Allow a validation function

[Richienb]

Instead of checking if a file exists, we just try to open it.
So, instead of searching for a port, try to use the port and try another if it fails.

So, for example:

import express from 'express';
import {getValidatedPort} from 'get-port';
import pDefer from 'p-defer';

const app = express();

function listen(port) {
	const {promise, resolve, reject} = pDefer();

	app
		.listen(port, () => {
			app.off('error', reject);
			resolve();
		})
		.on('error', reject);

	return promise;
}

await getValidatedPort(listen);

What should the getValidatedPort function return? If we return the port that is used, we can't access the return value of the validation function and vice versa.

How should we check if the validator function approves an option? In the example, we check for a resolved or rejected promise but we could instead be checking for a boolean value.

[sindresorhus]

What problem does this solve? The current way of getting ports is working fine for me, so not sure how this makes it better.

[Richienb]

Cross-process race conditions can be avoided by trying to use the port as part of the validation check. For example, when starting an express server, you could try opening it on a specific port directly and reacting to the errors instead of first checking if it's free and then opening it where some other script could take the port in the meantime.

[sindresorhus]

It is actually possible to reserve a port, and the kernel will then ensure it's not taken by other processes. I think the best first course of action would be to open a Node.js issue requesting an API for this.

• https://stackoverflow.com/questions/19374997/easier-way-to-reserve-port-number-in-c
• https://stackoverflow.com/questions/13308144/how-to-get-the-first-available-tcp-port-to-listen-to
• ability to reserve an available http port to use later on nodejs/node-v0.x-archive#7814

Specifically, the C bind() function.