mongrel.rb

if ['2.3.8', '2.3.9', '2.3.10', '2.3.11'].include?(Rails.version) && Gem.available?('mongrel', '~>1.1.5') && self.class.const_defined?(:Mongrel)
  
  # Pulled right from latest rack. Old looked like this in 1.1.0 version.
  # 
  # def [](k)
  #   super(@names[k] ||= @names[k.downcase])
  # end
  # 
  module Rack
    module Utils
      class HeaderHash < Hash
        def [](k)
          super(@names[k]) if @names[k]
          super(@names[k.downcase])
        end
      end
    end
  end
  
  # Code pulled from the ticket above.
  # 
  class Mongrel::CGIWrapper
    def header_with_rails_fix(options = 'text/html')
      @head['cookie'] = options.delete('cookie').flatten.map { |v| v.sub(/^\n/,'') } if options.class != String and options['cookie']
      header_without_rails_fix(options)
    end
    alias_method_chain :header, :rails_fix
  end
  
  # Pulled right from 2.3.8 ActionPack. Simple diff was
  # 
  # if headers.include?('Set-Cookie')
  #   headers['cookie'] = headers.delete('Set-Cookie').split("\n")
  # end
  # 
  # to 
  # 
  # if headers['Set-Cookie']
  #   headers['cookie'] = headers.delete('Set-Cookie').split("\n")
  # end
  #       
  module ActionController
    class CGIHandler
      def self.dispatch_cgi(app, cgi, out = $stdout)
        env = cgi.__send__(:env_table)
        env.delete "HTTP_CONTENT_LENGTH"
        cgi.stdinput.extend ProperStream
        env["SCRIPT_NAME"] = "" if env["SCRIPT_NAME"] == "/"
        env.update({
          "rack.version" => [0,1],
          "rack.input" => cgi.stdinput,
          "rack.errors" => $stderr,
          "rack.multithread" => false,
          "rack.multiprocess" => true,
          "rack.run_once" => false,
          "rack.url_scheme" => ["yes", "on", "1"].include?(env["HTTPS"]) ? "https" : "http"
        })
        env["QUERY_STRING"] ||= ""
        env["HTTP_VERSION"] ||= env["SERVER_PROTOCOL"]
        env["REQUEST_PATH"] ||= "/"
        env.delete "PATH_INFO" if env["PATH_INFO"] == ""
        status, headers, body = app.call(env)
        begin
          out.binmode if out.respond_to?(:binmode)
          out.sync = false if out.respond_to?(:sync=)
          headers['Status'] = status.to_s
          if headers['Set-Cookie']
            headers['cookie'] = headers.delete('Set-Cookie').split("\n")
          end
          out.write(cgi.header(headers))
          body.each { |part|
            out.write part
            out.flush if out.respond_to?(:flush)
          }
        ensure
          body.close if body.respond_to?(:close)
        end
      end
    end
  end
  
end

This fixed my problem while upgrading to Rails 2.3.15 (just comment out the if statement checking the Rails version because you dont need it). The error message that I had was:

Error calling Dispatcher.dispatch #<NoMethodError: private method `split' called for nil:NilClass>

Just copy the contents of this gist and paste into config/initializers/mongrel.rb