Revert "Reduce Worker indirection for bundlers"

This reverts commit a42a69f.

Unfortunately, this removes a useful optimisation, but is necessary to workaround the semver breakage caused by rustwasm/wasm-bindgen#4159.

src/lib.rs

@@ -52,13 +52,6 @@ extern "C" {
     fn start_workers(module: JsValue, memory: JsValue, builder: wbg_rayon_PoolBuilder) -> Promise;
 }
 
-#[cfg(not(feature = "no-bundler"))]
-#[allow(unused_must_use)]
-fn _ensure_worker_emitted() {
-    // Just ensure that the worker is emitted into the output folder, but don't actually use the URL.
-    wasm_bindgen::link_to!(module = "/src/workerHelpers.worker.js");
-}
-
 #[wasm_bindgen]
 impl wbg_rayon_PoolBuilder {
     fn new(num_threads: usize) -> Self {

src/workerHelpers.js

@@ -11,6 +11,52 @@
  * limitations under the License.
  */
 
+// Note: we use `wasm_bindgen_worker_`-prefixed message types to make sure
+// we can handle bundling into other files, which might happen to have their
+// own `postMessage`/`onmessage` communication channels.
+//
+// If we didn't take that into the account, we could send much simpler signals
+// like just `0` or whatever, but the code would be less resilient.
+
+function waitForMsgType(target, type) {
+  return new Promise(resolve => {
+    target.addEventListener('message', function onMsg({ data }) {
+      if (data?.type !== type) return;
+      target.removeEventListener('message', onMsg);
+      resolve(data);
+    });
+  });
+}
+
+waitForMsgType(self, 'wasm_bindgen_worker_init').then(async data => {
+  // # Note 1
+  // Our JS should have been generated in
+  // `[out-dir]/snippets/wasm-bindgen-rayon-[hash]/workerHelpers.js`,
+  // resolve the main module via `../../..`.
+  //
+  // This might need updating if the generated structure changes on wasm-bindgen
+  // side ever in the future, but works well with bundlers today. The whole
+  // point of this crate, after all, is to abstract away unstable features
+  // and temporary bugs so that you don't need to deal with them in your code.
+  //
+  // # Note 2
+  // This could be a regular import, but then some bundlers complain about
+  // circular deps.
+  //
+  // Dynamic import could be cheap if this file was inlined into the parent,
+  // which would require us just using `../../..` in `new Worker` below,
+  // but that doesn't work because wasm-pack unconditionally adds
+  // "sideEffects":false (see below).
+  //
+  // OTOH, even though it can't be inlined, it should be still reasonably
+  // cheap since the requested file is already in cache (it was loaded by
+  // the main thread).
+  const pkg = await import('../../..');
+  await pkg.default(data.module, data.memory);
+  postMessage({ type: 'wasm_bindgen_worker_ready' });
+  pkg.wbg_rayon_start_worker(data.receiver);
+});
+
 // Note: this is never used, but necessary to prevent a bug in Firefox
 // (https://bugzilla.mozilla.org/show_bug.cgi?id=1702191) where it collects
 // Web Workers that have a shared WebAssembly memory with the main thread,
@@ -26,6 +72,7 @@ export async function startWorkers(module, memory, builder) {
   }
 
   const workerInit = {
+    type: 'wasm_bindgen_worker_init',
     module,
     memory,
     receiver: builder.receiver()
@@ -39,16 +86,21 @@ export async function startWorkers(module, memory, builder) {
       // way to get asset URLs relative to the module across various bundlers
       // and browser, ideally we should switch to `import.meta.resolve`
       // once it becomes a standard.
-      const worker = new Worker(
-        new URL('./workerHelpers.worker.js', import.meta.url),
-        {
-          type: 'module'
-        }
-      );
+      //
+      // Note: we could use `../../..` as the URL here to inline workerHelpers.js
+      // into the parent entry instead of creating another split point -
+      // this would be preferable from optimization perspective -
+      // however, Webpack then eliminates all message handler code
+      // because wasm-pack produces "sideEffects":false in package.json
+      // unconditionally.
+      //
+      // The only way to work around that is to have side effect code
+      // in an entry point such as Worker file itself.
+      const worker = new Worker(new URL('./workerHelpers.js', import.meta.url), {
+        type: 'module'
+      });
       worker.postMessage(workerInit);
-      await new Promise(resolve =>
-        worker.addEventListener('message', resolve, { once: true })
-      );
+      await waitForMsgType(worker, 'wasm_bindgen_worker_ready');
       return worker;
     })
   );