Detaching child process from parent fails to execute child

[SamVerschueren]

We had an issue in Alfy haunting us for a while. Alfred only shows the results when the process exits. In order to detect if a workflow has an update, we spawn a new process which does this for us in the form of alfred-notifier.

This is how we did it first

execa('check.js');

Then we noticed what was going on and tried to unref the child process.

const cp = execa('check.js');
cp.unref();

Unfortunately, this is not enough. In order to detach the child process, you must set the detached flag to true and make sure there is no communication channel between child and parent.

const cp = execa('check.js', {
    detached: true,
    stdio: 'ignore'
});
cp.unref();

However, this results in check.js not being executed. Using child_process directly however, did the trick.

const subProcess = cp.spawn('check.js', [], {
    detached: true,
    stdio: 'ignore'
});
subProcess.unref();

---

How to reproduce

Create two files, child.js and index.js.

child.js

#!/usr/bin/env node
'use strict';
const name = process.argv[2];

setTimeout(() => {
	console.log(`Hello World from ${name}`);
}, 3000);

index.js

'use strict';
const path = require('path');
const fs = require('fs');
const cp = require('child_process');
const execa = require('execa');

const outCp = fs.openSync('out-cp.log', 'w');
const outExeca = fs.openSync('out-execa.log', 'w');

// Spawn the child process with `child_process`
const subProcess = cp.spawn(path.join(__dirname, 'child.js'), ['child_process'], {
	detached: true,
	stdio: ['ignore', outCp, outCp]
});
subProcess.unref();

// Spawn the child process with `execa`
const subProcess2 = execa(path.join(__dirname, 'child.js'), ['execa'], {
	detached: true,
	stdio: ['ignore', outExeca, outExeca]
});
subProcess2.unref();

Run node index.js. The process exits immediately and you will notice that two files are being created, out-cp.log and out-execa.log. After 3 seconds, the content of out-cp.log changes but the file out-execa.log keeps being empty.

[kasselTrankos]

hi, im using
node --version ::: v6.12.0
and when run node index.js
give me this error:

internal/child_process.js:328
throw errnoException(err, 'spawn');
^

Error: spawn EACCES
at exports._errnoException (util.js:1020:11)
at ChildProcess.spawn (internal/child_process.js:328:11)
at Object.exports.spawn (child_process.js:369:9)
at Object. (/home/vera/help-man/index.js:11:23)
at Module._compile (module.js:570:32)
at Object.Module._extensions..js (module.js:579:10)
at Module.load (module.js:487:32)
at tryModuleLoad (module.js:446:12)
at Function.Module._load (module.js:438:3)
at Module.runMain (module.js:604:10)
need upgrade my node ?
thanks

[SamVerschueren]

Run chmod +x child.js

[SamVerschueren]

I debugged this issue and found out that when I set cleanup to false, it works as expected. Probably because we unref the child process, causing the child to to think the parent exited and kills itself.

Should we set cleanup to false when detached is set to true? Guess it makes sense. Not sure what we shoul do if detached is set to true and cleanup is explicitely set to true as well.