do not append a second slash when using

actionpack/CHANGELOG.md

@@ -1,5 +1,17 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Do not append second slash to root_url when using `trailing_slash: true`
+    Fix #8700
+
+    Example:
+        # before
+        root_url # => http://test.host//
+
+        # after
+        root_url # => http://test.host/
+
+    *Yves Senn*
+
 *   Allow to toggle dumps on error pages.
 
     *Gosha Arinich*

actionpack/lib/action_dispatch/http/url.rb

@@ -32,7 +32,11 @@ def url_for(options = {})
           params.reject! { |_,v| v.to_param.nil? }
 
           result = build_host_url(options)
-          result << (options[:trailing_slash] ? path.sub(/\?|\z/) { "/" + $& } : path)
+          if options[:trailing_slash] && !path.ends_with?('/')
+            result << path.sub(/(\?|\z)/) { "/" + $& }
+          else
+            result << path
+          end
           result << "?#{params.to_query}" unless params.empty?
           result << "##{Journey::Router::Utils.escape_fragment(options[:anchor].to_param.to_s)}" if options[:anchor]
           result

actionpack/test/controller/routing_test.rb

@@ -57,13 +57,13 @@ def test_route_generation_allows_passing_non_string_values_to_generated_helper
 end
 
 class MockController
-  def self.build(helpers)
+  def self.build(helpers, additional_options = {})
     Class.new do
-      def url_options
-        options = super
+      define_method :url_options do
+        options = super()
         options[:protocol] ||= "http"
         options[:host] ||= "test.host"
-        options
+        options.merge(additional_options)
       end
 
       include helpers
@@ -428,8 +428,8 @@ def test_optimised_named_route_with_host
     routes.send(:pages_url)
   end
 
-  def setup_for_named_route
-    MockController.build(rs.url_helpers).new
+  def setup_for_named_route(options = {})
+    MockController.build(rs.url_helpers, options).new
   end
 
   def test_named_route_without_hash
@@ -456,6 +456,16 @@ def test_named_route_root_without_hash
     assert_equal("/", routes.send(:root_path))
   end
 
+  def test_named_route_root_with_trailing_slash
+    rs.draw do
+      root "hello#index"
+    end
+
+    routes = setup_for_named_route(trailing_slash: true)
+    assert_equal("http://test.host/", routes.send(:root_url))
+    assert_equal("http://test.host/?foo=bar", routes.send(:root_url, foo: :bar))
+  end
+
   def test_named_route_with_regexps
     rs.draw do
       get 'page/:year/:month/:day/:title' => 'page#show', :as => 'article',