events: allow use of AbortController with on

Signed-off-by: James M Snell <[email protected]>

Author: jasnell

doc/api/events.md

@@ -1000,7 +1000,7 @@ Value: `Symbol.for('nodejs.rejection')`
 
 See how to write a custom [rejection handler][rejection].
 
-## `events.on(emitter, eventName)`
+## `events.on(emitter, eventName[, options])`
 <!-- YAML
 added:
  - v13.6.0
@@ -1009,6 +1009,9 @@ added:
 
 * `emitter` {EventEmitter}
 * `eventName` {string|symbol} The name of the event being listened for
+* `options` {Object}
+  * `signal` {AbortSignal} An {AbortSignal} that can be used to cancel awaiting
+    events.
 * Returns: {AsyncIterator} that iterates `eventName` events emitted by the `emitter`
 
 ```js
@@ -1038,6 +1041,33 @@ if the `EventEmitter` emits `'error'`. It removes all listeners when
 exiting the loop. The `value` returned by each iteration is an array
 composed of the emitted event arguments.
 
+An {AbortSignal} may be used to cancel waiting on events:
+
+```js
+const { on, EventEmitter } = require('events');
+const ac = new AbortController();
+
+(async () => {
+  const ee = new EventEmitter();
+
+  // Emit later on
+  process.nextTick(() => {
+    ee.emit('foo', 'bar');
+    ee.emit('foo', 42);
+  });
+
+  for await (const event of on(ee, 'foo', { signal: ac.signal })) {
+    // The execution of this inner block is synchronous and it
+    // processes one event at a time (even with await). Do not use
+    // if concurrent execution is required.
+    console.log(event); // prints ['bar'] [42]
+  }
+  // Unreachable here
+})();
+
+process.nextTick(() => ac.abort());
+```
+
 ## `EventTarget` and `Event` API
 <!-- YAML
 added: v14.5.0

lib/events.js

@@ -730,7 +730,13 @@ function eventTargetAgnosticAddListener(emitter, name, listener, flags) {
   }
 }
 
-function on(emitter, event) {
+function on(emitter, event, options) {
+  const { signal } = { ...options };
+  validateAbortSignal(signal, 'options.signal');
+  if (signal && signal.aborted) {
+    throw lazyDOMException('The operation was aborted', 'AbortError');
+  }
+
   const unconsumedEvents = [];
   const unconsumedPromises = [];
   let error = null;
@@ -768,6 +774,15 @@ function on(emitter, event) {
     return() {
       eventTargetAgnosticRemoveListener(emitter, event, eventHandler);
       eventTargetAgnosticRemoveListener(emitter, 'error', errorHandler);
+
+      if (signal) {
+        eventTargetAgnosticRemoveListener(
+          signal,
+          'abort',
+          abortListener,
+          { once: true });
+      }
+
       finished = true;
 
       for (const promise of unconsumedPromises) {
@@ -797,9 +812,20 @@ function on(emitter, event) {
     addErrorHandlerIfEventEmitter(emitter, errorHandler);
   }
 
+  if (signal) {
+    eventTargetAgnosticAddListener(
+      signal,
+      'abort',
+      abortListener,
+      { once: true });
+  }
 
   return iterator;
 
+  function abortListener() {
+    errorHandler(lazyDOMException('The operation was aborted', 'AbortError'));
+  }
+
   function eventHandler(...args) {
     const promise = unconsumedPromises.shift();
     if (promise) {

test/parallel/test-event-on-async-iterator.js

@@ -1,4 +1,4 @@
-// Flags: --expose-internals
+// Flags: --expose-internals --no-warnings
 'use strict';
 
 const common = require('../common');
@@ -248,6 +248,117 @@ async function nodeEventTarget() {
   clearInterval(interval);
 }
 
+async function abortableOnBefore() {
+  const ee = new EventEmitter();
+  const ac = new AbortController();
+  ac.abort();
+  [1, {}, null, false, 'hi'].forEach((signal) => {
+    assert.throws(() => on(ee, 'foo', { signal }), {
+      code: 'ERR_INVALID_ARG_TYPE'
+    });
+  });
+  assert.throws(() => on(ee, 'foo', { signal: ac.signal }), {
+    name: 'AbortError'
+  });
+}
+
+async function eventTargetAbortableOnBefore() {
+  const et = new EventTarget();
+  const ac = new AbortController();
+  ac.abort();
+  [1, {}, null, false, 'hi'].forEach((signal) => {
+    assert.throws(() => on(et, 'foo', { signal }), {
+      code: 'ERR_INVALID_ARG_TYPE'
+    });
+  });
+  assert.throws(() => on(et, 'foo', { signal: ac.signal }), {
+    name: 'AbortError'
+  });
+}
+
+async function abortableOnAfter() {
+  const ee = new EventEmitter();
+  const ac = new AbortController();
+
+  const i = setInterval(() => ee.emit('foo', 'foo'), 10);
+
+  async function foo() {
+    for await (const f of on(ee, 'foo', { signal: ac.signal })) {
+      assert.strictEqual(f, 'foo');
+    }
+  }
+
+  foo().catch(common.mustCall((error) => {
+    assert.strictEqual(error.name, 'AbortError');
+  })).finally(() => {
+    clearInterval(i);
+  });
+
+  process.nextTick(() => ac.abort());
+}
+
+async function eventTargetAbortableOnAfter() {
+  const et = new EventTarget();
+  const ac = new AbortController();
+
+  const i = setInterval(() => et.dispatchEvent(new Event('foo')), 10);
+
+  async function foo() {
+    for await (const f of on(et, 'foo', { signal: ac.signal })) {
+      assert(f);
+    }
+  }
+
+  foo().catch(common.mustCall((error) => {
+    assert.strictEqual(error.name, 'AbortError');
+  })).finally(() => {
+    clearInterval(i);
+  });
+
+  process.nextTick(() => ac.abort());
+}
+
+async function eventTargetAbortableOnAfter2() {
+  const et = new EventTarget();
+  const ac = new AbortController();
+
+  const i = setInterval(() => et.dispatchEvent(new Event('foo')), 10);
+
+  async function foo() {
+    for await (const f of on(et, 'foo', { signal: ac.signal })) {
+      assert(f);
+      // Cancel after a single event has been triggered.
+      ac.abort();
+    }
+  }
+
+  foo().catch(common.mustCall((error) => {
+    assert.strictEqual(error.name, 'AbortError');
+  })).finally(() => {
+    clearInterval(i);
+  });
+}
+
+async function abortableOnAfterDone() {
+  const ee = new EventEmitter();
+  const ac = new AbortController();
+
+  const i = setInterval(() => ee.emit('foo', 'foo'), 1);
+  let count = 0;
+
+  async function foo() {
+    for await (const f of on(ee, 'foo', { signal: ac.signal })) {
+      assert.strictEqual(f[0], 'foo');
+      if (++count === 5)
+        break;
+    }
+    ac.abort();  // No error will occur
+  }
+
+  foo().finally(() => {
+    clearInterval(i);
+  });
+}
 
 async function run() {
   const funcs = [
@@ -260,7 +371,13 @@ async function run() {
     iterableThrow,
     eventTarget,
     errorListenerCount,
-    nodeEventTarget
+    nodeEventTarget,
+    abortableOnBefore,
+    abortableOnAfter,
+    eventTargetAbortableOnBefore,
+    eventTargetAbortableOnAfter,
+    eventTargetAbortableOnAfter2,
+    abortableOnAfterDone
   ];
 
   for (const fn of funcs) {