Automatically set Link header for each stylesheet and script

<link rel="preload"> elements[0] can be serialized in `Link`
headers[1] to allow the browser to preload them before
it parsed the HTML body.

It is particularly useful for scripts included at the bottom
of the document.

[0] https://developer.mozilla.org/en-US/docs/Web/HTML/Preloading_content
[1] https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Link

actionview/lib/action_view/helpers/asset_tag_helper.rb

@@ -86,11 +86,11 @@ module ActionView
       def javascript_include_tag(*sources)
         options = sources.extract_options!.stringify_keys
         path_options = options.extract!("protocol", "extname", "host", "skip_pipeline").symbolize_keys
-        early_hints_links = []
+        preload_links = []
 
         sources_tags = sources.uniq.map { |source|
           href = path_to_javascript(source, path_options)
-          early_hints_links << "<#{href}>; rel=preload; as=script"
+          preload_links << "<#{href}>; rel=preload; as=script"
           tag_options = {
             "src" => href
           }.merge!(options)
@@ -100,7 +100,7 @@ module ActionView
           content_tag("script", "", tag_options)
         }.join("\n").html_safe
 
-        request.send_early_hints("Link" => early_hints_links.join("\n")) if respond_to?(:request) && request
+        send_preload_links_header(preload_links)
 
         sources_tags
       end
@@ -136,11 +136,11 @@ module ActionView
       def stylesheet_link_tag(*sources)
         options = sources.extract_options!.stringify_keys
         path_options = options.extract!("protocol", "host", "skip_pipeline").symbolize_keys
-        early_hints_links = []
+        preload_links = []
 
         sources_tags = sources.uniq.map { |source|
           href = path_to_stylesheet(source, path_options)
-          early_hints_links << "<#{href}>; rel=preload; as=style"
+          preload_links << "<#{href}>; rel=preload; as=style"
           tag_options = {
             "rel" => "stylesheet",
             "media" => "screen",
@@ -149,7 +149,7 @@ module ActionView
           tag(:link, tag_options)
         }.join("\n").html_safe
 
-        request.send_early_hints("Link" => early_hints_links.join("\n")) if respond_to?(:request) && request
+        send_preload_links_header(preload_links)
 
         sources_tags
       end
@@ -281,12 +281,12 @@ module ActionView
           crossorigin: crossorigin
         }.merge!(options.symbolize_keys))
 
-        early_hints_link = "<#{href}>; rel=preload; as=#{as_type}"
-        early_hints_link += "; type=#{mime_type}" if mime_type
-        early_hints_link += "; crossorigin=#{crossorigin}" if crossorigin
-        early_hints_link += "; nopush" if nopush
+        preload_link = "<#{href}>; rel=preload; as=#{as_type}"
+        preload_link += "; type=#{mime_type}" if mime_type
+        preload_link += "; crossorigin=#{crossorigin}" if crossorigin
+        preload_link += "; nopush" if nopush
 
-        request.send_early_hints("Link" => early_hints_link) if respond_to?(:request) && request
+        send_preload_links_header([preload_link])
 
         link_tag
       end
@@ -482,6 +482,16 @@ module ActionView
             type
           end
         end
+
+        def send_preload_links_header(preload_links)
+          if respond_to?(:request) && request
+            request.send_early_hints("Link" => preload_links.join("\n"))
+          end
+
+          if respond_to?(:response) && response
+            response.headers["Link"] = [response.headers["Link"].presence, *preload_links].compact.join(",")
+          end
+        end
     end
   end
 end

actionview/test/abstract_unit.rb

@@ -84,7 +84,7 @@ class RoutedRackApp
 end
 
 class BasicController
-  attr_accessor :request
+  attr_accessor :request, :response
 
   def config
     @config ||= ActiveSupport::InheritableOptions.new(ActionController::Base.config).tap do |config|

actionview/test/template/asset_tag_helper_test.rb

@@ -9,23 +9,33 @@ ActionView::Template::Types.delegate_to Mime
 class AssetTagHelperTest < ActionView::TestCase
   tests ActionView::Helpers::AssetTagHelper
 
-  attr_reader :request
+  attr_reader :request, :response
+
+  class FakeRequest
+    attr_accessor :script_name
+    def protocol() "http://" end
+    def ssl?() false end
+    def host_with_port() "localhost" end
+    def base_url() "http://www.example.com" end
+    def send_early_hints(links) end
+  end
+
+  class FakeResponse
+    def headers
+      @headers ||= {}
+    end
+  end
 
   def setup
     super
 
     @controller = BasicController.new
 
-    @request = Class.new do
-      attr_accessor :script_name
-      def protocol() "http://" end
-      def ssl?() false end
-      def host_with_port() "localhost" end
-      def base_url() "http://www.example.com" end
-      def send_early_hints(links) end
-    end.new
-
+    @request = FakeRequest.new
     @controller.request = @request
+
+    @response = FakeResponse.new
+    @controller.response = @response
   end
 
   def url_for(*args)
@@ -499,6 +509,14 @@ class AssetTagHelperTest < ActionView::TestCase
     assert_dom_equal %(<script src="/javascripts/foo.js"></script>), javascript_include_tag("foo.js")
   end
 
+
+  def test_should_set_preload_links
+    stylesheet_link_tag("http://example.com/style.css")
+    javascript_include_tag("http://example.com/all.js")
+    expected = "<http://example.com/style.css>; rel=preload; as=style,<http://example.com/all.js>; rel=preload; as=script"
+    assert_equal expected, @response.headers["Link"]
+  end
+
   def test_image_path
     ImagePathToTag.each { |method, tag| assert_dom_equal(tag, eval(method)) }
   end