From 92f587c191c96e201984d47ccdaf43a1bff2fc17 Mon Sep 17 00:00:00 2001
From: Jean Boussier <jean.boussier@gmail.com>
Date: Thu, 16 Jul 2020 11:39:30 +0200
Subject: Add early hints support

While not part of the rack spec, this API is exposed
by both puma and falcon, and Rails use it when available.

The 103 Early Hints response code is specified in RFC 8297.
---
 lib/unicorn/configurator.rb |  9 +++++++++
 lib/unicorn/http_server.rb  | 31 +++++++++++++++++++++++++++++--
 test/unit/test_server.rb    | 30 ++++++++++++++++++++++++++++++
 3 files changed, 68 insertions(+), 2 deletions(-)

diff --git a/lib/unicorn/configurator.rb b/lib/unicorn/configurator.rb
index c3a4f2d..b0606af 100644
--- a/lib/unicorn/configurator.rb
+++ b/lib/unicorn/configurator.rb
@@ -276,6 +276,15 @@ class Unicorn::Configurator
     set_bool(:default_middleware, bool)
   end
 
+  # sets whether to enable the proposed early hints Rack API.
+  # If enabled, Rails 5.2+ will automatically send a 103 Early Hint
+  # for all the `javascript_include_tag` and `stylesheet_link_tag`
+  # in your response. See: https://api.rubyonrails.org/v5.2/classes/ActionDispatch/Request.html#method-i-send_early_hints
+  # See also https://tools.ietf.org/html/rfc8297
+  def early_hints(bool)
+    set_bool(:early_hints, bool)
+  end
+
   # sets listeners to the given +addresses+, replacing or augmenting the
   # current set.  This is for the global listener pool shared by all
   # worker processes.  For per-worker listeners, see the after_fork example
diff --git a/lib/unicorn/http_server.rb b/lib/unicorn/http_server.rb
index 45a2e97..05dad99 100644
--- a/lib/unicorn/http_server.rb
+++ b/lib/unicorn/http_server.rb
@@ -15,7 +15,7 @@ class Unicorn::HttpServer
                 :before_fork, :after_fork, :before_exec,
                 :listener_opts, :preload_app,
                 :orig_app, :config, :ready_pipe, :user,
-                :default_middleware
+                :default_middleware, :early_hints
   attr_writer   :after_worker_exit, :after_worker_ready, :worker_exec
 
   attr_reader :pid, :logger
@@ -588,6 +588,25 @@ class Unicorn::HttpServer
   rescue
   end
 
+  def e103_response_write(client, headers)
+    response = if @request.response_start_sent
+      "103 Early Hints\r\n"
+    else
+      "HTTP/1.1 103 Early Hints\r\n"
+    end
+
+    headers.each_pair do |k, vs|
+      next if !vs || vs.empty?
+      values = vs.to_s.split("\n".freeze)
+      values.each do |v|
+        response << "#{k}: #{v}\r\n"
+      end
+    end
+    response << "\r\n".freeze
+    response << "HTTP/1.1 ".freeze if @request.response_start_sent
+    client.write(response)
+  end
+
   def e100_response_write(client, env)
     # We use String#freeze to avoid allocations under Ruby 2.1+
     # Not many users hit this code path, so it's better to reduce the
@@ -602,7 +621,15 @@ class Unicorn::HttpServer
   # once a client is accepted, it is processed in its entirety here
   # in 3 easy steps: read request, call app, write app response
   def process_client(client)
-    status, headers, body = @app.call(env = @request.read(client))
+    env = @request.read(client)
+
+    if early_hints
+      env["rack.early_hints"] = lambda do |headers|
+        e103_response_write(client, headers)
+      end
+    end
+
+    status, headers, body = @app.call(env)
 
     begin
       return if @request.hijacked?
diff --git a/test/unit/test_server.rb b/test/unit/test_server.rb
index 8096955..d706243 100644
--- a/test/unit/test_server.rb
+++ b/test/unit/test_server.rb
@@ -23,6 +23,16 @@ class TestHandler
   end
 end
 
+class TestEarlyHintsHandler
+  def call(env)
+    while env['rack.input'].read(4096)
+    end
+    env['rack.early_hints'].call(
+      "Link" => "</style.css>; rel=preload; as=style\n</script.js>; rel=preload"
+    )
+    [200, { 'Content-Type' => 'text/plain' }, ['hello!\n']]
+  end
+end
 
 class WebServerTest < Test::Unit::TestCase
 
@@ -84,6 +94,26 @@ class WebServerTest < Test::Unit::TestCase
     tmp.close!
   end
 
+  def test_early_hints
+    teardown
+    redirect_test_io do
+      @server = HttpServer.new(TestEarlyHintsHandler.new,
+                               :listeners => [ "127.0.0.1:#@port"],
+                               :early_hints => true)
+      @server.start
+    end
+
+    sock = TCPSocket.new('127.0.0.1', @port)
+    sock.syswrite("GET / HTTP/1.0\r\n\r\n")
+
+    responses = sock.sysread(4096)
+    assert_match %r{\AHTTP/1.[01] 103\b}, responses
+    assert_match %r{^Link: </style\.css>}, responses
+    assert_match %r{^Link: </script\.js>}, responses
+
+    assert_match %r{^HTTP/1.[01] 200\b}, responses
+  end
+
   def test_broken_app
     teardown
     app = lambda { |env| raise RuntimeError, "hello" }
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