result.stdout is empty while result.all is not

[ehmicky]

Steps to reproduce:

import { promisify } from 'util'
import execa from 'execa'

(async () => {
  const childProcess = execa('echo', ['test'])
  await promisify(setTimeout)(1e3)

  const result = await childProcess
  console.log('stdout:', result.stdout)
  console.log('stderr:', result.stderr)
  console.log('all:', result.all)
})()

$ node index.js
stdout: 
stderr: 
all: test

Node v12.4.0
Ubuntu 19.10
Execa 2.0.0

[ehmicky]

Reduced the issue to the following code (which is based on what execa does internally):

const childProcess = require('child_process');
const getStream = require('get-stream');
const mergeStream = require('merge-stream');

const bug = async () => {
	const spawned = childProcess.spawn('echo', ['test']);

	const allStream = mergeStream(spawned.stdout);

	spawned.on('exit', async () => {
		const stdout = await getStream(spawned.stdout);
		const all = await getStream(allStream);

		console.log({stdout, all});
	});
};

bug();

$ node bug.js
{ stdout: '', all: 'test\n' }

[ehmicky]

The problem might be the following: when a childProcess exits (exit event), its stdout and stderr are flushed and closed by Node.js.

A closed stream cannot be read from nor piped from. However childProcess.stdout|stderr.pipe() is setup by childProcess.then(). We purposely defer that call in case users prefer using the streams directly instead of the returned promise. This means if childProcess.then() happens after childProcess exit event, stdout|stderr will be empty.

On one hand child process streams being cleaned up on exit is a good thing as it prevents memory leak. On the other hand there might be cases where users might want to defer a child process streams consumption, e.g. when it is passed to another consumer. We could stop deferring stream consumption, but that has issues as well. @sindresorhus I would need your feedback on this as this is tricky.

---

A second related (but distinct) issue is that this does not apply to childProcess.all because it's handled differently from childProcess.stdout|stderr. I think the reason might be either that:

• it is always piped before childProcess.then()
• or that it is piped slightly differently.

I think this we should deal with the first problem before checking out what's happening with childProcess.all.

[ehmicky]

The second issue seems to be because all is always piped before childProcess.then(). When a stream is piped before get-stream starts, it works.

[ehmicky]

PR at #351.

[ehmicky]

This is not an issue, see #351 (comment)