Use feature flag to toggle between MR base pipelines

This merge request allows a single backend feature flag to toggle the
base pipeline used for comparing CodeQuality artifacts in merge
requests between the MR diff base and the merge base from the target
branch.

app/models/merge_request.rb

@@ -1600,7 +1600,7 @@ class MergeRequest < ApplicationRecord
   end
 
   def merge_base_pipeline
-    @merge_target_pipeline ||= project.ci_pipelines
+    @merge_base_pipeline ||= project.ci_pipelines
       .order(id: :desc)
       .find_by(sha: actual_head_pipeline.target_sha, ref: target_branch)
   end

app/serializers/merge_request_widget_entity.rb

@@ -120,11 +120,11 @@ class MergeRequestWidgetEntity < Grape::Entity
     end
 
     expose :base_path do |merge_request|
-      base_pipeline_downloadable_path_for_report_type(:codequality)
-    end
-
-    expose :merge_base_path do |merge_request|
-      merge_target_pipeline_downloadable_path_for_report_type(:codequality)
+      if use_merge_base_with_merged_results?
+        merge_base_pipeline_downloadable_path_for_report_type(:codequality)
+      else
+        base_pipeline_downloadable_path_for_report_type(:codequality)
+      end
     end
   end
 
@@ -161,7 +161,12 @@ class MergeRequestWidgetEntity < Grape::Entity
       &.downloadable_path_for_report_type(file_type)
   end
 
-  def merge_target_pipeline_downloadable_path_for_report_type(file_type)
+  def use_merge_base_with_merged_results?
+    Feature.enabled?(:merge_base_pipelines, object.target_project) &&
+      object.actual_head_pipeline&.merge_request_event_type == :merged_result
+  end
+
+  def merge_base_pipeline_downloadable_path_for_report_type(file_type)
     object.merge_base_pipeline&.present(current_user: current_user)
       &.downloadable_path_for_report_type(file_type)
   end

config/feature_flags/development/merge_base_pipelines.yml

+---
+name: merge_base_pipelines
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/44648
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/263724
+type: development
+group: group::testing
+default_enabled: false

spec/models/merge_request_spec.rb

@@ -3525,15 +3525,17 @@ RSpec.describe MergeRequest, factory_default: :keep do
       create(:merge_request, :with_merge_request_pipeline)
     end
 
-    let(:pre_merge_target_pipeline) do
+    let(:merge_base_pipeline) do
       create(:ci_pipeline, ref: merge_request.target_branch, sha: merge_request.target_branch_sha)
     end
 
-    it 'returns a pipeline pointing to a commit on the target ref' do
-      pre_merge_target_pipeline
+    before do
+      merge_base_pipeline
       merge_request.update_head_pipeline
+    end
 
-      expect(merge_request.merge_base_pipeline).to eq(pre_merge_target_pipeline)
+    it 'returns a pipeline pointing to a commit on the target ref' do
+      expect(merge_request.merge_base_pipeline).to eq(merge_base_pipeline)
     end
   end
 

spec/serializers/merge_request_widget_entity_spec.rb

@@ -88,11 +88,13 @@ RSpec.describe MergeRequestWidgetEntity do
   end
 
   describe 'codequality report artifacts', :request_store do
+    let(:merge_base_pipeline) { create(:ci_pipeline, :with_codequality_report, project: project) }
+
     before do
       project.add_developer(user)
 
       allow(resource).to receive_messages(
-        merge_base_pipeline: pipeline,
+        merge_base_pipeline: merge_base_pipeline,
         base_pipeline: pipeline,
         head_pipeline: pipeline
       )
@@ -101,10 +103,29 @@ RSpec.describe MergeRequestWidgetEntity do
     context 'with report artifacts' do
       let(:pipeline) { create(:ci_pipeline, :with_codequality_report, project: project) }
 
-      it 'has entries for 3 reference artifacts' do
-        expect(subject[:codeclimate][:merge_base_path]).to be_present
-        expect(subject[:codeclimate][:base_path]).to be_present
+      it 'has head_path and base_path entries' do
         expect(subject[:codeclimate][:head_path]).to be_present
+        expect(subject[:codeclimate][:base_path]).to be_present
+      end
+
+      context 'on pipelines for merged results' do
+        let(:pipeline) { create(:ci_pipeline, :merged_result_pipeline, :with_codequality_report, project: project) }
+
+        context 'with merge_base_pipelines enabled' do
+          it 'returns URLs from the head_pipeline and merge_base_pipeline' do
+            expect(subject[:codeclimate][:head_path]).not_to eq(subject[:codeclimate][:base_path])
+          end
+        end
+
+        context 'with merge_base_pipelines disabled' do
+          before do
+            stub_feature_flags(merge_base_pipelines: false)
+          end
+
+          it 'returns URLs from the head_pipeline and base_pipeline' do
+            expect(subject[:codeclimate][:head_path]).to eq(subject[:codeclimate][:base_path])
+          end
+        end
       end
     end