fix service worker webpack assets not being found

templates/main.js

@@ -4,6 +4,10 @@ import '../routes/_utils/serviceWorkerClient'
 import '../routes/_utils/historyEvents'
 import '../routes/_utils/loadingMask'
 
+console.log('1')
+console.log('2')
+console.log('3')
+
 loadPolyfills().then(() => {
   console.log('init()')
   // `routes` is an array of route objects injected by Sapper

templates/service-worker.js

@@ -1,19 +1,21 @@
 const timestamp = '__timestamp__'
 const ASSETS = `assets_${timestamp}`
+const WEBPACK_ASSETS = `webpack_assets_${timestamp}`
 const ON_DEMAND = `ondemand_${timestamp}`
 
 // `assets` is an array of everything in the `assets` directory
 const assets = __assets__
   .map(file => file.startsWith('/') ? file : `/${file}`)
   .filter(filename => !filename.startsWith('/apple-icon'))
+  .concat(['/index.html'])
 
 // `shell` is an array of all the files generated by webpack
 // also contains '/index.html' for some reason
-const resources = __shell__
+const webpackAssets = __shell__
   .filter(filename => !filename.endsWith('.map'))
+  .filter(filename => filename !== '/index.html')
 
-const toCache = [].concat(assets).concat(resources)
-const cacheSet = new Set(toCache)
+const cacheSet = new Set([].concat(assets).concat(webpackAssets))
 
 // `routes` is an array of `{ pattern: RegExp }` objects that
 // match the pages in your app
@@ -21,8 +23,10 @@ const routes = __routes__
 
 self.addEventListener('install', event => {
   event.waitUntil((async () => {
-    let cache = await caches.open(ASSETS)
-    await cache.addAll(toCache)
+    await Promise.all([
+      caches.open(WEBPACK_ASSETS).then(cache => cache.addAll(webpackAssets)),
+      caches.open(ASSETS).then(cache => cache.addAll(assets))
+    ])
     self.skipWaiting()
   })())
 })
@@ -33,11 +37,30 @@ self.addEventListener('activate', event => {
 
     // delete old asset/ondemand caches
     for (let key of keys) {
-      if (key !== ASSETS && key !== ON_DEMAND) {
+      if (key !== ASSETS &&
+          key !== ON_DEMAND &&
+          !key.startsWith('webpack_assets_')) {
         await caches.delete(key)
       }
     }
 
+    // for webpack assets, keep the two latest builds because we may need
+    // them when the service worker has installed but the page has not
+    // yet reloaded (e.g. when it gives the toast saying "please reload"
+    // but then you don't refresh and instead load an async chunk)
+    let webpackKeysToDelete = keys
+      .filter(key => key.startsWith('webpack_assets_'))
+      .sort((a, b) => {
+        let aTimestamp = parseInt(a.substring(15), 10)
+        let bTimestamp = parseInt(b.substring(15), 10)
+        return bTimestamp < aTimestamp ? -1 : 1
+      })
+      .slice(2)
+
+    for (let key of webpackKeysToDelete) {
+      await caches.delete(key)
+    }
+
     await self.clients.claim()
   })())
 })