Only add one process.exit handler

[benlowry]

If you create short-lived tunnels then each time it binds a method to the process.on('exit') until error messages start being emitted -

(node:17328) MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 exit listeners added to [process]. Use emitter.setMaxListeners() to increase limit
(Use `node --trace-warnings ...` to show where the warning was created)

[bubenshchykov]

Thanks for reporting!

There is a guard that prevents spawning two processes in parallel https://github.com/bubenshchykov/ngrok/blob/master/process.js#L18

Thus, we can be assigning only one process.on('exit', .. ) handler at a time.

The use-case that you describe is: start process; kill process; start process; kill process - true, it will lead to multiple exit handlers and a memory leak.

So the problem boils down to simply calling process.removeListener(cb) inside exit callback, what do you think?

[philnash]

Thanks for this, I'm closing in favour of #240, in which we definitely only set up the listener once and don't need to introduce more state. I wouldn't have thought of it without this PR as inspiration though.