ConditionalGet middleware (Last-Modified/Etag)

Adapted from Michael Klishin's implementation for Merb:
http://github.com/wycats/merb-core/tree/master/lib/merb-core/rack/middleware/conditional_get.rb

Implemented by Ryan Tomayko.

lib/rack.rb

@@ -29,6 +29,7 @@ def self.release
   autoload :Builder, "rack/builder"
   autoload :Cascade, "rack/cascade"
   autoload :CommonLogger, "rack/commonlogger"
+  autoload :ConditionalGet, "rack/conditionalget"
   autoload :File, "rack/file"
   autoload :Deflater, "rack/deflater"
   autoload :Directory, "rack/directory"

lib/rack/conditionalget.rb

@@ -0,0 +1,42 @@
+module Rack
+
+  # Middleware that enables conditional GET using If-None-Match and
+  # If-Modified-Since. The application should set either or both of the
+  # Last-Modified or Etag response headers according to RFC 2616. When
+  # either of the conditions is met, the response body is set to be zero
+  # length and the response status is set to 304 Not Modified.
+  #
+  # Applications that defer response body generation until the body's each
+  # message is received will avoid response body generation completely when
+  # a conditional GET matches.
+  #
+  # Adapted from Michael Klishin's Merb implementation:
+  # http://github.com/wycats/merb-core/tree/master/lib/merb-core/rack/middleware/conditional_get.rb
+  class ConditionalGet
+    def initialize(app)
+      @app = app
+    end
+
+    def call(env)
+      return @app.call(env) unless %w[GET HEAD].include?(env['REQUEST_METHOD'])
+
+      status, headers, body = @app.call(env)
+      if etag_matches?(env, headers) || modified_since?(env, headers)
+        status = 304
+        body = []
+      end
+      [status, headers, body]
+    end
+
+  private
+    def etag_matches?(env, headers)
+      etag = headers['Etag'] and etag == env['HTTP_IF_NONE_MATCH']
+    end
+
+    def modified_since?(env, headers)
+      last_modified = headers['Last-Modified'] and
+        last_modified == env['HTTP_IF_MODIFIED_SINCE']
+    end
+  end
+
+end

test/spec_rack_conditionalget.rb

@@ -0,0 +1,41 @@
+require 'test/spec'
+require 'time'
+
+require 'rack/mock'
+require 'rack/conditionalget'
+
+context "Rack::ConditionalGet" do
+  specify "should set a 304 status and truncate body when If-Modified-Since hits" do
+    timestamp = Time.now.httpdate
+    app = Rack::ConditionalGet.new(lambda { |env|
+      [200, {'Last-Modified'=>timestamp}, 'TEST'] })
+
+    response = Rack::MockRequest.new(app).
+      get("/", 'HTTP_IF_MODIFIED_SINCE' => timestamp)
+
+    response.status.should.be == 304
+    response.body.should.be.empty
+  end
+
+  specify "should set a 304 status and truncate body when If-None-Match hits" do
+    app = Rack::ConditionalGet.new(lambda { |env|
+      [200, {'Etag'=>'1234'}, 'TEST'] })
+
+    response = Rack::MockRequest.new(app).
+      get("/", 'HTTP_IF_NONE_MATCH' => '1234')
+
+    response.status.should.be == 304
+    response.body.should.be.empty
+  end
+
+  specify "should not affect non-GET/HEAD requests" do
+    app = Rack::ConditionalGet.new(lambda { |env|
+      [200, {'Etag'=>'1234'}, 'TEST'] })
+
+    response = Rack::MockRequest.new(app).
+      post("/", 'HTTP_IF_NONE_MATCH' => '1234')
+
+    response.status.should.be == 200
+    response.body.should.be == 'TEST'
+  end
+end