timers: add promisify support

Add support for `util.promisify(setTimeout)` and
`util.promisify(setImmediate)` as a proof-of-concept implementation.
`clearTimeout()` and `clearImmediate()` do not work on those Promises.
Includes documentation and tests.

PR-URL: nodejs#12442
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
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/timers.md

@@ -85,6 +85,27 @@ next event loop iteration.
 
 If `callback` is not a function, a [`TypeError`][] will be thrown.
 
+*Note*: This method has a custom variant for promises that is available using
+[`util.promisify()`][]:
+
+```js
+const util = require('util');
+const setImmediatePromise = util.promisify(setImmediate);
+
+setImmediatePromise('foobar').then((value) => {
+  // value === 'foobar' (passing values is optional)
+  // This is executed after all I/O callbacks.
+});
+
+// or with async function
+async function timerExample() {
+  console.log('Before I/O callbacks');
+  await setImmediatePromise();
+  console.log('After I/O callbacks');
+}
+timerExample();
+```
+
 ### setInterval(callback, delay[, ...args])
 <!-- YAML
 added: v0.0.1
@@ -126,12 +147,28 @@ will be set to `1`.
 
 If `callback` is not a function, a [`TypeError`][] will be thrown.
 
+*Note*: This method has a custom variant for promises that is available using
+[`util.promisify()`][]:
+
+```js
+const util = require('util');
+const setTimeoutPromise = util.promisify(setTimeout);
+
+setTimeoutPromise(40, 'foobar').then((value) => {
+  // value === 'foobar' (passing values is optional)
+  // This is executed after about 40 milliseconds.
+});
+```
+
 ## Cancelling Timers
 
 The [`setImmediate()`][], [`setInterval()`][], and [`setTimeout()`][] methods
 each return objects that represent the scheduled timers. These can be used to
 cancel the timer and prevent it from triggering.
 
+It is not possible to cancel timers that were created using the promisified
+variants of [`setImmediate()`][], [`setTimeout()`][].
+
 ### clearImmediate(immediate)
 <!-- YAML
 added: v0.9.1
@@ -168,4 +205,5 @@ Cancels a `Timeout` object created by [`setTimeout()`][].
 [`setImmediate()`]: timers.html#timers_setimmediate_callback_args
 [`setInterval()`]: timers.html#timers_setinterval_callback_delay_args
 [`setTimeout()`]: timers.html#timers_settimeout_callback_delay_args
+[`util.promisify()`]: util.html#util_util_promisify_original
 [the Node.js Event Loop]: https://nodejs.org/en/docs/guides/event-loop-timers-and-nexttick

lib/timers.js

@@ -23,6 +23,8 @@
 
 const TimerWrap = process.binding('timer_wrap').Timer;
 const L = require('internal/linkedlist');
+const internalUtil = require('internal/util');
+const { createPromise, promiseResolve } = process.binding('util');
 const assert = require('assert');
 const util = require('util');
 const debug = util.debuglog('timer');
@@ -364,7 +366,7 @@ exports.enroll = function(item, msecs) {
  */
 
 
-exports.setTimeout = function(callback, after, arg1, arg2, arg3) {
+function setTimeout(callback, after, arg1, arg2, arg3) {
   if (typeof callback !== 'function') {
     throw new TypeError('"callback" argument must be a function');
   }
@@ -383,8 +385,16 @@ exports.setTimeout = function(callback, after, arg1, arg2, arg3) {
   }
 
   return createSingleTimeout(callback, after, args);
+}
+
+setTimeout[internalUtil.promisify.custom] = function(after, value) {
+  const promise = createPromise();
+  createSingleTimeout(promise, after, [value]);
+  return promise;
 };
 
+exports.setTimeout = setTimeout;
+
 function createSingleTimeout(callback, after, args) {
   after *= 1; // coalesce to number or NaN
   if (!(after >= 1 && after <= TIMEOUT_MAX))
@@ -403,6 +413,8 @@ function createSingleTimeout(callback, after, args) {
 function ontimeout(timer) {
   var args = timer._timerArgs;
   var callback = timer._onTimeout;
+  if (typeof callback !== 'function')
+    return promiseResolve(callback, args[0]);
   if (!args)
     callback.call(timer);
   else {
@@ -687,6 +699,8 @@ function tryOnImmediate(immediate, oldTail) {
 function runCallback(timer) {
   const argv = timer._argv;
   const argc = argv ? argv.length : 0;
+  if (typeof timer._callback !== 'function')
+    return promiseResolve(timer._callback, argv[0]);
   switch (argc) {
     // fast-path callbacks with 0-3 arguments
     case 0:
@@ -715,7 +729,7 @@ function Immediate() {
   this.domain = process.domain;
 }
 
-exports.setImmediate = function(callback, arg1, arg2, arg3) {
+function setImmediate(callback, arg1, arg2, arg3) {
   if (typeof callback !== 'function') {
     throw new TypeError('"callback" argument must be a function');
   }
@@ -740,8 +754,16 @@ exports.setImmediate = function(callback, arg1, arg2, arg3) {
       break;
   }
   return createImmediate(args, callback);
+}
+
+setImmediate[internalUtil.promisify.custom] = function(value) {
+  const promise = createPromise();
+  createImmediate([value], promise);
+  return promise;
 };
 
+exports.setImmediate = setImmediate;
+
 function createImmediate(args, callback) {
   // declaring it `const immediate` causes v6.0.0 to deoptimize this function
   var immediate = new Immediate();

test/parallel/test-timers-promisified.js

@@ -0,0 +1,40 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const timers = require('timers');
+const { promisify } = require('util');
+
+/* eslint-disable no-restricted-syntax */
+
+common.crashOnUnhandledRejection();
+
+const setTimeout = promisify(timers.setTimeout);
+const setImmediate = promisify(timers.setImmediate);
+
+{
+  const promise = setTimeout(1);
+  promise.then(common.mustCall((value) => {
+    assert.strictEqual(value, undefined);
+  }));
+}
+
+{
+  const promise = setTimeout(1, 'foobar');
+  promise.then(common.mustCall((value) => {
+    assert.strictEqual(value, 'foobar');
+  }));
+}
+
+{
+  const promise = setImmediate();
+  promise.then(common.mustCall((value) => {
+    assert.strictEqual(value, undefined);
+  }));
+}
+
+{
+  const promise = setImmediate('foobar');
+  promise.then(common.mustCall((value) => {
+    assert.strictEqual(value, 'foobar');
+  }));
+}