at /.../node_modules/promise-mysql/lib/connection.js:41:17
    at tryCatcher (/.../node_modules/bluebird/js/release/util.js:16:23)
    at Promise._settlePromiseFromHandler (/.../node_modules/bluebird/js/release/promise.js:517:31)
    at Promise._settlePromise (/.../node_modules/bluebird/js/release/promise.js:574:18)
    at Promise._settlePromiseCtx (/.../node_modules/bluebird/js/release/promise.js:611:10)
    at _drainQueueStep (/.../node_modules/bluebird/js/release/async.js:142:12)
    at _drainQueue (/.../node_modules/bluebird/js/release/async.js:131:9)
    at Async._drainQueues (/.../node_modules/bluebird/js/release/async.js:147:5)
    at Immediate.Async.drainQueues (/.../node_modules/bluebird/js/release/async.js:17:14)
    at processImmediate (internal/timers.js:439:21)
    at process.topLevelDomainCallback (domain.js:126:23)

Pool.getConnection create a new PoolConnection which call addReconnectHandler and add an error listener. But this listener is never removed and another will be added on the next Pool.getConnection call.

[nathan818fr]

Reconnect is a base feature of any pools system. Without reconnection a pool will be fundamentally broken. As you can see here, when acquiring a connection, if the connection is closed (or encounter any error):

• the acquirer is pushed back to the connection queue (and it will acquire a new connection - either another already open connection or a new one that will be created)
• and the broken connection is purged

Also note that when an existing connection is retrieved, a ping is made to check if the connection is still good.

–

In addition, currently, the reconnect feature of this module does not work with pools (so the fix will not even be a breaking change!). Explanation:

• When acquiring a connection from a pool, the connection is wrapped in a poolConnection which extends Connection.
• The config object passed to this Connection only contains a returnArgumentsArray property (no host, port or anything else - you can see this here)!
• So when a connection from the pool is wrapped, a reconnect handler is added with this almost empty config object. This config object will be used to reconnect.
• Then, the reconnection will fail because no host and port properties are available !

[nathan818fr]

Anyway the reconnect feature does not make any sense with connections Pools. They already have an reconnect feature built internally.

The best fix seem to add reconnect: false here: https://github.com/lukeb-uk/node-promise-mysql/blob/f2e72814d40146f3025b04c4b78d606c6aec8be2/lib/pool.js#L42:L46