Color not enabled when in a Node Worker

This is related to an ongoing issue/discussion in Node.js related to process.stdout and process.stderr not having isTTY === true when in a Node.js Worker thread.

This means that, if debug is used within a Worker thread, it does not print colors nor ms diff (but timestamps).

Code to reproduce it (requires Node >= 12 or Node 10 with node --experimental-worker flag):

foo.js:

process.env.DEBUG = '*';

const { Worker, parentPort, threadId, isMainThread } = require('worker_threads');
const tty = require('tty');
const debug = require('debug');

async function run()
{
	if (isMainThread)
	{
		const logger = debug('[parent]');

		logger(
			'run() in main thread [process.stderr.isTTY:%s, tty.isatty():%s]',
			process.stderr.isTTY,
			tty.isatty(process.stderr.fd));

		const worker = new Worker(__filename);

		worker.once('message', message => logger('message from child: %o', message));
		worker.postMessage('hi child!');
	}
	else
	{
		const logger = debug('[child]');

		logger(
			'run() in child thread [process.stderr.isTTY:%s, tty.isatty():%s]',
			process.stderr.isTTY,
			tty.isatty(process.stderr.fd));

		parentPort.once('message', message => logger('message from parent: %o', message));
		parentPort.postMessage('hi dad!');
	}
}

run();

Output:

$ node --experimental-worker foo.js

  [parent] run() in main thread [process.stderr.isTTY:true, tty.isatty():true] +0ms
2020-01-12T15:22:36.083Z [child] run() in child thread [process.stderr.isTTY:undefined, tty.isatty():false]
  [parent] message from child: 'hi dad!' +33ms
2020-01-12T15:22:36.085Z [child] message from parent: 'hi child!'

Of course it works if I set DEBUG_COLORS=true environment variable. Just wondering if a debug instance may expose a method to set “tty” mode.

[ibc]

The problem is that this may not even be “fixed” in Node. From the referenced issue:

IMO it make sense for .isTTY to be undefined because process.stdout and process.stderr in worker threads don’t actually write to the TTY. They postMessage() the data to the main thread which then writes it out on the worker thread’s behalf.

We could copy the .isTTY values from the main thread but that breaks the idiom where if (process.stdout.isTTY) guards are used around calls to process.stdout.setRawMode(true) or process.stdout.cursorTo() - and you can’t reasonably support those APIs in worker threads because the TTY is a per-process resource.

I’d be happy by just manually setting process.stderr.isTTY = true in the Worker thread. However this won’t make debug module produce colors since it looks at tty.isatty().

And another super ugly workaround is (in the top of the app):

// before loading debug and before creating Workers that also use debug.

const tty = require('tty');
if (tty.isatty(process.stderr.fd)) {
  process.env.DEBUG_COLORS = 'true';
}

[ibc]

Thanks for the explanation.