Fix sorting of helpers from different paths

When more than one directory for helpers is provided to a controller, it
should preserver the order of directories. Given 2 paths:

    MyController.helpers_paths = ["dir1/helpers", "dir2/helpers"]

helpers from dir1 should be loaded first. Before this commit, all
helpers were mixed and then sorted alphabetically, which essentially
would require to rename helpers to get desired order.

This is a problem especially for engines, where you would like to be
able to predict accurately which engine helpers will load first.

(closes #6496)

actionpack/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   change a way of ordering helpers from several directories. Previously,
+    when loading helpers from multiple paths, all of the helpers files were
+    gathered into one array an then they were sorted. Helpers from different
+    directories should not be mixed before loading them to make loading more
+    predictable. The most common use case for such behavior is loading helpers
+    from engines. When you load helpers from application and engine Foo, in
+    that order, first rails will load all of the helpers from application,
+    sorted alphabetically and then it will do the same for Foo engine.
+
+    *Piotr Sarnacki*
+
 *   `truncate` now always returns an escaped HTMl-safe string. The option `:escape` can be used as
     false to not escape the result.
 

actionpack/lib/action_controller/metal/helpers.rb

@@ -95,9 +95,9 @@ def all_helpers_from_path(path)
         helpers = []
         Array(path).each do |_path|
           extract  = /^#{Regexp.quote(_path.to_s)}\/?(.*)_helper.rb$/
-          helpers += Dir["#{_path}/**/*_helper.rb"].map { |file| file.sub(extract, '\1') }
+          names = Dir["#{_path}/**/*_helper.rb"].map { |file| file.sub(extract, '\1') }
+          helpers += names.sort
         end
-        helpers.sort!
         helpers.uniq!
         helpers
       end

actionpack/test/controller/helper_test.rb

@@ -46,12 +46,42 @@ def lib
 class MeTooController < JustMeController
 end
 
+class HelpersPathsController < ActionController::Base
+  paths = ["helpers2_pack", "helpers1_pack"].map do |path|
+    File.join(File.expand_path('../../fixtures', __FILE__), path)
+  end
+  $:.unshift(*paths)
+
+  self.helpers_path = paths
+  helper :all
+
+  def index
+    render :inline => "<%= conflicting_helper %>"
+  end
+end
+
 module LocalAbcHelper
   def a() end
   def b() end
   def c() end
 end
 
+class HelperPathsTest < ActiveSupport::TestCase
+  def setup
+    @request    = ActionController::TestRequest.new
+    @response   = ActionController::TestResponse.new
+  end
+
+  def test_helpers_paths_priority
+    request  = ActionController::TestRequest.new
+    responses = HelpersPathsController.action(:index).call(request.env)
+
+    # helpers1_pack was given as a second path, so pack1_helper should be
+    # included as the second one
+    assert_equal "pack1", responses.last.body
+  end
+end
+
 class HelperTest < ActiveSupport::TestCase
   class TestController < ActionController::Base
     attr_accessor :delegate_attr

actionpack/test/fixtures/helpers1_pack/pack1_helper.rb

+module Pack1Helper
+  def conflicting_helper
+    "pack1"
+  end
+end

actionpack/test/fixtures/helpers2_pack/pack2_helper.rb

+module Pack2Helper
+  def conflicting_helper
+    "pack2"
+  end
+end

guides/source/upgrading_ruby_on_rails.textile

@@ -46,6 +46,10 @@ h4(#action_pack4_0). ActionPack
 
 Rails 4.0 changed how <tt>assert_generates</tt>, <tt>assert_recognizes</tt>, and <tt>assert_routing</tt> work. Now all these assertions raise <tt>Assertion</tt> instead of <tt>RoutingError</tt>.
 
+h4(#helpers_order). Helpers loading order
+
+The loading order of helpers from more than one directory has changed in Rails 4.0. Previously, helpers from all directories were gathered and then sorted alphabetically. After upgrade to Rails 4.0 helpers will preserve the order of loaded directories and will be sorted alphabetically only within each directory. Unless you explicitly use <tt>helpers_path</tt> parameter, this change will only impact the way of loading helpers from engines. If you rely on the fact that particular helper from engine loads before or after another helper from application or another engine, you should check if correct methods are available after upgrade. If you would like to change order in which engines are loaded, you can use <tt>config.railties_order=</tt> method.
+
 h3. Upgrading from Rails 3.1 to Rails 3.2
 
 If your application is currently on any version of Rails older than 3.1.x, you should upgrade to Rails 3.1 before attempting an update to Rails 3.2.