timers: add experimental scheduler api

Adds experimental implementations of the yield and wait APIs being
explored at https://github.com/WICG/scheduling-apis.

When I asked the WHATWG folks about the possibility of standardizing the
[awaitable versions of setTimeout/setImmediate](whatwg/html#7340)
that we have implemented in `timers/promises`, they pointed at the work
in progress scheduling APIs draft as they direction they'll be going.
While there is definitely a few thing in that draft that have
questionable utility to Node.js, the yield and wait APIs map cleanly to
the setImmediate and setTimeout we already have.

Signed-off-by: James M Snell <jasnell@gmail.com>

Author: jasnell

doc/api/timers.md

@@ -472,7 +472,44 @@ const interval = 100;
 })();
 ```
 
+### `timersPromises.scheduler.wait(delay[, options])`
+
+> Stability: 1 - Experimental
+
+* `delay` {number} The number of milliseconds to wait before resolving the
+  promise.
+* `options` {object}
+  * `signal` {AbortSignal} An optional `AbortSignal` that can be used to
+    cancel waiting.
+* Returns: {Promise}
+
+An experimental API defined by the [Scheduling APIs][] draft specification
+being developed as a standard Web Platform API.
+
+Calling `timersPromises.scheduler.wait(delay, options)` is roughly equivalent
+to calling `timersPromises.setTimeout(delay, undefined, options)` except that
+the `ref` option is not supported.
+
+```mjs
+import { scheduler } from 'timers/promises';
+
+await scheduler.wait(1000); // Wait one second before continuing
+```
+
+### `timersPromises.scheduler.yield()`
+
+> Stability: 1 - Experimental
+
+* Returns: {Promise}
+
+An experimental API defined by the [Scheduling APIs][] draft specification
+being developed as a standard Web Platform API.
+
+Calling `timersPromises.scheduler.yield()` is equivalent to calling
+`timersPromises.setImmediate()` with no arguments.
+
 [Event Loop]: https://nodejs.org/en/docs/guides/event-loop-timers-and-nexttick/#setimmediate-vs-settimeout
+[Scheduling APIs]: https://github.com/WICG/scheduling-apis
 [`AbortController`]: globals.md#class-abortcontroller
 [`TypeError`]: errors.md#class-typeerror
 [`clearImmediate()`]: #clearimmediateimmediate

lib/timers/promises.js

@@ -170,8 +170,34 @@ async function* setInterval(after, value, options = {}) {
   }
 }
 
+// TODO(@jasnell): Scheduler is an API currently being discussed by WICG
+// for Web Platform standardization: https://github.com/WICG/scheduling-apis
+// The scheduler.yield() and scheduler.wait() methods correspond roughly to
+// the awaitable setTimeout and setImmediate implementations here. This api
+// should be considered to be experimental until the spec for these are
+// finalized. Note, also, that Scheduler is expected to be defined as a global,
+// but while the API is experimental we shouldn't expose it as such.
+class Scheduler {
+  /**
+   * @returns {Promise<void>}
+   */
+  yield() { return setImmediate(); }
+
+  /**
+   * @typedef {import('../internal/abort_controller').AbortSignal} AbortSignal
+   * @param {number} delay
+   * @param {{ signal? : AbortSignal }} [options]
+   * @returns {Promise<void>}
+   */
+  wait(delay, options) {
+
+    return setTimeout(delay, undefined, { signal: options?.signal });
+  }
+}
+
 module.exports = {
   setTimeout,
   setImmediate,
   setInterval,
+  scheduler: new Scheduler(),
 };

test/parallel/test-timers-promises-scheduler.js

@@ -0,0 +1,50 @@
+'use strict';
+
+const common = require('../common');
+
+const { scheduler } = require('timers/promises');
+const { setTimeout } = require('timers');
+const {
+  strictEqual,
+  rejects,
+} = require('assert');
+
+async function testYield() {
+  await scheduler.yield();
+  process.emit('foo');
+}
+testYield().then(common.mustCall());
+queueMicrotask(common.mustCall(() => {
+  process.addListener('foo', common.mustCall());
+}));
+
+async function testWait() {
+  let value = 0;
+  setTimeout(() => value++, 10);
+  await scheduler.wait(15);
+  strictEqual(value, 1);
+}
+
+testWait().then(common.mustCall());
+
+async function testCancelableWait1() {
+  const ac = new AbortController();
+  const wait = scheduler.wait(1e6, { signal: ac.signal });
+  ac.abort();
+  await rejects(wait, {
+    code: 'ABORT_ERR',
+    message: 'The operation was aborted',
+  });
+}
+
+testCancelableWait1().then(common.mustCall());
+
+async function testCancelableWait2() {
+  const wait = scheduler.wait(10000, { signal: AbortSignal.abort() });
+  await rejects(wait, {
+    code: 'ABORT_ERR',
+    message: 'The operation was aborted',
+  });
+}
+
+testCancelableWait2().then(common.mustCall());