fix: Worker hangs when posting "undefined" as message (#8920)

This commit fixes hang in web workers occuring when sending
"undefined" as message value. It is a temporary band-aid
until proper structured close is implemented.

Co-authored-by: Bartek Iwańczuk <biwanczuk@gmail.com>

cli/tests/worker_post_undefined.ts

@@ -0,0 +1,5 @@
+self.onmessage = (ev: MessageEvent) => {
+  console.log("received in worker", ev.data);
+  self.postMessage(undefined);
+  console.log("posted from worker");
+};

cli/tests/workers_test.ts

@@ -385,6 +385,31 @@ Deno.test({
   },
 });
 
+Deno.test({
+  name: "Worker post undefined",
+  fn: async function (): Promise<void> {
+    const promise = deferred();
+    const worker = new Worker(
+      new URL("./worker_post_undefined.ts", import.meta.url).href,
+      { type: "module" },
+    );
+
+    const handleWorkerMessage = (e: MessageEvent): void => {
+      console.log("main <- worker:", e.data);
+      worker.terminate();
+      promise.resolve();
+    };
+
+    worker.addEventListener("messageerror", () => console.log("message error"));
+    worker.addEventListener("error", () => console.log("error"));
+    worker.addEventListener("message", handleWorkerMessage);
+
+    console.log("\npost from parent");
+    worker.postMessage(undefined);
+    await promise;
+  },
+});
+
 Deno.test("Worker inherits permissions", async function () {
   const promise = deferred();
   const worker = new Worker(

runtime/js/11_workers.js

@@ -47,6 +47,13 @@
   }
 
   function decodeMessage(dataIntArray) {
+    // Temporary solution until structured clone arrives in v8.
+    // Current clone is made by parsing json to byte array and from byte array back to json.
+    // In that case "undefined" transforms to empty byte array, but empty byte array does not transform back to undefined.
+    // Thats why this special is statement is needed.
+    if (dataIntArray.length == 0) {
+      return undefined;
+    }
     const dataJson = decoder.decode(dataIntArray);
     return JSON.parse(dataJson);
   }