Test that postMessage doesn't block on event loop

Per spec postMessage should queue a message up on the target port immediately. Instead, when Workers are involved, some implementations queue a task up on current thread's event loop, which means that in case it gets blocked, some messages are never sent. Closes whatwg/html#5485.
web-platform-tests · Apr 28, 2020 · 3320205 · 3320205
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diff --git a/workers/postMessage_block.https.html b/workers/postMessage_block.https.html
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+<!DOCTYPE html>
+<meta charset=utf-8>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<body>
+<script>
+const t = async_test('postMessage and block');
+
+const w = new Worker('support/postMessage_block_worker.js');
+
+w.onmessage = t.step_func_done(() => {
+  const a = new Int32Array(new SharedArrayBuffer(4));
+  w.postMessage(a);
+  while (Atomics.load(a, 0) === 0);
+  assert_equals(Atomics.load(a, 0), 1);
+});
+</script>
+</body>
diff --git a/workers/postMessage_block.https.html.headers b/workers/postMessage_block.https.html.headers
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+Cross-Origin-Opener-Policy: same-origin
+Cross-Origin-Embedder-Policy: require-corp
diff --git a/workers/support/postMessage_block_worker.js b/workers/support/postMessage_block_worker.js
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+onmessage = e => Atomics.store(e.data, 0, 1);
+postMessage('ready');
diff --git a/workers/support/postMessage_block_worker.js.headers b/workers/support/postMessage_block_worker.js.headers
@@ -0,0 +1 @@
+Cross-Origin-Embedder-Policy: require-corp