Add a feature flag for switching pipeline stages

app/models/ci/pipeline.rb

@@ -250,13 +250,17 @@ module Ci
     end
 
     ##
-    # TODO consider switching to persisted stages only in pipelines table
-    # (not necessairly in the show pipeline page because of #23257.
-    # Hide this behind two feature flags - enabled / disabled and only
-    # gitlab-ce / everywhere.
+    # TODO We do not completely switch to persisted stages because of
+    # race conditions with setting statuses gitlab-ce#23257.
     #
-    def stages
-      super
+    def ordered_stages
+      return legacy_stages unless complete?
+
+      if Feature.enabled?('ci_pipeline_persisted_stages')
+        stages
+      else
+        legacy_stages
+      end
     end
 
     def legacy_stages

app/serializers/pipeline_details_entity.rb

 class PipelineDetailsEntity < PipelineEntity
   expose :details do
-    expose :stages, using: StageEntity
+    expose :ordered_stages, as: :stages, using: StageEntity
     expose :artifacts, using: BuildArtifactEntity
     expose :manual_actions, using: BuildActionEntity
   end

spec/models/ci/pipeline_spec.rb

@@ -500,6 +500,87 @@ describe Ci::Pipeline, :mailer do
         end
       end
     end
+
+    describe '#stages' do
+      before do
+        create(:ci_stage_entity, project: project,
+                                 pipeline: pipeline,
+                                 name: 'build')
+      end
+
+      it 'returns persisted stages' do
+        expect(pipeline.stages).not_to be_empty
+        expect(pipeline.stages).to all(be_persisted)
+      end
+    end
+
+    describe '#ordered_stages' do
+      before do
+        create(:ci_stage_entity, project: project,
+                                 pipeline: pipeline,
+                                 position: 4,
+                                 name: 'deploy')
+
+        create(:ci_build, project: project,
+                          pipeline: pipeline,
+                          stage: 'test',
+                          stage_idx: 3,
+                          name: 'test')
+
+        create(:ci_build, project: project,
+                          pipeline: pipeline,
+                          stage: 'build',
+                          stage_idx: 2,
+                          name: 'build')
+
+        create(:ci_stage_entity, project: project,
+                                 pipeline: pipeline,
+                                 position: 1,
+                                 name: 'sanity')
+
+        create(:ci_stage_entity, project: project,
+                                 pipeline: pipeline,
+                                 position: 5,
+                                 name: 'cleanup')
+      end
+
+      subject { pipeline.ordered_stages }
+
+      context 'when using legacy stages' do
+        before do
+          stub_feature_flags(ci_pipeline_persisted_stages: false)
+        end
+
+        it 'returns legacy stages in valid order' do
+          expect(subject.map(&:name)).to eq %w[build test]
+        end
+      end
+
+      context 'when using persisted stages' do
+        before do
+          stub_feature_flags(ci_pipeline_persisted_stages: true)
+        end
+
+        context 'when pipelines is not complete' do
+          it 'still returns legacy stages' do
+            expect(subject).to all(be_a Ci::LegacyStage)
+            expect(subject.map(&:name)).to eq %w[build test]
+          end
+        end
+
+        context 'when pipeline is complete' do
+          before do
+            pipeline.succeed!
+          end
+
+          it 'returns stages in valid order' do
+            expect(subject).to all(be_a Ci::Stage)
+            expect(subject.map(&:name))
+              .to eq %w[sanity build test deploy cleanup]
+          end
+        end
+      end
+    end
   end
 
   describe 'state machine' do