lib: add support for JSTransferable as a mixin

lib/internal/test/transfer.js

@@ -0,0 +1,42 @@
+'use strict';
+
+const {
+  makeTransferable,
+  kClone,
+  kDeserialize,
+} = require('internal/worker/js_transferable');
+
+process.emitWarning(
+  'These APIs are for internal testing only. Do not use them.',
+  'internal/test/transfer');
+
+// Used as part of parallel/test-messaging-maketransferable.
+// This has to exist within the lib/internal/ path in order
+// for deserialization to work.
+
+class E {
+  constructor(b) {
+    this.b = b;
+  }
+}
+
+class F extends E {
+  constructor(b) {
+    super(b);
+    /* eslint-disable-next-line no-constructor-return */
+    return makeTransferable(this);
+  }
+
+  [kClone]() {
+    return {
+      data: { b: this.b },
+      deserializeInfo: 'internal/test/transfer:F'
+    };
+  }
+
+  [kDeserialize]({ b }) {
+    this.b = b;
+  }
+}
+
+module.exports = { E, F };

lib/internal/worker/js_transferable.js

@@ -1,6 +1,11 @@
 'use strict';
 const {
   Error,
+  ObjectDefineProperties,
+  ObjectGetOwnPropertyDescriptors,
+  ObjectGetPrototypeOf,
+  ObjectSetPrototypeOf,
+  ReflectConstruct,
   StringPrototypeSplit,
 } = primordials;
 const {
@@ -22,21 +27,30 @@ function setup() {
     const { 0: module, 1: ctor } = StringPrototypeSplit(deserializeInfo, ':');
     const Ctor = require(module)[ctor];
     if (typeof Ctor !== 'function' ||
-        !(Ctor.prototype instanceof JSTransferable)) {
+        typeof Ctor.prototype[messaging_deserialize_symbol] !== 'function') {
       // Not one of the official errors because one should not be able to get
       // here without messing with Node.js internals.
       // eslint-disable-next-line no-restricted-syntax
       throw new Error(`Unknown deserialize spec ${deserializeInfo}`);
     }
+
     return new Ctor();
   });
 }
 
+function makeTransferable(obj) {
+  const inst = ReflectConstruct(JSTransferable, [], obj.constructor);
+  ObjectDefineProperties(inst, ObjectGetOwnPropertyDescriptors(obj));
+  ObjectSetPrototypeOf(inst, ObjectGetPrototypeOf(obj));
+  return inst;
+}
+
 module.exports = {
+  makeTransferable,
   setup,
   JSTransferable,
   kClone: messaging_clone_symbol,
   kDeserialize: messaging_deserialize_symbol,
   kTransfer: messaging_transfer_symbol,
-  kTransferList: messaging_transfer_list_symbol
+  kTransferList: messaging_transfer_list_symbol,
 };

node.gyp

@@ -229,6 +229,7 @@
       'lib/internal/source_map/source_map.js',
       'lib/internal/source_map/source_map_cache.js',
       'lib/internal/test/binding.js',
+      'lib/internal/test/transfer.js',
       'lib/internal/timers.js',
       'lib/internal/tls.js',
       'lib/internal/trace_events_async_hooks.js',

test/parallel/test-messaging-maketransferable.js

@@ -0,0 +1,27 @@
+// Flags: --expose-internals
+'use strict';
+
+const common = require('../common');
+
+const assert = require('assert');
+const {
+  JSTransferable,
+} = require('internal/worker/js_transferable');
+const { E, F } = require('internal/test/transfer');
+
+// Tests that F is transferable even tho it does not directly,
+// observably extend the JSTransferable class.
+
+const mc = new MessageChannel();
+
+mc.port1.onmessageerror = common.mustNotCall();
+
+mc.port1.onmessage = common.mustCall(({ data }) => {
+  assert(!(data instanceof JSTransferable));
+  assert(data instanceof F);
+  assert(data instanceof E);
+  assert.strictEqual(data.b, 1);
+  mc.port1.close();
+});
+
+mc.port2.postMessage(new F(1));