I think child_process.fork() should officially support { detached: true }

[mmorearty]

• Version: v9.2.1
• Platform: Linux ip-172-31-29-251 3.13.0-135-generic lib: micro-optimize url.resolve() #184-Ubuntu SMP Wed Oct 18 11:55:51 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
• Subsystem: child_process

child_process.spawn() supports an option detached which "makes it possible for the child process to continue running after the parent exits." child_process.fork() does not officially support detached as one of its options (by "officially support", I mean that it is not documented as a valid option); but I think it should be officially supported.

My reasons:

1. It works today. (details below)
2. It's useful. (details below)
3. I can't think of any reason that it shouldn't be supported.

If you agree, then no code changes would be required, but the documentation for child_process.fork() would need to list detached as a valid option. (Also, TypeScript's @types/node would need to be updated, but that would probably be a separate GitHub issue somewhere else.)

1. It works today: First of all, if you look at the current source for child_process.fork(), it's clear (and not surprising) that fork() is just a simple wrapper around spawn(). It passes most options through unchanged.

To prove that detached works with fork(): save this as demo.js:

// launch with "node demo.js" or "node demo.js detached"

const child_process = require('child_process')

if (process.argv.indexOf('--daemon') === -1) {
    let options = {};
    if (process.argv.indexOf('detached') >= 0) {
        options.detached = true;
    }

    const child = child_process.fork(__filename, ['--daemon'], options);
    console.log('hello from parent; press ^C to terminate parent')
    process.stdin.read()
} else {
    console.log(`hello from child, my pid is ${process.pid}`)
    setInterval(() => {}, 5000)
}

To see the NON-detached behavior, launch it with node demo.js. It will call fork(), so there are now two instances running. Then press ^C; if you do ps aux | grep demo.js you will see that both instances terminated.

To see the detached behavior, repeat the above but with node demo.js detached. In this case, after ^C, the child process is still running.

2. It's useful: child_process.fork() can be useful for starting daemon processes, and detached is certainly useful for daemons.

[Trott]

@bnoordhuis @cjihrig

[cjihrig]

I can think of at least one corner case, but it's probably unlikely to be affected since no code changes would be required. Code that checks for the existence of APIs like process.send() could break once the IPC channel is closed.

If it works already, people find it useful, and no one has a good reason not to support it, then we probably should document it (although I still think spawn() makes more sense here). We'd also need tests to make sure it works consistently on all supported platforms.

[ramsgoli]

I'd be happy to work on a PR for this if the idea goes through! 👍

[refack]

FTR: this is basically a "need to document and test" issue.

[nanomosfet]

Hi @refack. I would love to contribute to this issue as my first issue.

Would this be testing the functionality that @mmorearty documented?

const child_process = require('child_process')

if (process.argv.indexOf('--daemon') === -1) {
    let options = {};
    if (process.argv.indexOf('detached') >= 0) {
        options.detached = true;
    }

    const child = child_process.fork(__filename, ['--daemon'], options);
    console.log('hello from parent; press ^C to terminate parent')
    process.stdin.read()
} else {
    console.log(`hello from child, my pid is ${process.pid}`)
    setInterval(() => {}, 5000)
}

From some quick research I think the path would be to create a file named test-child-process-fork-detached.js under /test/parallel/.

There I would be creating the test for the above code. Then I might follow the test /test/paralell/test-child-process-detached.js as a guide for testing the functionality.

Did I miss anything?

Edit:
I went ahead and added a test demonstrating the above functionality.
master...nanomosfet:add-fork-detached-test

does this look right?

Also where would I update the documentation for this?

[Trott]

Fixed in f051737