test: test and docs for detached fork process

This tests child process fork component in detached mode
by spawning a parent process that creates a child process.
We kill the parent process and check if the child is still
running.
Fixes: #17592
PR-URL: #24524
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: nanomosfet

doc/api/child_process.md

@@ -325,6 +325,9 @@ changes:
 *`args` {string[]} List of string arguments.
 *`options` {Object}
 *`cwd` {string} Current working directory of the child process.
+*`detached` {boolean} Prepare child to run independently of its parent
+ process. Specific behavior depends on the platform, see
+[`options.detached`][]).
 *`env` {Object} Environment key-value pairs.
 *`execPath` {string} Executable used to create the child process.
 *`execArgv` {string[]} List of string arguments passed to the executable.

test/fixtures/parent-process-nonpersistent-fork.js

@@ -0,0 +1,12 @@
+constfork=require('child_process').fork;
+constpath=require('path');
+
+constchild=fork(
+path.join(__dirname,'child-process-persistent.js'),
+[],
+{detached: true,stdio: 'ignore'}
+);
+
+console.log(child.pid);
+
+child.unref();

test/parallel/test-child-process-fork-detached.js

@@ -0,0 +1,23 @@
+'use strict';
+require('../common');
+constassert=require('assert');
+constfork=require('child_process').fork;
+constfixtures=require('../common/fixtures');
+
+constnonPersistentNode=fork(
+fixtures.path('parent-process-nonpersistent-fork.js'),
+[],
+{silent: true});
+
+letchildId=-1;
+
+nonPersistentNode.stdout.on('data',(data)=>{
+childId=parseInt(data,10);
+nonPersistentNode.kill();
+});
+
+process.on('exit',()=>{
+assert.notStrictEqual(childId,-1);
+// Killing the child process should not throw an error
+process.kill(childId);
+});