worker: reduce MessagePort prototype to documented API

`MessagePort` is special because it has to be a C++ API
that is exposed to userland. Therefore, there is a number
of internal methods on its native prototype; this commit
reduces this set of methods to only what is documented in
the API.

PR-URL: #23037
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: addaleax

lib/internal/worker.js

@@ -14,18 +14,18 @@ const {
 
 const { internalBinding } = require('internal/bootstrap/loaders');
 const { MessagePort, MessageChannel } = internalBinding('messaging');
-const { handle_onclose } = internalBinding('symbols');
+const {
+  handle_onclose: handleOnCloseSymbol,
+  oninit: onInitSymbol
+} = internalBinding('symbols');
 const { clearAsyncIdStack } = require('internal/async_hooks');
 const { serializeError, deserializeError } = require('internal/error-serdes');
 const { pathToFileURL } = require('url');
 
-util.inherits(MessagePort, EventEmitter);
-
 const {
   Worker: WorkerImpl,
   getEnvMessagePort,
-  threadId,
-  oninit: oninit_symbol
+  threadId
 } = internalBinding('worker');
 
 const isMainThread = threadId === 0;
@@ -58,6 +58,23 @@ const messageTypes = {
   LOAD_SCRIPT: 'loadScript'
 };
 
+// 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
+// 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 = Object.create(
+  Object.getPrototypeOf(MessagePort.prototype),
+  Object.getOwnPropertyDescriptors(MessagePort.prototype));
+// Set up the new inheritance chain.
+Object.setPrototypeOf(MessagePort, EventEmitter);
+Object.setPrototypeOf(MessagePort.prototype, EventEmitter.prototype);
+// Finally, purge methods we don't want to be public.
+delete MessagePort.prototype.stop;
+delete MessagePort.prototype.drain;
+delete MessagePort.prototype.hasRef;
+delete MessagePort.prototype.getAsyncId;
+
 // 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
@@ -81,10 +98,10 @@ Object.defineProperty(MessagePort.prototype, 'onmessage', {
     this[kOnMessageListener] = value;
     if (typeof value === 'function') {
       this.ref();
-      this.start();
+      MessagePortPrototype.start.call(this);
     } else {
       this.unref();
-      this.stop();
+      MessagePortPrototype.stop.call(this);
     }
   }
 });
@@ -94,7 +111,7 @@ function oninit() {
   setupPortReferencing(this, this, 'message');
 }
 
-Object.defineProperty(MessagePort.prototype, oninit_symbol, {
+Object.defineProperty(MessagePort.prototype, onInitSymbol, {
   enumerable: true,
   writable: false,
   value: oninit
@@ -111,22 +128,18 @@ function onclose() {
   this.emit('close');
 }
 
-Object.defineProperty(MessagePort.prototype, handle_onclose, {
+Object.defineProperty(MessagePort.prototype, handleOnCloseSymbol, {
   enumerable: false,
   writable: false,
   value: onclose
 });
 
-const originalClose = MessagePort.prototype.close;
 MessagePort.prototype.close = function(cb) {
   if (typeof cb === 'function')
     this.once('close', cb);
-  originalClose.call(this);
+  MessagePortPrototype.close.call(this);
 };
 
-const drainMessagePort = MessagePort.prototype.drain;
-delete MessagePort.prototype.drain;
-
 Object.defineProperty(MessagePort.prototype, util.inspect.custom, {
   enumerable: false,
   writable: false,
@@ -135,7 +148,7 @@ Object.defineProperty(MessagePort.prototype, util.inspect.custom, {
     try {
       // This may throw when `this` does not refer to a native object,
       // e.g. when accessing the prototype directly.
-      ref = this.hasRef();
+      ref = MessagePortPrototype.hasRef.call(this);
     } catch { return this; }
     return Object.assign(Object.create(MessagePort.prototype),
                          ref === undefined ? {
@@ -157,12 +170,12 @@ function setupPortReferencing(port, eventEmitter, eventName) {
   eventEmitter.on('newListener', (name) => {
     if (name === eventName && eventEmitter.listenerCount(eventName) === 0) {
       port.ref();
-      port.start();
+      MessagePortPrototype.start.call(port);
     }
   });
   eventEmitter.on('removeListener', (name) => {
     if (name === eventName && eventEmitter.listenerCount(eventName) === 0) {
-      port.stop();
+      MessagePortPrototype.stop.call(port);
       port.unref();
     }
   });
@@ -304,7 +317,7 @@ class Worker extends EventEmitter {
 
   [kOnExit](code) {
     debug(`[${threadId}] hears end event for Worker ${this.threadId}`);
-    drainMessagePort.call(this[kPublicPort]);
+    MessagePortPrototype.drain.call(this[kPublicPort]);
     this[kDispose]();
     this.emit('exit', code);
     this.removeAllListeners();

src/node_worker.cc

@@ -502,10 +502,6 @@ void InitWorker(Local<Object> target,
               thread_id_string,
               Number::New(env->isolate(),
                           static_cast<double>(env->thread_id()))).FromJust();
-  Local<String> oninit_string = FIXED_ONE_BYTE_STRING(env->isolate(), "oninit");
-  target->Set(env->context(),
-              oninit_string,
-              env->oninit_symbol()).FromJust();
 }
 
 }  // anonymous namespace

test/parallel/test-heapdump-worker.js

@@ -19,8 +19,7 @@ validateSnapshotNodes('Worker', [
 validateSnapshotNodes('MessagePort', [
   {
     children: [
-      { name: 'MessagePortData' },
-      { name: 'MessagePort' }
+      { name: 'MessagePortData' }
     ]
   }
 ], { loose: true });

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

@@ -69,3 +69,12 @@ const { MessageChannel, MessagePort } = require('worker_threads');
     });
   });
 }
+
+{
+  assert.deepStrictEqual(
+    Object.getOwnPropertyNames(MessagePort.prototype).sort(),
+    [
+      'close', 'constructor', 'onmessage', 'postMessage', 'ref', 'start',
+      'unref'
+    ]);
+}