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 +132,10 @@ 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 finished,
+either normally or with an error (e.g. the client disconnected).
+Invoked with <tt>(env, status, headers, error)</tt> arguments. If there is no error sending the response,
+the error argument will be +nil+, otherwise you can expect an exception object, e.g. +IOError+.
 
 === 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 +363,18 @@ 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 finished,
+        ## either normally or with an error (e.g. the client disconnected).
+        ## Invoked with <tt>(env, status, headers, error)</tt> arguments. If there is no error sending the response,
+        ## the error argument will be +nil+, otherwise you can expect an exception object, e.g. +IOError+.
+        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
 
       ##
@@ -582,7 +594,7 @@ def check_hijack_response(headers, env)
           ## ignore the +body+ part of the response tuple when the
           ## +rack.hijack+ response header is present. Using an empty +Array+
           ## instance is recommended.
-        else       
+        else
           ##
           ## The special response header +rack.hijack+ must only be set
           ## if the request +env+ has a truthy +rack.hijack?+.

test/spec_lint.rb

@@ -253,6 +253,16 @@ 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/)
   end
 
   it "notice input errors" do
@@ -811,4 +821,15 @@ 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, status, headers, error)
+      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