Memory leak due to res.once("finish")

[owenallenaz]

At https://github.com/keymetrics/pm2-io-apm/blob/master/src/metrics/httpMetrics.ts#L178 you have the following passage:

return function (event: string, req: any, res: any) {
  // only handle http request
  if (event !== 'request') return original.apply(this, arguments)

  const meter: Meter | undefined = self.metrics.get(`${name}.meter`)
  if (meter !== undefined) {
    meter.mark()
  }
  const latency: Histogram | undefined = self.metrics.get(`${name}.latency`)
  if (latency === undefined) return original.apply(this, arguments)
  if (res === undefined || res === null) return original.apply(this, arguments)
  const startTime = Date.now()
  // wait for the response to set the metrics
  res.once('finish', _ => {
    latency.update(Date.now() - startTime)
  })
  return original.apply(this, arguments)
}

res.once is only called once the response has been flushed to the connecting socket. If the socket was to hang-up, such as a timeout on the client side, it results in the finish event never occurring. Instead, you likely want to use the close event. You can see a replication of this in the following gist: https://gist.github.com/owenallenaz/bc66547842e87a649b554b7c168a7314 . If you run that server and hit it with call, and then hit the /status/ route you'll see that finish is not called for every cancelled request. This has the potentiality to leak the res object in it's entirety.