fix: send BroadcastChannel messages before close (#34628)

Fixes a race where BroadcastChannel.postMessage deferred the
cross-worker send and then skipped it if the channel was closed before
the deferred callback ran. This made node:worker_threads
BroadcastChannel examples that call close() immediately after
postMessage() hang waiting for a message that was never sent.

The send is now committed to the shared broadcast queue before
postMessage() returns, while receiver dispatch remains asynchronous.
Added a node:worker_threads regression spec for the reported repro.

Closes denoland/divybot#362
Fixes #31134

Verification:
- timeout 10s target/debug/deno run --allow-all
--unstable-broadcast-channel /tmp/orchid-362-repro.mjs
- env -u RUSTC_WRAPPER cargo test -p specs_tests
specs::node::worker_threads::broadcast_channel -- --nocapture
- target/debug/deno fmt --check
tests/specs/node/worker_threads/__test__.jsonc
tests/specs/node/worker_threads/broadcast_channel.mjs
- git diff --check

Note: ./tools/format.js --check on the touched files spawned dprint
plugin processes that did not exit in this workspace after reporting the
initial import formatting diff; the reported diff was applied, and the
new spec files pass deno fmt --check.

Co-authored-by: divybot <divybot@users.noreply.github.com>
Co-authored-by: Divy Srivastava <me@littledivy.com>

Author: divybot

ext/web/01_broadcast_channel.js

@@ -138,12 +138,10 @@ class BroadcastChannel extends EventTarget {
     // Send to other listeners in this VM.
     dispatch(this, this[_name], new Uint8Array(data));
 
-    // Send to listeners in other VMs.
-    defer(() => {
-      if (!this[_closed]) {
-        op_broadcast_send(rid, this[_name], data);
-      }
-    });
+    // Send to listeners in other VMs. This must happen before returning from
+    // postMessage(), otherwise close() immediately after postMessage() can
+    // cancel the deferred send before other workers observe the message.
+    op_broadcast_send(rid, this[_name], data);
   }
 
   [refBroadcastChannel](ref) {

tests/specs/node/worker_threads/__test__.jsonc

@@ -34,6 +34,11 @@
       "output": "message_port_unref.out",
       "exitCode": 0
     },
+    "broadcast_channel": {
+      "args": "run --quiet --allow-read --unstable-broadcast-channel broadcast_channel.mjs",
+      "output": "hello from worker\n",
+      "exitCode": 0
+    },
     "parent_port_unref": {
       "args": "run --allow-env --allow-read message_port_unref.mjs",
       "envs": {

tests/specs/node/worker_threads/broadcast_channel.mjs

@@ -0,0 +1,14 @@
+import { BroadcastChannel, isMainThread, Worker } from "node:worker_threads";
+
+const bc = new BroadcastChannel("hello");
+
+if (isMainThread) {
+  bc.onmessage = (event) => {
+    console.log(event.data);
+    bc.close();
+  };
+  new Worker(new URL(import.meta.url));
+} else {
+  bc.postMessage("hello from worker");
+  bc.close();
+}