FEATURE: Preload resources via link header (#18475)

Experiment moving from preload tags in the document head to preload information the the response headers.

While this is a minor improvement in most browsers (headers are parsed before the response body), this allows smart proxies like Cloudflare to "learn" from those headers and build HTTP 103 Early Hints for subsequent requests to the same URI, which will allow the user agent to download and parse our JS/CSS while we are waiting for the server to generate and stream the HTML response.

Co-authored-by: Penar Musaraj <pmusaraj@gmail.com>

spec/helpers/application_helper_spec.rb

@@ -3,9 +3,8 @@
 
 RSpec.describe ApplicationHelper do
   describe "preload_script" do
-    def preload_link(url)
+    def script_tag(url)
       <<~HTML
-          <link rel="preload" href="#{url}" as="script">
           <script defer src="#{url}"></script>
       HTML
     end
@@ -32,7 +31,7 @@ RSpec.describe ApplicationHelper do
       helper.request.env["HTTP_ACCEPT_ENCODING"] = 'br'
       link = helper.preload_script('discourse')
 
-      expect(link).to eq(preload_link("https://awesome.com/brotli_asset/discourse.js"))
+      expect(link).to eq(script_tag("https://awesome.com/brotli_asset/discourse.js"))
     end
 
     context "with s3 CDN" do
@@ -61,36 +60,77 @@ RSpec.describe ApplicationHelper do
         helper.request.env["HTTP_ACCEPT_ENCODING"] = 'br'
         link = helper.preload_script('discourse')
 
-        expect(link).to eq(preload_link("https://s3cdn.com/assets/discourse.br.js"))
+        expect(link).to eq(script_tag("https://s3cdn.com/assets/discourse.br.js"))
       end
 
       it "gives s3 cdn if asset host is not set" do
         link = helper.preload_script('discourse')
 
-        expect(link).to eq(preload_link("https://s3cdn.com/assets/discourse.js"))
+        expect(link).to eq(script_tag("https://s3cdn.com/assets/discourse.js"))
       end
 
       it "can fall back to gzip compression" do
         helper.request.env["HTTP_ACCEPT_ENCODING"] = 'gzip'
         link = helper.preload_script('discourse')
-        expect(link).to eq(preload_link("https://s3cdn.com/assets/discourse.gz.js"))
+        expect(link).to eq(script_tag("https://s3cdn.com/assets/discourse.gz.js"))
       end
 
       it "gives s3 cdn even if asset host is set" do
         set_cdn_url "https://awesome.com"
         link = helper.preload_script('discourse')
 
-        expect(link).to eq(preload_link("https://s3cdn.com/assets/discourse.js"))
+        expect(link).to eq(script_tag("https://s3cdn.com/assets/discourse.js"))
       end
 
       it "gives s3 cdn but without brotli/gzip extensions for theme tests assets" do
         helper.request.env["HTTP_ACCEPT_ENCODING"] = 'gzip, br'
         link = helper.preload_script('discourse/tests/theme_qunit_ember_jquery')
-        expect(link).to eq(preload_link("https://s3cdn.com/assets/discourse/tests/theme_qunit_ember_jquery.js"))
+        expect(link).to eq(script_tag("https://s3cdn.com/assets/discourse/tests/theme_qunit_ember_jquery.js"))
       end
     end
   end
 
+  describe "add_resource_preload_list" do
+    it "adds resources to the preload list when it's available" do
+      @links_to_preload = []
+      add_resource_preload_list('/assets/discourse.js', 'script')
+      add_resource_preload_list('/assets/discourse.css', 'style')
+
+      expect(@links_to_preload.size).to eq(2)
+    end
+
+    it "doesn't add resources to the preload list when it's not available" do
+      @links_to_preload = nil
+      add_resource_preload_list('/assets/discourse.js', 'script')
+      add_resource_preload_list('/assets/discourse.css', 'style')
+
+      expect(@links_to_preload).to eq(nil)
+    end
+
+    it "adds resources to the preload list when preload_script is called" do
+      @links_to_preload = []
+      helper.preload_script('discourse')
+
+      expect(@links_to_preload.size).to eq(1)
+    end
+
+    it "adds resources to the preload list when discourse_stylesheet_link_tag is called" do
+      @links_to_preload = []
+      helper.discourse_stylesheet_link_tag(:desktop)
+
+      expect(@links_to_preload.size).to eq(1)
+    end
+
+    it "adds resources as the correct type" do
+      @links_to_preload = []
+      helper.discourse_stylesheet_link_tag(:desktop)
+      helper.preload_script('discourse')
+
+      expect(@links_to_preload[0]).to match(/as="style"/)
+      expect(@links_to_preload[1]).to match(/as="script"/)
+    end
+  end
+
   describe "escape_unicode" do
     it "encodes tags" do
       expect(helper.escape_unicode("<tag>")).to eq("\u003ctag>")