worker_threads: remove the global onmessage accessor inside a node worker

src/js/node/worker_threads.ts

@@ -755,23 +755,26 @@ function receiveMessageOnPort(port: MessagePort) {
 // TODO: parent port emulation is not complete
 function fakeParentPort() {
   const fake = Object.create(MessagePort.prototype);
-  Object.defineProperty(fake, "onmessage", {
-    get() {
-      return self.onmessage;
-    },
-    set(value) {
-      self.onmessage = value;
-    },
-  });
-
-  Object.defineProperty(fake, "onmessageerror", {
-    get() {
-      return self.onmessageerror;
-    },
-    set(value) {
-      self.onmessageerror = value;
-    },
-  });
+  function defineHandlerAttribute(name: string, event: string) {
+    let handler: any = null;
+    const wrapper = (e: Event) => {
+      if ($isCallable(handler)) handler.$call(fake, e);
+      else if ($isCallable(handler?.handleEvent)) handler.handleEvent.$call(handler, e);
+    };
+    Object.defineProperty(fake, name, {
+      get() {
+        return handler;
+      },
+      set(value) {
+        const next = $isCallable(value) || (value !== null && typeof value === "object") ? value : null;
+        if (handler === null && next !== null) self.addEventListener(event, wrapper);
+        else if (handler !== null && next === null) self.removeEventListener(event, wrapper);
+        handler = next;
+      },
+    });
+  }
+  defineHandlerAttribute("onmessage", "message");
+  defineHandlerAttribute("onmessageerror", "messageerror");
 
   const postMessage = $newCppFunction("ZigGlobalObject.cpp", "jsFunctionPostMessage", 1);
   Object.defineProperty(fake, "postMessage", {
@@ -832,6 +835,10 @@ let parentPort: MessagePort | null = isMainThread ? null : fakeParentPort();
 // Gate on _isNodeWorker so a raw `new globalThis.Worker` that transitively loads
 // this module does NOT have process.abort/chdir/setuid replaced.
 if (!isMainThread && _isNodeWorker) {
+  // Node has no global onmessage; parentPort shares this event target, so keeping it double-delivers.
+  try {
+    delete (globalThis as any).onmessage;
+  } catch {}
   applyWorkerProcessOverrides();
 }
 function applyWorkerProcessOverrides() {

test/js/node/worker_threads/worker_threads.test.ts

@@ -1756,3 +1756,59 @@ test("the SHARE_ENV founding thread's process.env stays live after the swap", as
   expect(stdout.trim()).toBe("yes,unset");
   expect(exitCode).toBe(0);
 });
+
+// https://github.com/oven-sh/bun/issues/7816
+// Emscripten's pthread worker shim registers parentPort.on("message", ...) and
+// then assigns self.onmessage. In Node the global onmessage is not an event
+// handler accessor, so only the parentPort listener fires; Bun was delivering
+// the message to both, so the pthread 'load' handler ran twice and aborted.
+test("globalThis.onmessage is not an event handler accessor inside a node:worker_threads worker", async () => {
+  const worker = new Worker(
+    /* js */ `
+      const { parentPort } = require("worker_threads");
+      let count = 0;
+      const hadOnMessage = "onmessage" in globalThis;
+      Object.assign(global, { self: global });
+      parentPort.on("message", data => {
+        globalThis.onmessage({ data });
+      });
+      self.onmessage = e => {
+        count++;
+        if (e.data === "ping") {
+          setImmediate(() => parentPort.postMessage({ count, hadOnMessage }));
+        }
+      };
+      parentPort.postMessage("ready");
+    `,
+    { eval: true },
+  );
+  try {
+    await once(worker, "message"); // "ready"
+    worker.postMessage("ping");
+    const [result] = await once(worker, "message");
+    expect(result).toEqual({ count: 1, hadOnMessage: false });
+  } finally {
+    await worker.terminate();
+  }
+});
+
+test("parentPort.onmessage works inside a node:worker_threads worker", async () => {
+  const worker = new Worker(
+    /* js */ `
+      const { parentPort } = require("worker_threads");
+      parentPort.onmessage = e => {
+        parentPort.postMessage({ echoed: e.data, handlerIsSet: typeof parentPort.onmessage === "function" });
+      };
+      parentPort.postMessage("ready");
+    `,
+    { eval: true },
+  );
+  try {
+    await once(worker, "message"); // "ready"
+    worker.postMessage("hello");
+    const [result] = await once(worker, "message");
+    expect(result).toEqual({ echoed: "hello", handlerIsSet: true });
+  } finally {
+    await worker.terminate();
+  }
+});

test/napi/node-napi-tests/test/common/index.js

@@ -356,8 +356,8 @@ if (typeof Bun === "object") {
     ResolveMessage,
     ErrorEvent,
     Worker,
-    onmessage,
-    onerror,
+    globalThis.onmessage,
+    globalThis.onerror,
   );
 }