KeepAliveAgent leaks memory when run under node 6

[ksheedlo]

cc @Raynos

The following test program exhibits a memory leak when run under node 6.

'use strict';

require('heapdump');
var Pool = require('lb_pool').Pool;
var http = require('http');

process.on('warning', warning => {
  console.error(warning.stack);
});

var httpClient = new Pool(http, ['localhost:3001'], {keep_alive: true});

function makeRequest() {
  httpClient.get('/foof', function handleGetCall(err, res, body) {
    if (err) {
      console.log('ERROR', err);
    } else {
      console.log('GET /foof ok');
    }

    setTimeout(makeRequest, 1000);
  });
}

makeRequest();

Here is the output when running on my laptop (note: I run a dummy server on localhost:3001 that just returns 200)

GET /foof ok
GET /foof ok
GET /foof ok
GET /foof ok
GET /foof ok
GET /foof ok
GET /foof ok
GET /foof ok
GET /foof ok
GET /foof ok
(node:27128) Warning: Possible EventEmitter memory leak detected. 11 timeout listeners added. Use emitter.setMaxListeners() to increase limit
Warning: Possible EventEmitter memory leak detected. 11 timeout listeners added. Use emitter.setMaxListeners() to increase limit
    at _addListener (events.js:259:19)
    at Socket.addListener (events.js:275:10)
    at Socket.Readable.on (_stream_readable.js:687:35)
    at Socket.once (events.js:301:8)
    at tickOnSocket (_http_client.js:566:12)
    at onSocketNT (_http_client.js:580:5)
    at _combinedTickCallback (internal/process/next_tick.js:74:11)
    at process._tickCallback (internal/process/next_tick.js:98:9)

Additionally, heapdump analysis shows a memory leak. Here are the heapdumps I collected: heapdumps.zip. Load them in the following order to see the leak.

1. heapdump-48854102.4747.heapsnapshot
2. heapdump-49041021.610678.heapsnapshot
3. heapdump-49196662.155354.heapsnapshot

If I instead pass {keep_alive: false} to lb_pool.Pool, the warning and leaking heap objects go away. So, I'm reasonably confident the leak is in the KeepAliveAgent.

[ksheedlo]

possibly a duplicate of nodejs/node#9268

[lxe]

From nodejs/node#9268 (comment):

This is merged in 6.10

I am indeed unable to reproduce this on node 6.10.0

[davidkearns]

There is a followup issue at nodejs/node#9668 that also seems related.