events: expand NodeEventTarget functionality

Enable `NodeEventTarget` as a base class for `MessagePort`,
by enabling special processing of events for Node.js listeners,
and removing implicit constructors/private properties so that
classes can be made subclasses of `NodeEventTarget` after they
are created.

PR-URL: #34057
Refs: https://twitter.com/addaleax/status/1276289101671608320
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: David Carlier <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>

lib/internal/event_target.js

@@ -31,8 +31,13 @@ const kEvents = Symbol('kEvents');
 const kStop = Symbol('kStop');
 const kTarget = Symbol('kTarget');
 
+const kHybridDispatch = Symbol.for('nodejs.internal.kHybridDispatch');
+const kCreateEvent = Symbol('kCreateEvent');
 const kNewListener = Symbol('kNewListener');
 const kRemoveListener = Symbol('kRemoveListener');
+const kIsNodeStyleListener = Symbol('kIsNodeStyleListener');
+const kMaxListeners = Symbol('kMaxListeners');
+const kMaxListenersWarned = Symbol('kMaxListenersWarned');
 
 // Lazy load perf_hooks to avoid the additional overhead on startup
 let perf_hooks;
@@ -157,7 +162,7 @@ Object.defineProperty(Event.prototype, SymbolToStringTag, {
 // the linked list makes dispatching faster, even if adding/removing is
 // slower.
 class Listener {
-  constructor(previous, listener, once, capture, passive) {
+  constructor(previous, listener, once, capture, passive, isNodeStyleListener) {
     this.next = undefined;
     if (previous !== undefined)
       previous.next = this;
@@ -166,6 +171,7 @@ class Listener {
     this.once = once;
     this.capture = capture;
     this.passive = passive;
+    this.isNodeStyleListener = isNodeStyleListener;
 
     this.callback =
       typeof listener === 'function' ?
@@ -185,13 +191,17 @@ class Listener {
   }
 }
 
+function initEventTarget(self) {
+  self[kEvents] = new Map();
+}
+
 class EventTarget {
   // Used in checking whether an object is an EventTarget. This is a well-known
   // symbol as EventTarget may be used cross-realm. See discussion in #33661.
   static [kIsEventTarget] = true;
 
   constructor() {
-    this[kEvents] = new Map();
+    initEventTarget(this);
   }
 
   [kNewListener](size, type, listener, once, capture, passive) {}
@@ -206,7 +216,8 @@ class EventTarget {
     const {
       once,
       capture,
-      passive
+      passive,
+      isNodeStyleListener
     } = validateEventListenerOptions(options);
 
     if (!shouldAddListener(listener)) {
@@ -228,7 +239,7 @@ class EventTarget {
     if (root === undefined) {
       root = { size: 1, next: undefined };
       // This is the first handler in our linked list.
-      new Listener(root, listener, once, capture, passive);
+      new Listener(root, listener, once, capture, passive, isNodeStyleListener);
       this[kNewListener](root.size, type, listener, once, capture, passive);
       this[kEvents].set(type, root);
       return;
@@ -247,7 +258,8 @@ class EventTarget {
       return;
     }
 
-    new Listener(previous, listener, once, capture, passive);
+    new Listener(previous, listener, once, capture, passive,
+                 isNodeStyleListener);
     root.size++;
     this[kNewListener](root.size, type, listener, once, capture, passive);
   }
@@ -290,39 +302,61 @@ class EventTarget {
     if (event[kTarget] !== null)
       throw new ERR_EVENT_RECURSION(event.type);
 
-    const root = this[kEvents].get(event.type);
+    this[kHybridDispatch](event, event.type, event);
+
+    return event.defaultPrevented !== true;
+  }
+
+  [kHybridDispatch](nodeValue, type, event) {
+    const createEvent = () => {
+      if (event === undefined) {
+        event = this[kCreateEvent](nodeValue, type);
+        event[kTarget] = this;
+      }
+      return event;
+    };
+
+    const root = this[kEvents].get(type);
     if (root === undefined || root.next === undefined)
       return true;
 
-    event[kTarget] = this;
+    if (event !== undefined)
+      event[kTarget] = this;
 
     let handler = root.next;
     let next;
 
     while (handler !== undefined &&
-           (handler.passive || event[kStop] !== true)) {
+           (handler.passive || event?.[kStop] !== true)) {
       // Cache the next item in case this iteration removes the current one
       next = handler.next;
 
       if (handler.once) {
         handler.remove();
         root.size--;
+        const { listener, capture } = handler;
+        this[kRemoveListener](root.size, type, listener, capture);
       }
 
       try {
-        const result = handler.callback.call(this, event);
+        let arg;
+        if (handler.isNodeStyleListener) {
+          arg = nodeValue;
+        } else {
+          arg = createEvent();
+        }
+        const result = handler.callback.call(this, arg);
         if (result !== undefined && result !== null)
-          addCatch(this, result, event);
+          addCatch(this, result, createEvent());
       } catch (err) {
-        emitUnhandledRejectionOrErr(this, err, event);
+        emitUnhandledRejectionOrErr(this, err, createEvent());
       }
 
       handler = next;
     }
 
-    event[kTarget] = undefined;
-
-    return event.defaultPrevented !== true;
+    if (event !== undefined)
+      event[kTarget] = undefined;
   }
 
   [customInspectSymbol](depth, options) {
@@ -350,15 +384,19 @@ Object.defineProperty(EventTarget.prototype, SymbolToStringTag, {
   value: 'EventTarget',
 });
 
-const kMaxListeners = Symbol('maxListeners');
-const kMaxListenersWarned = Symbol('maxListenersWarned');
+function initNodeEventTarget(self) {
+  initEventTarget(self);
+  // eslint-disable-next-line no-use-before-define
+  self[kMaxListeners] = NodeEventTarget.defaultMaxListeners;
+  self[kMaxListenersWarned] = false;
+}
+
 class NodeEventTarget extends EventTarget {
   static defaultMaxListeners = 10;
 
   constructor() {
     super();
-    this[kMaxListeners] = NodeEventTarget.defaultMaxListeners;
-    this[kMaxListenersWarned] = false;
+    initNodeEventTarget(this);
   }
 
   [kNewListener](size, type, listener, once, capture, passive) {
@@ -410,17 +448,18 @@ class NodeEventTarget extends EventTarget {
   }
 
   on(type, listener) {
-    this.addEventListener(type, listener);
+    this.addEventListener(type, listener, { [kIsNodeStyleListener]: true });
     return this;
   }
 
   addListener(type, listener) {
-    this.addEventListener(type, listener);
+    this.addEventListener(type, listener, { [kIsNodeStyleListener]: true });
     return this;
   }
 
   once(type, listener) {
-    this.addEventListener(type, listener, { once: true });
+    this.addEventListener(type, listener,
+                          { once: true, [kIsNodeStyleListener]: true });
     return this;
   }
 
@@ -470,6 +509,7 @@ function validateEventListenerOptions(options) {
     once: Boolean(options.once),
     capture: Boolean(options.capture),
     passive: Boolean(options.passive),
+    isNodeStyleListener: Boolean(options[kIsNodeStyleListener])
   };
 }
 
@@ -520,4 +560,9 @@ module.exports = {
   EventTarget,
   NodeEventTarget,
   defineEventHandler,
+  initEventTarget,
+  initNodeEventTarget,
+  kCreateEvent,
+  kNewListener,
+  kRemoveListener,
 };