Add rack.response_finished to Rack::Lint

This updates Rack::Lint to validate that `rack.response_finished` is an
array of callables when present in the `env`. e.g. procs, lambdas, or
objects that respond to `call`.

This validates that:

* `rack.response_finished` is an array
* The contents of the array all respond to `call`

SPEC.rdoc

@@ -132,6 +132,8 @@ There are the following restrictions:
   set. <tt>PATH_INFO</tt> should be <tt>/</tt> if
   <tt>SCRIPT_NAME</tt> is empty.
   <tt>SCRIPT_NAME</tt> never should be <tt>/</tt>, but instead be empty.
+<tt>rack.response_finished</tt>:: An array of callables run after the HTTP response has been sent.
+Invoked with <tt>(env, status, headers, error)</tt> arguments.
 
 === The Input Stream
 

lib/rack/constants.rb

@@ -51,6 +51,7 @@ module Rack
   RACK_RECURSIVE_INCLUDE              = 'rack.recursive.include'
   RACK_MULTIPART_BUFFER_SIZE          = 'rack.multipart.buffer_size'
   RACK_MULTIPART_TEMPFILE_FACTORY     = 'rack.multipart.tempfile_factory'
+  RACK_RESPONSE_FINISHED              = 'rack.response_finished'
   RACK_REQUEST_FORM_INPUT             = 'rack.request.form_input'
   RACK_REQUEST_FORM_HASH              = 'rack.request.form_hash'
   RACK_REQUEST_FORM_VARS              = 'rack.request.form_vars'

lib/rack/lint.rb

@@ -363,6 +363,16 @@ def check_environment(env)
         unless env[SCRIPT_NAME] != "/"
           raise LintError, "SCRIPT_NAME cannot be '/', make it '' and PATH_INFO '/'"
         end
+
+        ## <tt>rack.response_finished</tt>:: An array of callables run after the HTTP response has been sent.
+        ## Invoked with <tt>(env, status, headers, error)</tt> arguments.
+        if callables = env[RACK_RESPONSE_FINISHED]
+          raise LintError, "rack.response_finished must be an array of callable objects" unless callables.is_a?(Array)
+
+          callables.each do |callable|
+            raise LintError, "rack.response_finished values must respond to call(env, status, headers, error)" unless callable.respond_to?(:call)
+          end
+        end
       end
 
       ##

test/spec_lint.rb

@@ -253,6 +253,31 @@ def obj.fatal(*) end
       Rack::Lint.new(nil).call(env("SCRIPT_NAME" => "/"))
     }.must_raise(Rack::Lint::LintError).
       message.must_match(/cannot be .* make it ''/)
+
+    lambda {
+      Rack::Lint.new(nil).call(env("rack.response_finished" => "not a callable"))
+    }.must_raise(Rack::Lint::LintError).
+    message.must_match(/rack.response_finished must be an array of callable objects/)
+
+    lambda {
+      Rack::Lint.new(nil).call(env("rack.response_finished" => [-> (env) {}, "not a callable"]))
+    }.must_raise(Rack::Lint::LintError).
+    message.must_match(/rack.response_finished values must respond to call/)
+
+    lambda {
+      Rack::Lint.new(nil).call(env("rack.response_finished" => [-> () {}]))
+    }.must_raise(Rack::Lint::LintError).
+    message.must_match(/rack.response_finished values must accept an env argument/)
+
+    callable_object = Class.new do
+      def call
+      end
+    end.new
+
+    lambda {
+      Rack::Lint.new(nil).call(env("rack.response_finished" => [callable_object]))
+    }.must_raise(Rack::Lint::LintError).
+    message.must_match(/rack.response_finished values must accept an env argument/)
   end
 
   it "notice input errors" do
@@ -811,4 +836,15 @@ def assert_lint(*args)
     }.must_raise(Rack::Lint::LintError).
       message.must_equal 'rack.hijack header must respond to #call'
   end
+
+  it "pass valid rack.response_finished" do
+    callable_object = Class.new do
+      def call(env)
+      end
+    end.new
+
+    Rack::Lint.new(lambda { |env|
+                     [200, {}, ["foo"]]
+                   }).call(env({ "rack.response_finished" => [-> (env) {}, lambda { |env| }, callable_object], "content-length" => "3" })).first.must_equal 200
+  end
 end