Allow fallback to basic non-ES search

Setting query param basic_search=true on any search now should not use
Elasticsearch. Introduce Search::Elasticsearchable to not duplicate the
logic of whether or not to use elasticsearch.

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

+module Search
+  module Elasticsearchable
+    def use_elasticsearch?
+      return false if params[:basic_search]
+
+      ::Gitlab::CurrentSettings.search_using_elasticsearch?(scope: elasticsearchable_scope)
+    end
+
+    def elasticsearchable_scope
+      raise NotImplementedError
+    end
+  end
+end

ee/app/services/ee/search/global_service.rb

@@ -5,6 +5,7 @@ module EE
     module GlobalService
       extend ::Gitlab::Utils::Override
       include ::Gitlab::Utils::StrongMemoize
+      include ::Search::Elasticsearchable
 
       override :execute
       def execute
@@ -19,8 +20,8 @@ module EE
         )
       end
 
-      def use_elasticsearch?
-        ::Gitlab::CurrentSettings.search_using_elasticsearch?(scope: nil)
+      def elasticsearchable_scope
+        nil
       end
 
       def elastic_projects

ee/app/services/ee/search/group_service.rb

@@ -5,9 +5,9 @@ module EE
     module GroupService
       extend ::Gitlab::Utils::Override
 
-      override :use_elasticsearch?
-      def use_elasticsearch?
-        ::Gitlab::CurrentSettings.search_using_elasticsearch?(scope: group)
+      override :elasticsearchable_scope
+      def elasticsearchable_scope
+        group
       end
 
       override :elastic_projects

ee/app/services/ee/search/project_service.rb

@@ -4,6 +4,7 @@ module EE
   module Search
     module ProjectService
       extend ::Gitlab::Utils::Override
+      include ::Search::Elasticsearchable
 
       override :execute
       def execute
@@ -17,9 +18,8 @@ module EE
         )
       end
 
-      # This method is used in the top-level SearchService, so cannot be in-lined into #execute
-      def use_elasticsearch?
-        ::Gitlab::CurrentSettings.search_using_elasticsearch?(scope: project)
+      def elasticsearchable_scope
+        project
       end
     end
   end

ee/app/services/ee/search/snippet_service.rb

@@ -12,9 +12,9 @@ module EE
         ::Gitlab::Elastic::SnippetSearchResults.new(current_user, params[:search], elastic_projects, nil, true)
       end
 
-      # This method is used in the top-level SearchService, so cannot be in-lined into #execute
-      def use_elasticsearch?
-        ::Gitlab::CurrentSettings.search_using_elasticsearch?(scope: nil)
+      override :elasticsearchable_scope
+      def elasticsearchable_scope
+        nil
       end
     end
   end

ee/spec/services/search/global_service_spec.rb

@@ -15,7 +15,7 @@ describe Search::GlobalService do
 
   it_behaves_like 'EE search service shared examples', ::Gitlab::SearchResults, ::Gitlab::Elastic::SearchResults do
     let(:scope) { nil }
-    let(:service) { described_class.new(user, search: '*') }
+    let(:service) { described_class.new(user, params) }
   end
 
   context 'visibility', :elastic do

ee/spec/services/search/group_service_spec.rb

@@ -16,7 +16,7 @@ describe Search::GroupService, :elastic do
 
   it_behaves_like 'EE search service shared examples', ::Gitlab::GroupSearchResults, ::Gitlab::Elastic::GroupSearchResults do
     let(:scope) { create(:group) }
-    let(:service) { described_class.new(user, scope, search: '*') }
+    let(:service) { described_class.new(user, scope, params) }
   end
 
   describe 'group search' do

ee/spec/services/search/project_service_spec.rb

@@ -14,7 +14,7 @@ describe Search::ProjectService do
   it_behaves_like 'EE search service shared examples', ::Gitlab::ProjectSearchResults, ::Gitlab::Elastic::ProjectSearchResults do
     let(:user) { scope.owner }
     let(:scope) { create(:project) }
-    let(:service) { described_class.new(scope, user, search: '*') }
+    let(:service) { described_class.new(scope, user, params) }
   end
 
   context 'visibility', :elastic do

ee/spec/services/search/snippet_service_spec.rb

@@ -6,6 +6,6 @@ describe Search::SnippetService do
   it_behaves_like 'EE search service shared examples', ::Gitlab::SnippetSearchResults, ::Gitlab::Elastic::SnippetSearchResults do
     let(:user) { create(:user) }
     let(:scope) { nil }
-    let(:service) { described_class.new(user, search: '*') }
+    let(:service) { described_class.new(user, params) }
   end
 end

ee/spec/support/shared_examples/services/search_service_shared_examples.rb

 # frozen_string_literal: true
 
 RSpec.shared_examples 'EE search service shared examples' do |normal_results, elasticsearch_results|
+  let(:params) { { search: '*' } }
+
   describe '#use_elasticsearch?' do
     it 'delegates to Gitlab::CurrentSettings.search_using_elasticsearch?' do
       expect(Gitlab::CurrentSettings)
@@ -10,6 +12,17 @@ RSpec.shared_examples 'EE search service shared examples' do |normal_results, el
 
       expect(service.use_elasticsearch?).to eq(:value)
     end
+
+    context 'when requesting basic_search' do
+      let(:params) { { search: '*', basic_search: 'true' } }
+
+      it 'returns false' do
+        expect(Gitlab::CurrentSettings)
+          .not_to receive(:search_using_elasticsearch?)
+
+        expect(service.use_elasticsearch?).to eq(false)
+      end
+    end
   end
 
   describe '#execute' do