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)

1 files changed, 31 insertions, 0 deletions

diff --git a/tests/unit_node/worker_threads_test.ts b/tests/unit_node/worker_threads_test.ts
index d9fe9f77d..ac797601f 100644
--- a/tests/unit_node/worker_threads_test.ts
+++ b/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();
+  },
+});