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>

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 @@
+const fork = require('child_process').fork;
+const path = require('path');
+
+const child = 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');
+const assert = require('assert');
+const fork = require('child_process').fork;
+const fixtures = require('../common/fixtures');
+
+const nonPersistentNode = fork(
+  fixtures.path('parent-process-nonpersistent-fork.js'),
+  [],
+  { silent: true });
+
+let childId = -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);
+});