Add render_ee and override render_ce

Use `render_ee` and `render_ce` to achieve
layer-by-layer expansion of views:
1.  jh/app/views/layouts/_page.html.haml
2.  ee/app/views/layouts/_page.html.haml
3.  app/views/layouts/_page.html.haml

See: https://gitlab.com/gitlab-jh/gitlab/-/merge_requests/52

jh/app/helpers/jh/application_helper.rb

@@ -3,6 +3,7 @@
 module JH
   module ApplicationHelper
     extend ActiveSupport::Concern
+    extend ::Gitlab::Utils::Override
 
     class_methods do
       extend ::Gitlab::Utils::Override
@@ -12,5 +13,43 @@ def promo_host
         'about.gitlab.cn'
       end
     end
+
+    def render_ee(partial, locals = {})
+      render template: find_ee_template(partial), locals: locals
+    end
+
+    # rubocop: disable CodeReuse/ActiveRecord
+    def find_ee_template(name)
+      prefixes = [] # So don't create extra [] garbage
+
+      if ee_lookup_context.exists?(name, prefixes, true)
+        ee_lookup_context.find(name, prefixes, true)
+      else
+        ee_lookup_context.find(name, prefixes, false)
+      end
+    end
+    # rubocop: enable CodeReuse/ActiveRecord
+
+    def ee_lookup_context
+      @ee_lookup_context ||= fetch_lookup_context(::Gitlab.extensions.last)
+    end
+
+    override :ce_lookup_context
+    def ce_lookup_context
+      @ce_lookup_context ||= fetch_lookup_context(::Gitlab.extensions.reverse)
+    end
+
+    private
+
+    def fetch_lookup_context(folders)
+      raise 'folders should not be blank' if folders.blank?
+
+      folder_paths = Array(folders).map { |folder| "#{Rails.root}/#{folder}" }
+      view_paths = lookup_context.view_paths.paths.reject do |resolver|
+        resolver.to_path.start_with?(*folder_paths)
+      end
+
+      ActionView::LookupContext.new(view_paths)
+    end
   end
 end

jh/app/views/layouts/_page.html.haml

+= render_ee("layouts/page")

jh/spec/helpers/application_helper_spec.rb

@@ -8,4 +8,27 @@
       expect(helper.promo_host).to eq('about.gitlab.cn')
     end
   end
+
+  context 'when both JH and EE and CE has partials with the same name' do
+    let(:partial) { 'layouts/page' }
+    let(:ce_partial_path) { 'app/views/layouts/_page.html.haml' }
+    let(:jh_partial_path) { "jh/#{ce_partial_path}" }
+
+    describe 'find template' do
+      it 'finds the JH template' do
+        template = helper.lookup_context.find(partial, [], true)
+        expect(template.short_identifier).to eq(jh_partial_path)
+      end
+
+      it 'finds the CE template' do
+        template = helper.find_ce_template(partial)
+        expect(template.short_identifier).to eq(ce_partial_path)
+      end
+
+      it 'finds the nonexistent EE template' do
+        template = helper.find_ee_template(partial)
+        expect(template.short_identifier).to eq(ce_partial_path)
+      end
+    end
+  end
 end