Fix code search pagination on a custom branch

When we use non default code search with advanced global search enabled
we have a broken pagination. This change moves default_branch? condition
into the ProjectService to account for that difference.

changelogs/unreleased/207623-fix-code-search-pagination.yml

+---
+title: Fix code search pagination on a custom branch
+merge_request: 25984
+author:
+type: fixed

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

@@ -8,16 +8,26 @@ module EE
 
       override :execute
       def execute
-        return super unless use_elasticsearch?
+        return super unless use_elasticsearch? && default_branch?
 
         ::Gitlab::Elastic::ProjectSearchResults.new(
           current_user,
           params[:search],
           project.id,
-          params[:repository_ref]
+          repository_ref
         )
       end
 
+      def repository_ref
+        params[:repository_ref]
+      end
+
+      def default_branch?
+        return true if repository_ref.blank?
+
+        project.root_ref?(repository_ref)
+      end
+
       def elasticsearchable_scope
         project
       end

ee/lib/gitlab/elastic/project_search_results.rb

@@ -44,23 +44,14 @@ module Gitlab
 
       def blobs(page: 1, per_page: 20)
         return Kaminari.paginate_array([]) unless Ability.allowed?(@current_user, :download_code, project)
-
-        if project.empty_repo? || query.blank?
-          Kaminari.paginate_array([])
-        else
-          # We use elastic for default branch only
-          if root_ref?
-            project.repository.__elasticsearch__.elastic_search_as_found_blob(
-              query,
-              page: (page || 1).to_i,
-              per: per_page
-            )
-          else
-            Kaminari.paginate_array(
-              Gitlab::FileFinder.new(project, repository_ref).find(query)
-            )
-          end
-        end
+        return Kaminari.paginate_array([]) if project.empty_repo? || query.blank?
+        return Kaminari.paginate_array([]) unless root_ref?
+
+        project.repository.__elasticsearch__.elastic_search_as_found_blob(
+          query,
+          page: (page || 1).to_i,
+          per: per_page
+        )
       end
 
       def wiki_blobs(page: 1, per_page: 20)

ee/spec/lib/gitlab/elastic/project_search_results_spec.rb

@@ -126,18 +126,10 @@ describe Gitlab::Elastic::ProjectSearchResults, :elastic do
 
     subject(:blobs) { result.objects('blobs') }
 
-    it 'uses FileFinder instead of ES search' do
-      project.add_reporter(user)
-
-      expect_any_instance_of(Gitlab::FileFinder).to receive(:find).with('initial').and_return([])
-
-      _ = blobs
-    end
-
-    it 'respects project visibility' do
+    it 'always returns zero results' do
       expect_any_instance_of(Gitlab::FileFinder).to receive(:find).never
 
-      is_expected.to be_empty
+      expect(blobs).to be_empty
     end
   end
 

ee/spec/services/search/project_service_spec.rb

@@ -17,6 +17,23 @@ describe Search::ProjectService do
     let(:service) { described_class.new(scope, user, params) }
   end
 
+  context 'code search' do
+    let(:user) { scope.owner }
+    let(:scope) { create(:project) }
+    let(:service) { described_class.new(scope, user, params) }
+    let(:params) { { search: '*' } }
+
+    describe '#execute' do
+      subject { service.execute }
+
+      it 'returns ordinary results when searching non-default branch' do
+        expect(service).to receive(:default_branch?).and_return(false)
+
+        is_expected.to be_a(::Gitlab::ProjectSearchResults)
+      end
+    end
+  end
+
   context 'visibility', :elastic, :sidekiq_inline do
     include_context 'ProjectPolicyTable context'