child_process: add string shortcut for fork stdio

Add string shortcut option for stdio parameter.

Fixes: nodejs#10793
PR-URL: nodejs#10866
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Brian White <mscdex@mscdex.net>

doc/api/child_process.md

@@ -265,10 +265,10 @@ added: v0.5.0
     piped to the parent, otherwise they will be inherited from the parent, see
     the `'pipe'` and `'inherit'` options for [`child_process.spawn()`][]'s
     [`stdio`][] for more details (Default: `false`)
-  * `stdio` {Array} Supports the array version of [`child_process.spawn()`][]'s
-    [`stdio`][] option. When this option is provided, it overrides `silent`.
-    The array must contain exactly one item with value `'ipc'` or an error will
-    be thrown. For instance `[0, 1, 2, 'ipc']`.
+  * `stdio` {Array|String} See [`child_process.spawn()`][]'s [`stdio`][].
+    When this option is provided, it overrides `silent`. If the array variant
+    is used, it must contain exactly one item with value `'ipc'` or an error
+    will be thrown. For instance `[0, 1, 2, 'ipc']`.
   * `uid` {Number} Sets the user identity of the process. (See setuid(2).)
   * `gid` {Number} Sets the group identity of the process. (See setgid(2).)
 * Returns: {ChildProcess}

lib/child_process.js

@@ -17,6 +17,10 @@ const _validateStdio = child_process._validateStdio;
 const setupChannel = child_process.setupChannel;
 const ChildProcess = exports.ChildProcess = child_process.ChildProcess;
 
+function stdioStringToArray(option) {
+  return [option, option, option, 'ipc'];
+}
+
 exports.fork = function(modulePath /*, args, options*/) {
 
   // Get options and args arguments.
@@ -50,11 +54,21 @@ exports.fork = function(modulePath /*, args, options*/) {
 
   args = execArgv.concat([modulePath], args);
 
-  if (!Array.isArray(options.stdio)) {
+  if (typeof options.stdio === 'string') {
+    switch (options.stdio) {
+      case 'ignore':
+      case 'pipe':
+      case 'inherit':
+        options.stdio = stdioStringToArray(options.stdio);
+        break;
+      default:
+        throw new TypeError('Unknown stdio option');
+    }
+  } else if (!Array.isArray(options.stdio)) {
     // Use a separate fd=3 for the IPC channel. Inherit stdin, stdout,
     // and stderr from the parent if silent isn't set.
-    options.stdio = options.silent ? ['pipe', 'pipe', 'pipe', 'ipc'] :
-        [0, 1, 2, 'ipc'];
+    options.stdio = options.silent ? stdioStringToArray('pipe') :
+        stdioStringToArray('inherit');
   } else if (options.stdio.indexOf('ipc') === -1) {
     throw new TypeError('Forked processes must have an IPC channel');
   }

test/parallel/test-child-process-fork-stdio-string-variant.js

@@ -0,0 +1,27 @@
+'use strict';
+const common = require('../common');
+
+// Ensures that child_process.fork can accept string
+// variant of stdio parameter in options object and
+// throws a TypeError when given an unexpected string
+
+const assert = require('assert');
+const fork = require('child_process').fork;
+
+const childScript = `${common.fixturesDir}/child-process-spawn-node`;
+const errorRegexp = /^TypeError: Unknown stdio option$/;
+const malFormedOpts = {stdio: '33'};
+const payload = {hello: 'world'};
+const stringOpts = {stdio: 'pipe'};
+
+assert.throws(() => fork(childScript, malFormedOpts), errorRegexp);
+
+const child = fork(childScript, stringOpts);
+
+child.on('message', (message) => {
+  assert.deepStrictEqual(message, {foo: 'bar'});
+});
+
+child.send(payload);
+
+child.on('exit', common.mustCall((code) => assert.strictEqual(code, 0)));