child_process: support promisified exec(File)

Author: Benjamin Gruenbaum <inglor@gmail.com>
Author: Anna Henningsen <anna@addaleax.net>

PR-URL: nodejs#12442
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: William Kapke <william.kapke@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>

doc/api/child_process.md

@@ -214,6 +214,23 @@ child runs longer than `timeout` milliseconds.
 *Note: Unlike the exec(3) POSIX system call, `child_process.exec()` does not
 replace the existing process and uses a shell to execute the command.*
 
+If this method is invoked as its [`util.promisify()`][]ed version, it returns
+a Promise for an object with `stdout` and `stderr` properties.
+
+For example:
+
+```js
+const util = require('util');
+const exec = util.promisify(require('child_process').exec);
+
+async function lsExample() {
+  const {stdout, stderr} = await exec('ls');
+  console.log('stdout:', stdout);
+  console.log('stderr:', stderr);
+}
+lsExample();
+```
+
 ### child_process.execFile(file[, args][, options][, callback])
 <!-- YAML
 added: v0.1.91
@@ -263,6 +280,19 @@ can be used to specify the character encoding used to decode the stdout and
 stderr output. If `encoding` is `'buffer'`, or an unrecognized character
 encoding, `Buffer` objects will be passed to the callback instead.
 
+If this method is invoked as its [`util.promisify()`][]ed version, it returns
+a Promise for an object with `stdout` and `stderr` properties.
+
+```js
+const util = require('util');
+const execFile = util.promisify(require('child_process').execFile);
+async function getVersion() {
+  const {stdout} = await execFile('node', ['--version']);
+  console.log(stdout);
+}
+getVersion();
+```
+
 ### child_process.fork(modulePath[, args][, options])
 <!-- YAML
 added: v0.5.0
@@ -1269,4 +1299,5 @@ to `stdout` although there are only 4 characters.
 [`process.on('message')`]: process.html#process_event_message
 [`process.send()`]: process.html#process_process_send_message_sendhandle_options_callback
 [`stdio`]: #child_process_options_stdio
+[`util.promisify()`]: util.html#util_util_promisify_original
 [synchronous counterparts]: #child_process_synchronous_process_creation

lib/child_process.js

@@ -22,7 +22,9 @@
 'use strict';
 
 const util = require('util');
-const { deprecate, convertToValidSignal } = require('internal/util');
+const {
+  deprecate, convertToValidSignal, customPromisifyArgs
+} = require('internal/util');
 const debug = util.debuglog('child_process');
 
 const uv = process.binding('uv');
@@ -138,6 +140,9 @@ exports.exec = function(command /*, options, callback*/) {
                           opts.callback);
 };
 
+Object.defineProperty(exports.exec, customPromisifyArgs,
+                      { value: ['stdout', 'stderr'], enumerable: false });
+
 
 exports.execFile = function(file /*, args, options, callback*/) {
   var args = [];
@@ -333,6 +338,9 @@ exports.execFile = function(file /*, args, options, callback*/) {
   return child;
 };
 
+Object.defineProperty(exports.execFile, customPromisifyArgs,
+                      { value: ['stdout', 'stderr'], enumerable: false });
+
 const _deprecatedCustomFds = deprecate(
   function deprecateCustomFds(options) {
     options.stdio = options.customFds.map(function mapCustomFds(fd) {

test/parallel/test-child-process-promisified.js

@@ -0,0 +1,34 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const child_process = require('child_process');
+const { promisify } = require('util');
+
+common.crashOnUnhandledRejection();
+
+const exec = promisify(child_process.exec);
+const execFile = promisify(child_process.execFile);
+
+{
+  exec(`${process.execPath} -p 42`).then(common.mustCall((obj) => {
+    assert.deepStrictEqual(obj, { stdout: '42\n', stderr: '' });
+  }));
+}
+
+{
+  execFile(process.execPath, ['-p', '42']).then(common.mustCall((obj) => {
+    assert.deepStrictEqual(obj, { stdout: '42\n', stderr: '' });
+  }));
+}
+
+{
+  exec('doesntexist').catch(common.mustCall((err) => {
+    assert(err.message.includes('doesntexist'));
+  }));
+}
+
+{
+  execFile('doesntexist', ['-p', '42']).catch(common.mustCall((err) => {
+    assert(err.message.includes('doesntexist'));
+  }));
+}