From 0e3aa54dc8a33a65a2f0a44d8def14fcdb494ae6 Mon Sep 17 00:00:00 2001
From: Ingvar Stepanyan <rreverser@google.com>
Date: Wed, 29 Apr 2020 16:03:04 +0100
Subject: [PATCH] Add some comments

---
 codecs/oxipng/spawn.ts  | 23 +++++++++++++++++++++++
 codecs/oxipng/worker.ts |  4 ++++
 2 files changed, 27 insertions(+)

diff --git a/codecs/oxipng/spawn.ts b/codecs/oxipng/spawn.ts
index 75f5c0fc..ab6b8b53 100644
--- a/codecs/oxipng/spawn.ts
+++ b/codecs/oxipng/spawn.ts
@@ -15,11 +15,34 @@ function initWorker(worker: Worker, workerInit: WorkerInit) {
 
 async function startMainThread() {
   const num = navigator.hardwareConcurrency;
+
+  // First, let browser fetch and spawn Workers for our pool in the background.
+  // This is fairly expensive, so we want to start it as early as possible.
   const workers = Array.from({ length: num }, () => new Worker('./worker', { type: 'module' }));
+
+  // Meanwhile, asynchronously compile, instantiate and initialise Wasm on our main thread.
   await initOxiPNG(fetch(wasmUrl), undefined as any);
+
+  // Get module+memory from the Wasm instance.
+  //
+  // Ideally we wouldn't go via Wasm bindings here, since both are just JS variables, but memory is
+  // currently not exposed on the Wasm instance correctly by wasm-bindgen.
   const workerInit: WorkerInit = worker_initializer(num);
+
+  // Once done, we want to send module+memory to each Worker so that they instantiate Wasm too.
+  // While doing so, we need to wait for Workers to acknowledge that they have received our message.
+  // Ideally this shouldn't be necessary, but Chromium currently doesn't conform to the spec:
+  // https://bugs.chromium.org/p/chromium/issues/detail?id=1075645
+  //
+  // If we didn't do this ping-pong game, the `start_main_thread` below would block the current
+  // thread on an atomic before even *sending* the `postMessage` containing memory,
+  // so Workers would never be able to unblock us back.
   await Promise.all(workers.map(worker => initWorker(worker, workerInit)));
+
+  // Finally, instantiate rayon pool - this will use shared Wasm memory to send tasks to the
+  // Workers and then block until they're all ready.
   start_main_thread();
+
   return {
     optimise,
   };
diff --git a/codecs/oxipng/worker.ts b/codecs/oxipng/worker.ts
index 9a08f4c9..69f60501 100644
--- a/codecs/oxipng/worker.ts
+++ b/codecs/oxipng/worker.ts
@@ -11,6 +11,10 @@ addEventListener(
     //
     // At this point, the "main" thread can run Wasm that
     // will synchronously block waiting on other atomics.
+    //
+    // Note that we don't need to wait for Wasm instantiation here - it's
+    // better to start main thread as early as possible, and then it blocks
+    // on a shared atomic anyway until Worker is fully ready.
     postMessage(null);
 
     await initOxiPNG(...(event.data as WorkerInit));