wasm-bindgen-rayon and new OMT plugin (#1007)

* WIP: wasm-bindgen-rayon and new OMT plugin

* Bump package-lock

* Make OMT work again

* Update year

* Restore accidental change

* Prevent minification of `nextDefineUri`

* Delay loading external deps

This gives inline `define` calls a chance to define dependencies for earlier `define` calls on the same page.

* Add comment

codecs/oxipng/pkg-parallel/snippets/wasm-bindgen-rayon-3d2df09ebec17a22/src/workerHelpers.js

@@ -0,0 +1,98 @@
+/**
+ * Copyright 2021 Google Inc. All Rights Reserved.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * 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 == null || 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);
+});
+
+export async function startWorkers(module, memory, builder) {
+  const workerInit = {
+    type: 'wasm_bindgen_worker_init',
+    module,
+    memory,
+    receiver: builder.receiver()
+  };
+
+  try {
+    await Promise.all(
+      Array.from({ length: builder.numThreads() }, () => {
+        // Self-spawn into a new Worker.
+        //
+        // TODO: while `new URL('...', import.meta.url) becomes a semi-standard
+        // 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.
+        //
+        // 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);
+        return waitForMsgType(worker, 'wasm_bindgen_worker_ready');
+      })
+    );
+    builder.build();
+  } finally {
+    builder.free();
+  }
+}