Load index.json from the CDN instead of copying it to the app origin

The service worker precache and the runtime index loader used a same-origin
/docs path for index.json, on the assumption that the cache couldn't hold
the CDN's CORS responses. That assumption dates to the applicationCache era
(2013) and is false for service workers, which cache CORS responses fine;
documents.devdocs.io already serves index.json with access-control-allow-origin.

Point both doc_index_urls (SW precache) and Doc#indexUrl at docs_origin so
they load index.json straight from the CDN, and drop the now-dead index_host
special-case and index_path config. prepare_deploy no longer needs to copy
index.json into the app's public/docs, so stop downloading it — restoring the
deploy-size/timeout win from 45867a2 without breaking the service worker.

See #2686.

Author: simon04

assets/javascripts/app/app.js

@@ -282,16 +282,6 @@ class App extends Events {
     document.documentElement.classList.remove("_booting");
   }
 
-  indexHost() {
-    // Can't load the index files from the host/CDN when service worker is
-    // enabled because it doesn't support caching URLs that use CORS.
-    return this.config[
-      this.serviceWorker && this.settings.hasDocs()
-        ? "index_path"
-        : "docs_origin"
-    ];
-  }
-
   onBootError(...args) {
     this.trigger("bootError");
     this.hideLoadingScreen();

assets/javascripts/app/config.js.erb

@@ -6,7 +6,6 @@ app.config = {
   env: '<%= App.environment %>',
   history_cache_size: 10,
   index_filename: 'index.json',
-  index_path: '/<%= App.docs_prefix %>',
   max_results: 50,
   production_host: 'devdocs.io',
   search_param: 'q',

assets/javascripts/models/doc.js

@@ -51,9 +51,9 @@ app.models.Doc = class Doc extends app.Model {
   }
 
   indexUrl() {
-    return `${app.indexHost()}/${this.slug}/${app.config.index_filename}?${
-      this.mtime
-    }`;
+    return `${app.config.docs_origin}/${this.slug}/${
+      app.config.index_filename
+    }?${this.mtime}`;
   }
 
   toEntry() {

lib/app.rb

@@ -181,7 +181,7 @@ def user_has_docs?(slug)
     def doc_index_urls
       docs.each_with_object [] do |slug, result|
         if doc = settings.docs[slug]
-          result << File.join('', settings.docs_prefix, slug, 'index.json') + "?#{doc['mtime']}"
+          result << "#{settings.docs_origin}/#{slug}/index.json?#{doc['mtime']}"
         end
       end
     end

lib/tasks/docs.thor

@@ -249,12 +249,10 @@ class DocsCLI < Thor
           dir = File.join(Docs.store_path, doc.path)
           FileUtils.mkpath(dir)
 
-          # meta.json is needed to build the manifest. index.json must also be
-          # copied to the app's own origin: when the service worker is enabled
-          # (the production default) clients load index.json from same-origin
-          # /docs rather than the CDN, because the service worker can't cache
-          # the CDN's CORS responses (see App#indexHost / app.js).
-          ['index.json', 'meta.json'].each do |filename|
+          # Only meta.json is needed to build the manifest; clients (and the
+          # service worker precache) load index.json directly from the CDN,
+          # which serves it with CORS headers.
+          ['meta.json'].each do |filename|
             json = "https://documents.devdocs.io/#{doc.path}/#{filename}?#{time}"
             begin
               attempts = 0