Import Config by jcrosby (Jon Crosby) into core

lib/rack.rb

@@ -28,6 +28,7 @@ def self.release
   autoload :Chunked, "rack/chunked"
   autoload :CommonLogger, "rack/commonlogger"
   autoload :ConditionalGet, "rack/conditionalget"
+  autoload :Config, "rack/config"
   autoload :ContentLength, "rack/content_length"
   autoload :ContentType, "rack/content_type"
   autoload :File, "rack/file"

lib/rack/config.rb

@@ -0,0 +1,15 @@
+module Rack
+  # Rack::Config modifies the environment using the block given during
+  # initialization.
+  class Config
+    def initialize(app, &block)
+      @app = app
+      @block = block
+    end
+
+    def call(env)
+      @block.call(env)
+      @app.call(env)
+    end
+  end
+end

test/spec_rack_config.rb

@@ -0,0 +1,24 @@
+require 'test/spec'
+require 'rack/mock'
+require 'rack/builder'
+require 'rack/content_length'
+require 'rack/config'
+
+context "Rack::Config" do
+
+  specify "should accept a block that modifies the environment" do
+    app = Rack::Builder.new do
+      use Rack::Lint
+      use Rack::ContentLength
+      use Rack::Config do |env|
+        env['greeting'] = 'hello'
+      end
+      run lambda { |env|
+        [200, {'Content-Type' => 'text/plain'}, [env['greeting'] || '']]
+      }
+    end
+    response = Rack::MockRequest.new(app).get('/')
+    response.body.should.equal('hello')
+  end
+
+end