Browse files

Implement H2 Early Hints for Rails

When puma/puma#1403 is merged Puma will support the Early Hints status
code for sending assets before a request has finished.

While the Early Hints spec is still in draft, this PR prepares Rails to
allowing this status code.

If the proxy server supports Early Hints, it will send H2 pushes to the

This PR adds a method for setting Early Hints Link headers via Rails,
and also automatically sends Early Hints if supported from the
`stylesheet_link_tag` and the `javascript_include_tag`.

Once puma supports Early Hints the `--early-hints` argument can be
passed to the server to enable this or set in the puma config with
`early_hints(true)`. Note that for Early Hints to work
in the browser the requirements are 1) a proxy that can handle H2,
and 2) HTTPS.

To start the server with Early Hints enabled pass `--early-hints` to
`rails s`.

This has been verified to work with h2o, Puma, and Rails with Chrome.

The commit adds a new option to the rails server to enable early hints
for Puma.

Early Hints spec:

[Eileen M. Uchitelle, Aaron Patterson]
  • Loading branch information...
eileencodes committed Sep 26, 2017
1 parent f1e8962 commit 59a02fb7bcbe68f26e1e7fdcec45c00c66e4a065
@@ -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
@@ -199,6 +199,23 @@ def headers
@headers ||=
# 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"]
# Returns a +String+ with the last requested path including their params.
# # get '/foo'
@@ -1304,3 +1304,18 @@ class RequestFormData < BaseRequestTest
assert !request.form_data?
class EarlyHintsRequestTest < BaseRequestTest
def setup
@env["rack.early_hints"] = lambda { |links| links }
@request = stub_request
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
@@ -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 { |source|
early_hints_links = []
sources_tags = { |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
content_tag("script".freeze, "", tag_options)
request.send_early_hints("Link" => early_hints_links.join("\n"))
# 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 { |source|
early_hints_links = []
sources_tags = { |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
tag(:link, tag_options)
request.send_early_hints("Link" => early_hints_links.join("\n"))
# Returns a link tag that browsers and feed readers can use to auto-detect
@@ -19,6 +19,7 @@ def protocol() "http://" end
def ssl?() false end
def host_with_port() "localhost" end
def base_url() "" end
def send_early_hints(links) end
@controller.request = @request
@@ -653,7 +654,9 @@ def setup
@controller =
@controller.config.relative_url_root = "/collaboration/hieraki"
@request =, :base_url).new("gopher://", "gopher://")
@request =, :base_url) do
def send_early_hints(links); end"gopher://", "gopher://")
@controller.request = @request
@@ -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 = do
def send_early_hints(links) end
def teardown
@@ -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
@@ -227,6 +229,10 @@ def restart_command
"bin/rails server #{@server} #{@original_options.join(" ")} --restart"
def early_hints
def pid
@@ -81,6 +81,18 @@ def test_caching_with_option
assert_equal false, options[:caching]
def test_early_hints_with_option
args = ["--early-hints"]
options = parse_arguments(args)
assert_equal true, options[:early_hints]
def test_early_hints_is_nil_by_default
args = []
options = parse_arguments(args)
assert_nil options[:early_hints]
def test_log_stdout
with_rack_env nil do
with_rails_env nil do

0 comments on commit 59a02fb

Please sign in to comment.