fix(ext/node): emit online event after worker thread is initialized (#25243)

Fixes #23281. Part of #20613.

We were emitting the `online` event in the constructor, so the caller
could never receive it (since there was no time for them to add a
listener). Instead, emit the event where it's intended – after the
worker is initialized.

---

After this parcel no longer freezes, but still will fail due to other
bugs (which will be fixed in other PRs)

ext/node/polyfills/worker_threads.ts

@@ -52,6 +52,16 @@ function debugWT(...args) {
   }
 }
 
+interface WorkerOnlineMsg {
+  type: "WORKER_ONLINE";
+}
+
+function isWorkerOnlineMsg(data: unknown): data is WorkerOnlineMsg {
+  return typeof data === "object" && data !== null &&
+    ObjectHasOwn(data, "type") &&
+    (data as { "type": unknown })["type"] === "WORKER_ONLINE";
+}
+
 export interface WorkerOptions {
   // only for typings
   argv?: unknown[];
@@ -81,6 +91,7 @@ class NodeWorker extends EventEmitter {
   #refCount = 1;
   #messagePromise = undefined;
   #controlPromise = undefined;
+  #workerOnline = false;
   // "RUNNING" | "CLOSED" | "TERMINATED"
   // "TERMINATED" means that any controls or messages received will be
   // discarded. "CLOSED" means that we have received a control
@@ -141,6 +152,7 @@ class NodeWorker extends EventEmitter {
       workerData: options?.workerData,
       environmentData: environmentData,
       env: env_,
+      isWorkerThread: true,
     }, options?.transferList ?? []);
     const id = op_create_worker(
       {
@@ -159,8 +171,6 @@ class NodeWorker extends EventEmitter {
     this.threadId = id;
     this.#pollControl();
     this.#pollMessages();
-    // https://nodejs.org/api/worker_threads.html#event-online
-    this.emit("online");
   }
 
   [privateWorkerRef](ref) {
@@ -243,7 +253,17 @@ class NodeWorker extends EventEmitter {
         this.emit("messageerror", err);
         return;
       }
-      this.emit("message", message);
+      if (
+        // only emit "online" event once, and since the message
+        // has to come before user messages, we are safe to assume
+        // it came from us
+        !this.#workerOnline && isWorkerOnlineMsg(message)
+      ) {
+        this.#workerOnline = true;
+        this.emit("online");
+      } else {
+        this.emit("message", message);
+      }
     }
   };
 
@@ -358,10 +378,12 @@ internals.__initWorkerThreads = (
 
     parentPort = globalThis as ParentPort;
     threadId = workerId;
+    let isWorkerThread = false;
     if (maybeWorkerMetadata) {
       const { 0: metadata, 1: _ } = maybeWorkerMetadata;
       workerData = metadata.workerData;
       environmentData = metadata.environmentData;
+      isWorkerThread = metadata.isWorkerThread;
       const env = metadata.env;
       if (env) {
         process.env = env;
@@ -425,6 +447,15 @@ internals.__initWorkerThreads = (
     parentPort.ref = () => {
       parentPort[unrefPollForMessages] = false;
     };
+
+    if (isWorkerThread) {
+      // Notify the host that the worker is online
+      parentPort.postMessage(
+        {
+          type: "WORKER_ONLINE",
+        } satisfies WorkerOnlineMsg,
+      );
+    }
   }
 };
 

tests/unit_node/worker_threads_test.ts

@@ -590,3 +590,34 @@ Deno.test({
     channel.port2.close();
   },
 });
+
+Deno.test({
+  name: "[node/worker_threads] Emits online event",
+  async fn() {
+    const worker = new workerThreads.Worker(
+      `
+      import { parentPort } from "node:worker_threads";
+      const p = Promise.withResolvers();
+      let ok = false;
+      parentPort.on("message", () => {
+        ok = true;
+        p.resolve();
+      });
+      await Promise.race([p.promise, new Promise(resolve => setTimeout(resolve, 20000))]);
+      if (ok) {
+        parentPort.postMessage("ok");
+      } else {
+        parentPort.postMessage("timed out");
+      }
+      `,
+      {
+        eval: true,
+      },
+    );
+    worker.on("online", () => {
+      worker.postMessage("ok");
+    });
+    assertEquals((await once(worker, "message"))[0], "ok");
+    worker.terminate();
+  },
+});