Implement H2 Early Hints for Rails

actionpack/CHANGELOG.md

@@ -1,3 +1,11 @@
+*   Add ability to enable Early Hints for HTTP/2
+
+    If supported by the server, and enabled in Puma this allows H2 Early Hints to be used.
+
+    The `javascript_include_tag` and the `stylesheet_link_tag` automatically add Early Hints if requested.
+
+    *Eileen M. Uchitelle*, *Aaron Patterson*
+
 *   Simplify cookies middleware with key rotation support
 
     Use the `rotate` method for both `MessageEncryptor` and

actionpack/lib/action_dispatch/http/request.rb

@@ -199,6 +199,23 @@ def headers
       @headers ||= Http::Headers.new(self)
     end
 
+    # Early Hints is an HTTP/2 status code that indicates hints to help a client start
+    # making preparations for processing the final response.
+    #
+    # If the env contains +rack.early_hints+ then the server accepts HTTP2 push for Link headers.
+    #
+    # The +send_early_hints+ method accepts an hash of links as follows:
+    #
+    #   send_early_hints("Link" => "</style.css>; rel=preload; as=style\n</script.js>; rel=preload")
+    #
+    # If you are using +javascript_include_tag+ or +stylesheet_link_tag+ the
+    # Early Hints headers are included by default if supported.
+    def send_early_hints(links)
+      return unless env["rack.early_hints"]
+
+      env["rack.early_hints"].call(links)
+    end
+
     # Returns a +String+ with the last requested path including their params.
     #
     #    # get '/foo'

actionpack/test/dispatch/request_test.rb

@@ -1304,3 +1304,18 @@ class RequestFormData < BaseRequestTest
     assert !request.form_data?
   end
 end
+
+class EarlyHintsRequestTest < BaseRequestTest
+  def setup
+    super
+    @env["rack.early_hints"] = lambda { |links| links }
+    @request = stub_request
+  end
+
+  test "when early hints is set in the env link headers are sent" do
+    early_hints = @request.send_early_hints("Link" => "</style.css>; rel=preload; as=style\n</script.js>; rel=preload")
+    expected_hints = { "Link" => "</style.css>; rel=preload; as=style\n</script.js>; rel=preload" }
+
+    assert_equal expected_hints, early_hints
+  end
+end

actionview/lib/action_view/helpers/asset_tag_helper.rb

@@ -37,6 +37,9 @@ module AssetTagHelper
       # When the Asset Pipeline is enabled, you can pass the name of your manifest as
       # source, and include other JavaScript or CoffeeScript files inside the manifest.
       #
+      # If the server supports Early Hints header links for these assets will be
+      # automatically pushed.
+      #
       # ==== Options
       #
       # When the last parameter is a hash you can add HTML attributes using that
@@ -77,12 +80,20 @@ module AssetTagHelper
       def javascript_include_tag(*sources)
         options = sources.extract_options!.stringify_keys
         path_options = options.extract!("protocol", "extname", "host", "skip_pipeline").symbolize_keys
-        sources.uniq.map { |source|
+        early_hints_links = []
+
+        sources_tags = sources.uniq.map { |source|
+          href = path_to_javascript(source, path_options)
+          early_hints_links << "<#{href}>; rel=preload; as=script"
           tag_options = {
-            "src" => path_to_javascript(source, path_options)
+            "src" => href
           }.merge!(options)
           content_tag("script".freeze, "", tag_options)
         }.join("\n").html_safe
+
+        request.send_early_hints("Link" => early_hints_links.join("\n"))
+
+        sources_tags
       end
 
       # Returns a stylesheet link tag for the sources specified as arguments. If
@@ -92,6 +103,9 @@ def javascript_include_tag(*sources)
       # to "screen", so you must explicitly set it to "all" for the stylesheet(s) to
       # apply to all media types.
       #
+      # If the server supports Early Hints header links for these assets  will be
+      # automatically pushed.
+      #
       #   stylesheet_link_tag "style"
       #   # => <link href="/assets/style.css" media="screen" rel="stylesheet" />
       #
@@ -113,14 +127,22 @@ def javascript_include_tag(*sources)
       def stylesheet_link_tag(*sources)
         options = sources.extract_options!.stringify_keys
         path_options = options.extract!("protocol", "host", "skip_pipeline").symbolize_keys
-        sources.uniq.map { |source|
+        early_hints_links = []
+
+        sources_tags = sources.uniq.map { |source|
+          href = path_to_stylesheet(source, path_options)
+          early_hints_links << "<#{href}>; rel=preload; as=stylesheet"
           tag_options = {
             "rel" => "stylesheet",
             "media" => "screen",
-            "href" => path_to_stylesheet(source, path_options)
+            "href" => href
           }.merge!(options)
           tag(:link, tag_options)
         }.join("\n").html_safe
+
+        request.send_early_hints("Link" => early_hints_links.join("\n"))
+
+        sources_tags
       end
 
       # Returns a link tag that browsers and feed readers can use to auto-detect

actionview/test/template/asset_tag_helper_test.rb

@@ -19,6 +19,7 @@ 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
 
     @controller.request = @request
@@ -653,7 +654,9 @@ def setup
     @controller = BasicController.new
     @controller.config.relative_url_root = "/collaboration/hieraki"
 
-    @request = Struct.new(:protocol, :base_url).new("gopher://", "gopher://www.example.com")
+    @request = Struct.new(:protocol, :base_url) do
+      def send_early_hints(links); end
+    end.new("gopher://", "gopher://www.example.com")
     @controller.request = @request
   end
 

actionview/test/template/javascript_helper_test.rb

@@ -6,11 +6,15 @@ class JavaScriptHelperTest < ActionView::TestCase
   tests ActionView::Helpers::JavaScriptHelper
 
   attr_accessor :output_buffer
+  attr_reader :request
 
   setup do
     @old_escape_html_entities_in_json = ActiveSupport.escape_html_entities_in_json
     ActiveSupport.escape_html_entities_in_json = true
     @template = self
+    @request = Class.new do
+      def send_early_hints(links) end
+    end.new
   end
 
   def teardown

railties/lib/rails/commands/server/server_command.rb

@@ -127,6 +127,7 @@ class ServerCommand < Base # :nodoc:
       class_option "dev-caching", aliases: "-C", type: :boolean, default: nil,
         desc: "Specifies whether to perform caching in development."
       class_option "restart", type: :boolean, default: nil, hide: true
+      class_option "early_hints", type: :boolean, default: nil, desc: "Enables HTTP/2 early hints."
 
       def initialize(args = [], local_options = {}, config = {})
         @original_options = local_options
@@ -161,7 +162,8 @@ def server_options
             daemonize:             options[:daemon],
             pid:                   pid,
             caching:               options["dev-caching"],
-            restart_cmd:           restart_command
+            restart_cmd:           restart_command,
+            early_hints:           early_hints
           }
         end
       end
@@ -227,6 +229,10 @@ def restart_command
           "bin/rails server #{@server} #{@original_options.join(" ")} --restart"
         end
 
+        def early_hints
+          options[:early_hints]
+        end
+
         def pid
           File.expand_path(options[:pid])
         end

railties/test/commands/server_test.rb

@@ -81,6 +81,18 @@ def test_caching_with_option
     assert_equal false, options[:caching]
   end
 
+  def test_early_hints_with_option
+    args = ["--early-hints"]
+    options = parse_arguments(args)
+    assert_equal true, options[:early_hints]
+  end
+
+  def test_early_hints_is_nil_by_default
+    args = []
+    options = parse_arguments(args)
+    assert_nil options[:early_hints]
+  end
+
   def test_log_stdout
     with_rack_env nil do
       with_rails_env nil do