events: allow use of AbortController with once

Allows an AbortSignal to be passed in to events.once() to cancel waiting on an event. Signed-off-by: James M Snell <[email protected]> PR-URL: #34911 Backport-PR-URL: #38386 Reviewed-By: Denys Otrishko <[email protected]> Reviewed-By: Benjamin Gruenbaum <[email protected]> Reviewed-By: Matteo Collina <[email protected]>
nodejs · Apr 30, 2021 · 1fefb5c · 1fefb5c
1 parent 4b04bb8
commit 1fefb5c
Show file tree

Hide file tree

Showing 4 changed files with 179 additions and 4 deletions.
diff --git a/doc/api/events.md b/doc/api/events.md
@@ -829,7 +829,7 @@ class MyClass extends EventEmitter {
 }
 ```
 
-## `events.once(emitter, name)`
+## `events.once(emitter, name[, options])`
 <!-- YAML
 added:
  - v11.13.0
@@ -838,6 +838,9 @@ added:
 
 * `emitter` {EventEmitter}
 * `name` {string}
+* `options` {Object}
+  * `signal` {AbortSignal} An {AbortSignal} that may be used to cancel waiting
+    for the event.
 * Returns: {Promise}
 
 Creates a `Promise` that is fulfilled when the `EventEmitter` emits the given
@@ -896,6 +899,31 @@ ee.emit('error', new Error('boom'));
 // Prints: ok boom
 ```
 
+An {AbortSignal} may be used to cancel waiting for the event early:
+
+```js
+const { EventEmitter, once } = require('events');
+
+const ee = new EventEmitter();
+const ac = new AbortController();
+
+async function foo(emitter, event, signal) {
+  try {
+    await once(emitter, event, { signal });
+    console.log('event emitted!');
+  } catch (error) {
+    if (error.name === 'AbortError') {
+      console.error('Waiting for the event was canceled!');
+    } else {
+      console.error('There was an error', error.message);
+    }
+  }
+}
+
+foo(ee, 'foo', ac.signal);
+ac.abort(); // Abort waiting for the event
+```
+
 ### Awaiting multiple events emitted on `process.nextTick()`
 
 There is an edge case worth noting when using the `events.once()` function

diff --git a/lib/events.js b/lib/events.js
@@ -46,6 +46,7 @@ const kRejection = SymbolFor('nodejs.rejection');
 let spliceOne;
 
 const {
+  hideStackFrames,
   kEnhanceStackBeforeInspector,
   codes
 } = require('internal/errors');
@@ -59,9 +60,20 @@ const {
   inspect
 } = require('internal/util/inspect');
 
+const {
+  validateAbortSignal
+} = require('internal/validators');
+
 const kCapture = Symbol('kCapture');
 const kErrorMonitor = Symbol('events.errorMonitor');
 
+let DOMException;
+const lazyDOMException = hideStackFrames((message, name) => {
+  if (DOMException === undefined)
+    DOMException = internalBinding('messaging').DOMException;
+  return new DOMException(message, name);
+});
+
 function EventEmitter(opts) {
   EventEmitter.init.call(this, opts);
 }
@@ -622,22 +634,61 @@ function unwrapListeners(arr) {
   return ret;
 }
 
-function once(emitter, name) {
+async function once(emitter, name, options = {}) {
+  const signal = options ? options.signal : undefined;
+  validateAbortSignal(signal, 'options.signal');
+  if (signal && signal.aborted)
+    throw lazyDOMException('The operation was aborted', 'AbortError');
   return new Promise((resolve, reject) => {
     const errorListener = (err) => {
       emitter.removeListener(name, resolver);
+      if (signal != null) {
+        eventTargetAgnosticRemoveListener(
+          signal,
+          'abort',
+          abortListener,
+          { once: true });
+      }
       reject(err);
     };
     const resolver = (...args) => {
       if (typeof emitter.removeListener === 'function') {
         emitter.removeListener('error', errorListener);
       }
+      if (signal != null) {
+        eventTargetAgnosticRemoveListener(
+          signal,
+          'abort',
+          abortListener,
+          { once: true });
+      }
       resolve(args);
     };
     eventTargetAgnosticAddListener(emitter, name, resolver, { once: true });
     if (name !== 'error') {
       addErrorHandlerIfEventEmitter(emitter, errorListener, { once: true });
     }
+    function abortListener() {
+      if (typeof emitter.removeListener === 'function') {
+        emitter.removeListener(name, resolver);
+        emitter.removeListener('error', errorListener);
+      } else {
+        eventTargetAgnosticRemoveListener(
+          emitter,
+          name,
+          resolver,
+          { once: true });
+        eventTargetAgnosticRemoveListener(
+          emitter,
+          'error',
+          errorListener,
+          { once: true });
+      }
+      reject(lazyDOMException('The operation was aborted', 'AbortError'));
+    }
+    if (signal != null) {
+      signal.addEventListener('abort', abortListener, { once: true });
+    }
   });
 }
 

diff --git a/lib/internal/validators.js b/lib/internal/validators.js
@@ -218,6 +218,15 @@ const validateCallback = hideStackFrames((callback) => {
     throw new ERR_INVALID_CALLBACK(callback);
 });
 
+const validateAbortSignal = hideStackFrames((signal, name) => {
+  if (signal !== undefined &&
+      (signal === null ||
+       typeof signal !== 'object' ||
+       !('aborted' in signal))) {
+    throw new ERR_INVALID_ARG_TYPE(name, 'AbortSignal', signal);
+  }
+});
+
 module.exports = {
   isInt32,
   isUint32,
@@ -236,4 +245,5 @@ module.exports = {
   validateString,
   validateUint32,
   validateCallback,
+  validateAbortSignal,
 };
diff --git a/test/parallel/test-events-once.js b/test/parallel/test-events-once.js
@@ -1,9 +1,14 @@
 'use strict';
-// Flags: --expose-internals
+// Flags: --expose-internals --no-warnings --experimental-abortcontroller
 
 const common = require('../common');
 const { once, EventEmitter } = require('events');
-const { strictEqual, deepStrictEqual, fail } = require('assert');
+const {
+  strictEqual,
+  deepStrictEqual,
+  fail,
+  rejects,
+} = require('assert');
 const { EventTarget, Event } = require('internal/event_target');
 
 async function onceAnEvent() {
@@ -114,6 +119,81 @@ async function prioritizesEventEmitter() {
   process.nextTick(() => ee.emit('foo'));
   await once(ee, 'foo');
 }
+
+async function abortSignalBefore() {
+  const ee = new EventEmitter();
+  const ac = new AbortController();
+  ee.on('error', common.mustNotCall());
+  ac.abort();
+
+  await Promise.all([1, {}, 'hi', null, false].map((signal) => {
+    return rejects(once(ee, 'foo', { signal }), {
+      code: 'ERR_INVALID_ARG_TYPE'
+    });
+  }));
+
+  return rejects(once(ee, 'foo', { signal: ac.signal }), {
+    name: 'AbortError'
+  });
+}
+
+async function abortSignalAfter() {
+  const ee = new EventEmitter();
+  const ac = new AbortController();
+  ee.on('error', common.mustNotCall());
+  const r = rejects(once(ee, 'foo', { signal: ac.signal }), {
+    name: 'AbortError'
+  });
+  process.nextTick(() => ac.abort());
+  return r;
+}
+
+async function abortSignalAfterEvent() {
+  const ee = new EventEmitter();
+  const ac = new AbortController();
+  process.nextTick(() => {
+    ee.emit('foo');
+    ac.abort();
+  });
+  await once(ee, 'foo', { signal: ac.signal });
+}
+
+async function eventTargetAbortSignalBefore() {
+  const et = new EventTarget();
+  const ac = new AbortController();
+  ac.abort();
+
+  await Promise.all([1, {}, 'hi', null, false].map((signal) => {
+    return rejects(once(et, 'foo', { signal }), {
+      code: 'ERR_INVALID_ARG_TYPE'
+    });
+  }));
+
+  return rejects(once(et, 'foo', { signal: ac.signal }), {
+    name: 'AbortError'
+  });
+}
+
+async function eventTargetAbortSignalAfter() {
+  const et = new EventTarget();
+  const ac = new AbortController();
+  const r = rejects(once(et, 'foo', { signal: ac.signal }), {
+    name: 'AbortError'
+  });
+  process.nextTick(() => ac.abort());
+  return r;
+}
+
+async function eventTargetAbortSignalAfterEvent() {
+  const et = new EventTarget();
+  const ac = new AbortController();
+  process.nextTick(() => {
+    et.dispatchEvent(new Event('foo'));
+    ac.abort();
+  });
+  await once(et, 'foo', { signal: ac.signal });
+}
+
 Promise.all([
   onceAnEvent(),
   onceAnEventWithTwoArgs(),
@@ -123,4 +203,10 @@ Promise.all([
   onceWithEventTarget(),
   onceWithEventTargetError(),
   prioritizesEventEmitter(),
+  abortSignalBefore(),
+  abortSignalAfter(),
+  abortSignalAfterEvent(),
+  eventTargetAbortSignalBefore(),
+  eventTargetAbortSignalAfter(),
+  eventTargetAbortSignalAfterEvent(),
 ]).then(common.mustCall());