worker: make MessagePort inherit from EventTarget

Use `NodeEventTarget` to provide a mixed `EventEmitter`/`EventTarget`
API interface.

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]>

Author: addaleax

benchmark/worker/messageport.js

@@ -4,6 +4,7 @@ const common = require('../common.js');
 const { MessageChannel } = require('worker_threads');
 const bench = common.createBenchmark(main, {
   payload: ['string', 'object'],
+  style: ['eventtarget', 'eventemitter'],
   n: [1e6]
 });
 
@@ -25,14 +26,26 @@ function main(conf) {
   const { port1, port2 } = new MessageChannel();
 
   let messages = 0;
-  port2.onmessage = () => {
+  function listener() {
     if (messages++ === n) {
       bench.end(n);
       port1.close();
     } else {
       write();
     }
-  };
+  }
+
+  switch (conf.style) {
+    case 'eventtarget':
+      port2.onmessage = listener;
+      break;
+    case 'eventemitter':
+      port2.on('message', listener);
+      break;
+    default:
+      throw new Error('Unsupported listener type');
+  }
+
   bench.start();
   write();
 

lib/internal/per_context/messageport.js

@@ -1,12 +1,31 @@
 'use strict';
+const {
+  SymbolFor,
+} = primordials;
+
 class MessageEvent {
-  constructor(data, target) {
+  constructor(data, target, type) {
     this.data = data;
     this.target = target;
+    this.type = type;
   }
 }
 
-exports.emitMessage = function(data) {
-  if (typeof this.onmessage === 'function')
-    this.onmessage(new MessageEvent(data, this));
+const kHybridDispatch = SymbolFor('nodejs.internal.kHybridDispatch');
+
+exports.emitMessage = function(data, type) {
+  if (typeof this[kHybridDispatch] === 'function') {
+    this[kHybridDispatch](data, type, undefined);
+    return;
+  }
+
+  const event = new MessageEvent(data, this, type);
+  if (type === 'message') {
+    if (typeof this.onmessage === 'function')
+      this.onmessage(event);
+  } else {
+    // eslint-disable-next-line no-lonely-if
+    if (typeof this.onmessageerror === 'function')
+      this.onmessageerror(event);
+  }
 };

lib/internal/worker/io.js

@@ -24,20 +24,23 @@ const {
   stopMessagePort
 } = internalBinding('messaging');
 const {
-  threadId,
   getEnvMessagePort
 } = internalBinding('worker');
 
 const { Readable, Writable } = require('stream');
-const EventEmitter = require('events');
+const {
+  Event,
+  NodeEventTarget,
+  defineEventHandler,
+  initNodeEventTarget,
+  kCreateEvent,
+  kNewListener,
+  kRemoveListener,
+} = require('internal/event_target');
 const { inspect } = require('internal/util/inspect');
-let debug = require('internal/util/debuglog').debuglog('worker', (fn) => {
-  debug = fn;
-});
 
 const kIncrementsPortRef = Symbol('kIncrementsPortRef');
 const kName = Symbol('kName');
-const kOnMessageListener = Symbol('kOnMessageListener');
 const kPort = Symbol('kPort');
 const kWaitingStreams = Symbol('kWaitingStreams');
 const kWritableCallbacks = Symbol('kWritableCallbacks');
@@ -54,55 +57,47 @@ const messageTypes = {
 };
 
 // We have to mess with the MessagePort prototype a bit, so that a) we can make
-// it inherit from EventEmitter, even though it is a C++ class, and b) we do
+// it inherit from NodeEventTarget, even though it is a C++ class, and b) we do
 // not provide methods that are not present in the Browser and not documented
 // on our side (e.g. hasRef).
 // Save a copy of the original set of methods as a shallow clone.
 const MessagePortPrototype = ObjectCreate(
   ObjectGetPrototypeOf(MessagePort.prototype),
   ObjectGetOwnPropertyDescriptors(MessagePort.prototype));
 // Set up the new inheritance chain.
-ObjectSetPrototypeOf(MessagePort, EventEmitter);
-ObjectSetPrototypeOf(MessagePort.prototype, EventEmitter.prototype);
+ObjectSetPrototypeOf(MessagePort, NodeEventTarget);
+ObjectSetPrototypeOf(MessagePort.prototype, NodeEventTarget.prototype);
 // Copy methods that are inherited from HandleWrap, because
 // changing the prototype of MessagePort.prototype implicitly removed them.
 MessagePort.prototype.ref = MessagePortPrototype.ref;
 MessagePort.prototype.unref = MessagePortPrototype.unref;
 
-// A communication channel consisting of a handle (that wraps around an
-// uv_async_t) which can receive information from other threads and emits
-// .onmessage events, and a function used for sending data to a MessagePort
-// in some other thread.
-MessagePort.prototype[kOnMessageListener] = function onmessage(event) {
-  if (event.data && event.data.type !== messageTypes.STDIO_WANTS_MORE_DATA)
-    debug(`[${threadId}] received message`, event);
-  // Emit the deserialized object to userland.
-  this.emit('message', event.data);
-};
-
-// This is for compatibility with the Web's MessagePort API. It makes sense to
-// provide it as an `EventEmitter` in Node.js, but if somebody overrides
-// `onmessage`, we'll switch over to the Web API model.
-ObjectDefineProperty(MessagePort.prototype, 'onmessage', {
-  enumerable: true,
-  configurable: true,
-  get() {
-    return this[kOnMessageListener];
-  },
-  set(value) {
-    this[kOnMessageListener] = value;
-    if (typeof value === 'function') {
-      this.ref();
-      MessagePortPrototype.start.call(this);
-    } else {
-      this.unref();
-      stopMessagePort(this);
-    }
+class MessageEvent extends Event {
+  constructor(data, target, type) {
+    super(type);
+    this.data = data;
   }
-});
+}
+
+ObjectDefineProperty(
+  MessagePort.prototype,
+  kCreateEvent,
+  {
+    value: function(data, type) {
+      return new MessageEvent(data, this, type);
+    },
+    configurable: false,
+    writable: false,
+    enumerable: false,
+  });
 
 // This is called from inside the `MessagePort` constructor.
 function oninit() {
+  initNodeEventTarget(this);
+  // TODO(addaleax): This should be on MessagePort.prototype, but
+  // defineEventHandler() does not support that.
+  defineEventHandler(this, 'message');
+  defineEventHandler(this, 'messageerror');
   setupPortReferencing(this, this, 'message');
 }
 
@@ -112,9 +107,15 @@ ObjectDefineProperty(MessagePort.prototype, onInitSymbol, {
   value: oninit
 });
 
+class MessagePortCloseEvent extends Event {
+  constructor() {
+    super('close');
+  }
+}
+
 // This is called after the underlying `uv_async_t` has been closed.
 function onclose() {
-  this.emit('close');
+  this.dispatchEvent(new MessagePortCloseEvent());
 }
 
 ObjectDefineProperty(MessagePort.prototype, handleOnCloseSymbol, {
@@ -156,18 +157,36 @@ function setupPortReferencing(port, eventEmitter, eventName) {
   // If there are none or all are removed, unref() the channel so the worker
   // can shutdown gracefully.
   port.unref();
-  eventEmitter.on('newListener', (name) => {
-    if (name === eventName && eventEmitter.listenerCount(eventName) === 0) {
+  eventEmitter.on('newListener', function(name) {
+    if (name === eventName) newListener(eventEmitter.listenerCount(name));
+  });
+  eventEmitter.on('removeListener', function(name) {
+    if (name === eventName) removeListener(eventEmitter.listenerCount(name));
+  });
+  const origNewListener = eventEmitter[kNewListener];
+  eventEmitter[kNewListener] = function(size, type, ...args) {
+    if (type === eventName) newListener(size - 1);
+    return origNewListener.call(this, size, type, ...args);
+  };
+  const origRemoveListener = eventEmitter[kRemoveListener];
+  eventEmitter[kRemoveListener] = function(size, type, ...args) {
+    if (type === eventName) removeListener(size);
+    return origRemoveListener.call(this, size, type, ...args);
+  };
+
+  function newListener(size) {
+    if (size === 0) {
       port.ref();
       MessagePortPrototype.start.call(port);
     }
-  });
-  eventEmitter.on('removeListener', (name) => {
-    if (name === eventName && eventEmitter.listenerCount(eventName) === 0) {
+  }
+
+  function removeListener(size) {
+    if (size === 0) {
       stopMessagePort(port);
       port.unref();
     }
-  });
+  }
 }
 
 

src/node_messaging.cc

@@ -747,6 +747,8 @@ void MessagePort::OnMessage() {
 
     Local<Value> payload;
     Local<Value> message_error;
+    Local<Value> argv[2];
+
     {
       // Catch any exceptions from parsing the message itself (not from
       // emitting it) as 'messageeror' events.
@@ -765,16 +767,15 @@ void MessagePort::OnMessage() {
       continue;
     }
 
-    if (MakeCallback(emit_message, 1, &payload).IsEmpty()) {
+    argv[0] = payload;
+    argv[1] = env()->message_string();
+
+    if (MakeCallback(emit_message, arraysize(argv), argv).IsEmpty()) {
     reschedule:
       if (!message_error.IsEmpty()) {
-        // This should become a `messageerror` event in the sense of the
-        // EventTarget API at some point.
-        Local<Value> argv[] = {
-          env()->messageerror_string(),
-          message_error
-        };
-        USE(MakeCallback(env()->emit_string(), arraysize(argv), argv));
+        argv[0] = message_error;
+        argv[1] = env()->messageerror_string();
+        USE(MakeCallback(emit_message, arraysize(argv), argv));
       }
 
       // Re-schedule OnMessage() execution in case of failure.

test/parallel/test-bootstrap-modules.js

@@ -99,6 +99,7 @@ if (!common.isMainThread) {
     'NativeModule _stream_transform',
     'NativeModule _stream_writable',
     'NativeModule internal/error_serdes',
+    'NativeModule internal/event_target',
     'NativeModule internal/process/worker_thread_only',
     'NativeModule internal/streams/buffer_list',
     'NativeModule internal/streams/destroy',
@@ -109,6 +110,7 @@ if (!common.isMainThread) {
     'NativeModule internal/worker',
     'NativeModule internal/worker/io',
     'NativeModule stream',
+    'NativeModule util',
     'NativeModule worker_threads',
   ].forEach(expectedModules.add.bind(expectedModules));
 }

test/parallel/test-crypto-key-objects-messageport.js

@@ -75,9 +75,9 @@ for (const [key, repr] of keys) {
 
     // TODO(addaleax): Switch this to a 'messageerror' event once MessagePort
     // implements EventTarget fully and in a cross-context manner.
-    port2moved.emit = common.mustCall((name, err) => {
-      assert.strictEqual(name, 'messageerror');
-      assert.strictEqual(err.code, 'ERR_MESSAGE_TARGET_CONTEXT_UNAVAILABLE');
+    port2moved.onmessageerror = common.mustCall((event) => {
+      assert.strictEqual(event.data.code,
+                         'ERR_MESSAGE_TARGET_CONTEXT_UNAVAILABLE');
     });
 
     port2moved.start();

test/parallel/test-worker-message-port-inspect-during-init-hook.js

@@ -10,8 +10,9 @@ const { MessageChannel } = require('worker_threads');
 
 async_hooks.createHook({
   init: common.mustCall((id, type, triggerId, resource) => {
-    assert.strictEqual(util.inspect(resource),
-                       'MessagePort { active: true, refed: false }');
+    assert.strictEqual(
+      util.inspect(resource),
+      'MessagePort [EventTarget] { active: true, refed: false }');
   }, 2)
 }).enable();
 

test/parallel/test-worker-message-port-transfer-filehandle.js

@@ -57,9 +57,9 @@ const { once } = require('events');
   });
   // TODO(addaleax): Switch this to a 'messageerror' event once MessagePort
   // implements EventTarget fully and in a cross-context manner.
-  port2moved.emit = common.mustCall((name, err) => {
-    assert.strictEqual(name, 'messageerror');
-    assert.strictEqual(err.code, 'ERR_MESSAGE_TARGET_CONTEXT_UNAVAILABLE');
+  port2moved.onmessageerror = common.mustCall((event) => {
+    assert.strictEqual(event.data.code,
+                       'ERR_MESSAGE_TARGET_CONTEXT_UNAVAILABLE');
   });
   port2moved.start();
 

test/parallel/test-worker-message-port.js

@@ -154,7 +154,9 @@ const { MessageChannel, MessagePort } = require('worker_threads');
   assert.deepStrictEqual(
     Object.getOwnPropertyNames(MessagePort.prototype).sort(),
     [
-      'close', 'constructor', 'onmessage', 'postMessage', 'ref', 'start',
+      // TODO(addaleax): This should include onmessage (and eventually
+      // onmessageerror).
+      'close', 'constructor', 'postMessage', 'ref', 'start',
       'unref'
     ]);
 }