Add link to basic search when no results

For https://gitlab.com/gitlab-org/gitlab/issues/33678 this will allow us
to roll out Elasticsearch more confidently as users can always revert
back if there is a problem with the Elasticsearch results.

app/views/search/_results.html.haml

 - if @search_objects.to_a.empty?
   = render partial: "search/results/empty"
   = render_if_exists 'shared/promotions/promote_advanced_search'
+  = render_if_exists 'search/form_revert_to_basic'
 - else
   .row-content-block.d-md-flex.text-left.align-items-center
     - unless @search_objects.is_a?(Kaminari::PaginatableWithoutCount)

ee/app/services/concerns/search/elasticsearchable.rb

+# frozen_string_literal: true
+
 module Search
   module Elasticsearchable
     def use_elasticsearch?

ee/app/views/search/_form_revert_to_basic.html.haml

+- if search_service.use_elasticsearch?
+  - query_string = URI.parse(request.url).query
+  - query_params = Rack::Utils.parse_query(query_string)
+  - url = url_for :only_path => false, :params => query_params.merge(basic_search: 'true')
+  - begin_link = '<a href="%{url}">'.html_safe % { url: url }
+  - end_link = '</a>'.html_safe
+  %p.text-center
+    = _('You can try again using %{begin_link}basic search%{end_link}').html_safe % { begin_link: begin_link, end_link: end_link }

ee/spec/features/search/elastic/global_search_spec.rb

@@ -219,4 +219,24 @@ describe 'Global elastic search', :elastic do
       expect(page).to have_content('Users 0')
     end
   end
+
+  describe 'When requesting basic search' do
+    it 'does not use Elasticsearch' do
+      create(:issue, project: project, title: 'project issue')
+
+      visit dashboard_projects_path
+
+      submit_search('project')
+
+      # Project won't be found since ES index is not up to date
+      expect(page).not_to have_content('Projects 1')
+
+      # Since there are no results you have the option to instead use basic
+      # search
+      click_link 'basic search'
+
+      # Project is found now that we are using basic search
+      expect(page).to have_content('Projects 1')
+    end
+  end
 end

locale/gitlab.pot

@@ -19073,6 +19073,9 @@ msgstr ""
 msgid "You can test your .gitlab-ci.yml in %{linkStart}CI Lint%{linkEnd}."
 msgstr ""
 
+msgid "You can try again using %{begin_link}basic search%{end_link}"
+msgstr ""
+
 msgid "You cannot access the raw file. Please wait a minute."
 msgstr ""