events: addAbortListener ignores disposal and passes undefined event when signal is already aborted

Version

Tested on v24.7.0, reproduces on main (commit a48e33fb6f).

Platform

All platforms (pure JS logic in `lib/internal/events/abort_listener.js`).

Subsystem

events

What steps will reproduce the bug?

'use strict';
const { addAbortListener } = require('node:events');

// --- Bug 1: Disposing the listener does not cancel it if the signal was already aborted ---
const ac = new AbortController();
ac.abort();

let listenerCalledAfterDispose = false;
{
  using _ = addAbortListener(ac.signal, (event) => {
    listenerCalledAfterDispose = true;
    console.log('Event argument received:', event); // Bug 2: logs 'undefined' instead of an Event object
  });
  // Scope exits here -> disposable[Symbol.dispose]() is invoked synchronously.
}

queueMicrotask(() => {
  console.log('Listener called after disposal:', listenerCalledAfterDispose);
  // Prints: true (Expected: false)
});

How often does it reproduce? Is there a required condition?

100% reproducible whenever addAbortListener() is called on an AbortSignal where signal.aborted === true.

What is the expected behavior? Why is that the expected behavior?

1. Calling disposable[Symbol.dispose]() (or exiting a using scope) before the microtask executes should disarm/cancel the scheduled listener. The main motivation for addAbortListener() is TC39 explicit resource management, so disposal should always prevent the callback from firing.
2. The listener callback should receive an Event object (type === 'abort'), consistent with when the listener is triggered by an active signal.

What do you see instead?

1. The listener runs unconditionally in the microtask, ignoring Symbol.dispose().
2. The listener receives undefined instead of an Event instance. Code following the documented (e) => { ... } signature that accesses properties like e.type crashes with a TypeError inside the microtask.

Additional information

In lib/internal/events/abort_listener.js:

  let removeEventListener;
  if (signal.aborted) {
    queueMicrotask ??= require('internal/process/task_queues').queueMicrotask;
    queueMicrotask(() => listener()); // <-- 1. Listener called with no argument
  } else {
    kResistStopPropagation ??= require('internal/event_target').kResistStopPropagation;
    abortListenerOptions ??= ObjectFreeze({ __proto__: null, once: true, [kResistStopPropagation]: true });
    signal.addEventListener('abort', listener, abortListenerOptions);
    removeEventListener = () => {
      signal.removeEventListener('abort', listener);
    };
  }
  return {
    __proto__: null,
    [SymbolDispose]() {
      removeEventListener?.(); // <-- 2. removeEventListener is undefined on the aborted branch
    },
  };

When signal.aborted is true:

• removeEventListener is left undefined, making SymbolDispose a no-op.
• queueMicrotask(() => listener()) invokes listener without passing an Event object.

A possible fix is tracking a disposed flag for the microtask and creating an Event('abort') instance:

  let removeEventListener;
  if (signal.aborted) {
    queueMicrotask ??= require('internal/process/task_queues').queueMicrotask;
    let disposed = false;
    queueMicrotask(() => {
      if (!disposed) {
        const { Event } = require('internal/event_target');
        listener(new Event('abort', { cancelable: false, bubbles: false }));
      }
    });
    removeEventListener = () => {
      disposed = true;
    };
  } else {
    // ...

まず提供されたスクリプトを再現し、次に lib/internal/events/abort_listener.js を読み、signal.aborted ブランチと SymbolDispose の処理に注目します。キューに入れられたマイクロタスクの前の破棄とコールバック引数について回帰テストを追加し、その後、破棄後は listener が抑制され、それ以外の場合は abort Event を受け取ることを確認します。