Add a linter for rack.early_hints

SPEC.rdoc

@@ -138,6 +138,7 @@ There are the following restrictions:
 * There must be a valid input stream in <tt>rack.input</tt>.
 * There must be a valid error stream in <tt>rack.errors</tt>.
 * There may be a valid hijack stream in <tt>rack.hijack_io</tt>
+* There may be a valid early hints callback in <tt>rack.early_hints</tt>
 * The <tt>REQUEST_METHOD</tt> must be a valid token.
 * The <tt>SCRIPT_NAME</tt>, if non-empty, must start with <tt>/</tt>
 * The <tt>PATH_INFO</tt>, if non-empty, must start with <tt>/</tt>
@@ -244,6 +245,15 @@ if the request env has <tt>rack.hijack?</tt> <tt>true</tt>.
 * Middleware may wrap the IO object for the response pattern.
 * Middleware should not wrap the IO object for the request pattern. The
   request pattern is intended to provide the hijacker with "raw tcp".
+=== Early Hints
+
+The application or any middleware may call the <tt>rack.early_hints</tt>
+with pairs of headers that will be sent as a 103 Early Hints provisional
+response.
+
+If rack.early_hints is present it must respond to #call.
+The headers must respond to #each_pair
+The headers keys and values must respond to #to_s
 == The Response
 === The Status
 This is an HTTP status. It must be an Integer greater than or equal to

lib/rack.rb

@@ -62,6 +62,7 @@ module Rack
   RACK_HIJACK                         = 'rack.hijack'
   RACK_IS_HIJACK                      = 'rack.hijack?'
   RACK_HIJACK_IO                      = 'rack.hijack_io'
+  RACK_EARLY_HINTS                    = 'rack.early_hints'
   RACK_RECURSIVE_INCLUDE              = 'rack.recursive.include'
   RACK_MULTIPART_BUFFER_SIZE          = 'rack.multipart.buffer_size'
   RACK_MULTIPART_TEMPFILE_FACTORY     = 'rack.multipart.tempfile_factory'

lib/rack/lint.rb

@@ -335,6 +335,8 @@ def check_env(env)
       check_error env[RACK_ERRORS]
       ## * There may be a valid hijack stream in <tt>rack.hijack_io</tt>
       check_hijack env
+      ## * There may be a valid early hints callback in <tt>rack.early_hints</tt>
+      check_early_hints env
 
       ## * The <tt>REQUEST_METHOD</tt> must be a valid token.
       assert("REQUEST_METHOD unknown: #{env[REQUEST_METHOD]}") {
@@ -655,6 +657,29 @@ def check_hijack_response(headers, env)
     ## * Middleware should not wrap the IO object for the request pattern. The
     ##   request pattern is intended to provide the hijacker with "raw tcp".
 
+    ## === Early Hints
+    ##
+    ## The application or any middleware may call the <tt>rack.early_hints</tt>
+    ## with pairs of headers that will be sent as a 103 Early Hints provisional
+    ## response.
+    def check_early_hints(env)
+      if env[RACK_EARLY_HINTS]
+        ##
+        ## If rack.early_hints is present it must respond to #call.
+        assert("rack.early_hints must respond to call") { env[RACK_EARLY_HINTS].respond_to?(:call) }
+        original_callback = env[RACK_EARLY_HINTS]
+        env[RACK_EARLY_HINTS] = lambda do |headers|
+          ## The headers must respond to #each_pair
+          assert("rack.early_hints headers must respond to each_pair") { headers.respond_to?(:each_pair) }
+          headers.each_pair do |key, value|
+            ## The headers keys and values must respond to #to_s
+            assert("rack.early_hints headers keys must respond to to_s") { key.respond_to?(:to_s) }
+            assert("rack.early_hints headers values must respond to to_s") { value.respond_to?(:to_s) }
+          end
+        end
+      end
+    end
+
     ## == The Response
 
     ## === The Status