Got overhead down from 127 to 85. All tests pass:

* Tentatively replaced HeaderHash with SimpleHeaderHash, which does not preserve
    case but does handle converting Arrays to Strings in to_hash. This requires
    further discussion.
  * Moved default_charset to ActionDispatch::Response to avoid having to hop over
    to ActionController. Ideally, this would be a constant on AD::Response, but
    some tests expect to be able to change it dynamically and I didn't want to change
    them yet.
  * Completely override #initialize from Rack::Response. Previously, it was creating
    a HeaderHash, and then we were creating an entirely new one. There is no way to
    call super without incurring the overhead of creating a HeaderHash.
  * Override #write from Rack::Response. Its implementation tracks Content-Length,
    and doing so adds additional overhead that could be mooted if other middleware
    changes the body. It is more efficiently done at the top-level server.
  * Change sending_file to an instance_variable instead of header inspection. In
    general, if a state is important, it should be set as a property of the response
    not reconstructed later.
  * Set the Etag to @body instead of .body. AS::Cache.expand_cache_key handles
    Arrays fine, and it's more efficient to let it handle the body parts, since
    it is not forced to create a joined String.
  * If we detect the default cache control case, just set it, rather than setting
    the constituent parts and then running the normal (expensive) code to generate
    the string.

actionpack/lib/action_controller/metal/compatibility.rb

@@ -25,8 +25,9 @@ module ActionController
       cattr_accessor :relative_url_root
       self.relative_url_root = ENV['RAILS_RELATIVE_URL_ROOT']
 
-      cattr_accessor :default_charset
-      self.default_charset = "utf-8"
+      class << self
+        delegate :default_charset=, :to => "ActionDispatch::Response"
+      end
 
       # cattr_reader :protected_instance_variables
       cattr_accessor :protected_instance_variables

actionpack/lib/action_controller/metal/streaming.rb

@@ -145,7 +145,6 @@ module ActionController #:nodoc:
       def send_data(data, options = {}) #:doc:
         logger.info "Sending data #{options[:filename]}" if logger
         send_file_headers! options.merge(:length => data.bytesize)
-        @performed_render = false
         render :status => options[:status], :text => data
       end
 
@@ -175,6 +174,8 @@ module ActionController #:nodoc:
           'Content-Transfer-Encoding' => 'binary'
         )
 
+        response.sending_file = true
+
         # Fix a problem with IE 6.0 on opening downloaded files:
         # If Cache-Control: no-cache is set (which Rails does by default),
         # IE removes the file it just downloaded from its cache immediately

actionpack/lib/action_controller/testing/process.rb

@@ -52,7 +52,7 @@ module ActionController #:nodoc:
   class TestResponse < ActionDispatch::TestResponse
     def recycle!
       @status = 200
-      @header = Rack::Utils::HeaderHash.new
+      @header = ActionDispatch::Response::SimpleHeaderHash.new
       @writer = lambda { |x| @body << x }
       @block = nil
       @length = 0

actionpack/lib/action_dispatch/http/response.rb

@@ -32,18 +32,42 @@ module ActionDispatch # :nodoc:
   #    end
   #  end
   class Response < Rack::Response
+    class SimpleHeaderHash < Hash
+      def to_hash
+        result = {}
+        each do |k,v|
+          v = v.join("\n") if v.is_a?(Array)
+          result[k] = v
+        end
+        result
+      end
+    end
+
     attr_accessor :request
     attr_reader :cache_control
 
-    attr_writer :header
+    attr_writer :header, :sending_file
     alias_method :headers=, :header=
 
-    delegate :default_charset, :to => 'ActionController::Base'
-
     def initialize
-      super
+      @status = 200
+      @header = SimpleHeaderHash.new
       @cache_control = {}
-      @header = Rack::Utils::HeaderHash.new
+
+      @writer = lambda { |x| @body << x }
+      @block = nil
+      @length = 0
+
+      @body = []
+      @sending_file = false
+
+      yield self if block_given?
+    end
+
+    def write(str)
+      s = str.to_s
+      @writer.call s
+      str
     end
 
     def status=(status)
@@ -128,20 +152,20 @@ module ActionDispatch # :nodoc:
       end
     end
 
-    def sending_file?
-      headers["Content-Transfer-Encoding"] == "binary"
-    end
+    CONTENT_TYPE    = "Content-Type"
+
+    cattr_accessor(:default_charset) { "utf-8" }
 
     def assign_default_content_type_and_charset!
-      return if !headers["Content-Type"].blank?
+      return if !headers[CONTENT_TYPE].blank?
 
       @content_type ||= Mime::HTML
-      @charset      ||= default_charset
+      @charset      ||= self.class.default_charset
 
       type = @content_type.to_s.dup
-      type << "; charset=#{@charset}" unless sending_file?
+      type << "; charset=#{@charset}" unless @sending_file
 
-      headers["Content-Type"] = type
+      headers[CONTENT_TYPE] = type
     end
 
     def prepare!
@@ -168,17 +192,6 @@ module ActionDispatch # :nodoc:
       str
     end
 
-    def set_cookie(key, value)
-      if value.has_key?(:http_only)
-        ActiveSupport::Deprecation.warn(
-          "The :http_only option in ActionController::Response#set_cookie " +
-          "has been renamed. Please use :httponly instead.", caller)
-        value[:httponly] ||= value.delete(:http_only)
-      end
-
-      super(key, value)
-    end
-
     # Returns the response cookies, converted to a Hash of (name => value) pairs
     #
     #   assert_equal 'AuthorOfNewPage', r.cookies['author']
@@ -201,7 +214,7 @@ module ActionDispatch # :nodoc:
         if etag? || last_modified? || !cache_control.empty?
           set_conditional_cache_control!
         elsif nonempty_ok_response?
-          self.etag = body
+          self.etag = @body
 
           if request && request.etag_matches?(etag)
             self.status = 304
@@ -214,30 +227,37 @@ module ActionDispatch # :nodoc:
         end
       end
 
+      EMPTY_RESPONSE = [" "]
+
       def nonempty_ok_response?
         ok = !@status || @status == 200
-        ok && string_body?
+        ok && string_body? && @body != EMPTY_RESPONSE
       end
 
       def string_body?
         !body_parts.respond_to?(:call) && body_parts.any? && body_parts.all? { |part| part.is_a?(String) }
       end
 
-      def set_conditional_cache_control!
-        if cache_control.empty?
-          cache_control.merge!(:public => false, :max_age => 0, :must_revalidate => true)
-        end
+      DEFAULT_CACHE_CONTROL = "max-age=0, private, must-revalidate"
 
-        public_cache, max_age, must_revalidate, extras =
-          cache_control.values_at(:public, :max_age, :must_revalidate, :extras)
+      def set_conditional_cache_control!
+        control = cache_control
+
+        if control.empty?
+          headers["Cache-Control"] = DEFAULT_CACHE_CONTROL
+        else
+          extras  = control[:extras]
+          max_age = control[:max_age]
 
           options = []
           options << "max-age=#{max_age}" if max_age
-        options << (public_cache ? "public" : "private")
-        options << "must-revalidate" if must_revalidate
+          options << (control[:public] ? "public" : "private")
+          options << "must-revalidate" if control[:must_revalidate]
           options.concat(extras) if extras
 
           headers["Cache-Control"] = options.join(", ")
         end
+
+      end
   end
 end

activesupport/lib/active_support/core_ext/class/attribute_accessors.rb

@@ -46,11 +46,12 @@ class Class
         end                                            # end
         " unless options[:instance_writer] == false }  # # instance writer above is generated unless options[:instance_writer] == false
       EOS
+      self.send("#{sym}=", yield) if block_given?
     end
   end
 
-  def cattr_accessor(*syms)
+  def cattr_accessor(*syms, &blk)
     cattr_reader(*syms)
-    cattr_writer(*syms)
+    cattr_writer(*syms, &blk)
   end
 end