Add early hints feature (#1403)

This commit adds the early hints lambda to the headers hash. Users can
call it to emit the early hints headers. For example:

```
class Server
  def call env
    if env["REQUEST_PATH"] == "/"
      env['rack.early_hints'].call("Link" => "</style.css>; rel=preload; as=style\n</script.js>; rel=preload")
      [200, { "X-Hello" => "World" }, ["Hello world!"]]
    else
      [200, { "X-Hello" => "World" }, ["NEAT!"]]
    end
  end
end

run Server.new
```

In this example, the server sends stylesheet and javascript early hints
if the proxy supports it, it will send H2 pushes to the client.

Of course not every proxy server supports early hints, so to enable the
early hints feature with puma you have to pass the configuration variable,
`--early-hints`.

If `ENV['rack.early_hints']` is not set then early hints is not
supported by the webserver. Early hints is off by default.

lib/puma/cli.rb

@@ -181,6 +181,10 @@ module Puma
             user_config.tcp_mode!
           end
 
+          o.on "--early-hints", "Enable early hints support" do
+            user_config.early_hints
+          end
+
           o.on "-V", "--version", "Print the version information" do
             puts "puma version #{Puma::Const::VERSION}"
             exit 0

lib/puma/const.rb

@@ -221,5 +221,7 @@ module Puma
     HIJACK_P = "rack.hijack?".freeze
     HIJACK = "rack.hijack".freeze
     HIJACK_IO = "rack.hijack_io".freeze
+
+    EARLY_HINTS = "rack.early_hints".freeze
   end
 end

lib/puma/dsl.rb

@@ -241,6 +241,10 @@ module Puma
       @options[:mode] = :tcp
     end
 
+    def early_hints(answer=true)
+      @options[:early_hints] = answer
+    end
+
     # Redirect STDOUT and STDERR to files specified.
     def stdout_redirect(stdout=nil, stderr=nil, append=false)
       @options[:redirect_stdout] = stdout

lib/puma/runner.rb

@@ -161,6 +161,10 @@ module Puma
         server.tcp_mode!
       end
 
+      if @options[:early_hints]
+        server.early_hints = true
+      end
+
       unless development?
         server.leak_stack_on_error = false
       end

lib/puma/server.rb

@@ -81,7 +81,7 @@ module Puma
       @precheck_closing = true
     end
 
-    attr_accessor :binder, :leak_stack_on_error
+    attr_accessor :binder, :leak_stack_on_error, :early_hints
 
     forward :add_tcp_listener,  :@binder
     forward :add_ssl_listener,  :@binder
@@ -595,6 +595,24 @@ module Puma
       env[RACK_INPUT] = body
       env[RACK_URL_SCHEME] =  env[HTTPS_KEY] ? HTTPS : HTTP
 
+      if @early_hints
+        env[EARLY_HINTS] = lambda { |headers|
+          fast_write client, "HTTP/1.1 103 Early Hints\r\n".freeze
+
+          headers.each_pair do |k, vs|
+            if vs.respond_to?(:to_s) && !vs.to_s.empty?
+              vs.to_s.split(NEWLINE).each do |v|
+                fast_write client, "#{k}: #{v}\r\n"
+              end
+            else
+              fast_write client, "#{k}: #{v}\r\n"
+            end
+          end
+
+          fast_write client, "\r\n".freeze
+        }
+      end
+
       # A rack extension. If the app writes #call'ables to this
       # array, we will invoke them when the request is done.
       #

test/test_puma_server.rb

@@ -159,6 +159,61 @@ class TestPumaServer < Minitest::Test
     assert_equal "HTTP/1.0 200 OK\r\nContent-Length: 0\r\n\r\n", data
   end
 
+  def test_early_hints_works
+    @server.app = proc { |env|
+     env['rack.early_hints'].call("Link" => "</style.css>; rel=preload; as=style\n</script.js>; rel=preload")
+     [200, { "X-Hello" => "World" }, ["Hello world!"]]
+    }
+
+    @server.add_tcp_listener @host, @port
+    @server.early_hints = true
+    @server.run
+
+    sock = TCPSocket.new @host, @server.connected_port
+    sock << "HEAD / HTTP/1.0\r\n\r\n"
+
+    data = sock.read
+
+    expected_data = (<<EOF
+HTTP/1.1 103 Early Hints
+Link: </style.css>; rel=preload; as=style
+Link: </script.js>; rel=preload
+
+HTTP/1.0 200 OK
+X-Hello: World
+Content-Length: 12
+EOF
+).split("\n").join("\r\n") + "\r\n\r\n"
+
+    assert_equal true, @server.early_hints
+    assert_equal expected_data, data
+  end
+
+  def test_early_hints_is_off_by_default
+    @server.app = proc { |env|
+     assert_nil env['rack.early_hints']
+     [200, { "X-Hello" => "World" }, ["Hello world!"]]
+    }
+
+    @server.add_tcp_listener @host, @port
+    @server.run
+
+    sock = TCPSocket.new @host, @server.connected_port
+    sock << "HEAD / HTTP/1.0\r\n\r\n"
+
+    data = sock.read
+
+    expected_data = (<<EOF
+HTTP/1.0 200 OK
+X-Hello: World
+Content-Length: 12
+EOF
+).split("\n").join("\r\n") + "\r\n\r\n"
+
+    assert_nil @server.early_hints
+    assert_equal expected_data, data
+  end
+
   def test_GET_with_no_body_has_sane_chunking
     @server.app = proc { |env| [200, {}, []] }