Guard against empty caches due to a previously aborted install handler

service-worker.tmpl

@@ -1,5 +1,5 @@
 /**
- * Copyright 2015 Google Inc. All rights reserved.
+ * Copyright 2016 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.
@@ -65,33 +65,42 @@ self.addEventListener('install', function(event) {
   var now = Date.now();
 
   event.waitUntil(
-    caches.keys().then(function(allCacheNames) {
-      return Promise.all(
-        Object.keys(CurrentCacheNamesToAbsoluteUrl).filter(function(cacheName) {
-          return allCacheNames.indexOf(cacheName) === -1;
-        }).map(function(cacheName) {
-          var urlWithCacheBusting = getCacheBustedUrl(CurrentCacheNamesToAbsoluteUrl[cacheName],
-            now);
-
-          return caches.open(cacheName).then(function(cache) {
-            var request = new Request(urlWithCacheBusting, {credentials: 'same-origin'});
+    // Take a look at each of the cache names we expect for this version.
+    Promise.all(Object.keys(CurrentCacheNamesToAbsoluteUrl).map(function(cacheName) {
+      return caches.open(cacheName).then(function(cache) {
+        // Get a list of all the entries in the specific named cache.
+        // For caches that are already populated for a given version of a
+        // resource, there should be 1 entry.
+        return cache.keys().then(function(keys) {
+          if (keys.length === 0) {
+            // If there are 0 entries, either because this is a brand new version
+            // of a resource or because the install step was interrupted the
+            // last time it ran, then we need to populate the cache.
+            var urlWithCacheBusting = getCacheBustedUrl(
+              CurrentCacheNamesToAbsoluteUrl[cacheName], now);
+
+            var request = new Request(urlWithCacheBusting,
+              {credentials: 'same-origin'});
             return fetch(request).then(function(response) {
               if (response.ok) {
-                return cache.put(CurrentCacheNamesToAbsoluteUrl[cacheName], response);
+                return cache.put(CurrentCacheNamesToAbsoluteUrl[cacheName],
+                  response);
               }
 
-              console.error('Request for %s returned a response with status %d, so not attempting to cache it.',
+              console.error('Request for %s returned a response status %d, ' +
+                'so not attempting to cache it.',
                 urlWithCacheBusting, response.status);
               // Get rid of the empty cache if we can't add a successful response to it.
               return caches.delete(cacheName);
             });
-          });
-        })
-      ).then(function() {
-        return Promise.all(
-          allCacheNames.filter(function(cacheName) {
-            return cacheName.indexOf(CacheNamePrefix) === 0 &&
-                   !(cacheName in CurrentCacheNamesToAbsoluteUrl);
+          }
+        });
+      });
+    })).then(function() {
+      return caches.keys().then(function(allCacheNames) {
+        return Promise.all(allCacheNames.filter(function(cacheName) {
+          return cacheName.indexOf(CacheNamePrefix) === 0 &&
+            !(cacheName in CurrentCacheNamesToAbsoluteUrl);
           }).map(function(cacheName) {
             return caches.delete(cacheName);
           })