Always Refresh Merge Requests in Merge Train from the beginning

This commit adds the new service to improve merge train refresh
service.

config/feature_flags/development/ci_always_refresh_merge_requests_from_beginning.yml

+---
+name: ci_always_refresh_merge_requests_from_beginning
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/45232
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/268215
+type: development
+group: group::continuous integration
+default_enabled: false

ee/app/services/merge_trains/refresh_merge_requests_service.rb

@@ -19,7 +19,7 @@ module MergeTrains
       result = queue.safe_execute([merge_request.id], lock_timeout: 15.minutes) do |items|
         logging("Successfuly obtained the exclusive lock. Found merge requests to be refreshed", merge_request_ids: items.map(&:to_i))
 
-        first_merge_request = MergeTrain.first_in_train_from(items)
+        first_merge_request = get_first_in_train(items)
         unsafe_refresh(first_merge_request)
       end
 
@@ -30,7 +30,7 @@ module MergeTrains
       if result[:status] == :finished && result[:new_items].present?
         logging("Found more merge requests to be refreshed", merge_request_ids: result[:new_items].map(&:to_i))
 
-        MergeTrain.first_in_train_from(result[:new_items]).try do |first_merge_request|
+        get_first_in_train(result[:new_items]).try do |first_merge_request|
           logging("Rescheduled to refresh the merge train from", merge_request_ids: [first_merge_request.id])
 
           AutoMergeProcessWorker.perform_async(first_merge_request.id)
@@ -42,6 +42,14 @@ module MergeTrains
 
     attr_reader :merge_request
 
+    def get_first_in_train(items)
+      if Feature.enabled?(:ci_always_refresh_merge_requests_from_beginning, merge_request.target_project)
+        MergeTrain.first_in_train(merge_request.target_project, merge_request.target_branch)
+      else
+        MergeTrain.first_in_train_from(items)
+      end
+    end
+
     def unsafe_refresh(first_merge_request)
       require_next_recreate = false
 

ee/spec/services/merge_trains/refresh_merge_requests_service_spec.rb

@@ -174,45 +174,70 @@ RSpec.describe MergeTrains::RefreshMergeRequestsService do
     context 'when merge request 2 is passed' do
       let(:merge_request) { merge_request_2 }
 
-      it 'executes RefreshMergeRequestService to all the following merge requests' do
-        expect(refresh_service_1).not_to receive(:execute).with(merge_request_1)
+      it 'executes RefreshMergeRequestService to all the merge requests from beginning' do
+        expect(refresh_service_1).to receive(:execute).with(merge_request_1)
         expect(refresh_service_2).to receive(:execute).with(merge_request_2)
 
         subject
       end
 
-      it_behaves_like 'logging results', 2
-
-      context 'when merge request 1 was tried to be refreshed while the system is refreshing merge request 2' do
+      context 'when ci_always_refresh_merge_requests_from_beginning is disabled' do
         before do
-          allow_any_instance_of(described_class).to receive(:unsafe_refresh).with(merge_request_2) do
-            service.execute(merge_request_1)
-          end
+          stub_feature_flags(ci_always_refresh_merge_requests_from_beginning: false)
         end
 
-        it 'refreshes the merge request 1 later with AutoMergeProcessWorker' do
-          expect(AutoMergeProcessWorker).to receive(:perform_async).with(merge_request_1.id).once
+        it 'executes RefreshMergeRequestService to all the following merge requests' do
+          expect(refresh_service_1).not_to receive(:execute).with(merge_request_1)
+          expect(refresh_service_2).to receive(:execute).with(merge_request_2)
 
           subject
         end
 
-        it_behaves_like 'logging results', 4
-
-        context 'when merge request 1 has already been merged' do
+        context 'when merge request 1 was tried to be refreshed while the system is refreshing merge request 2' do
           before do
-            allow(merge_request_1.merge_train).to receive(:cleanup_ref)
-            merge_request_1.merge_train.update_column(:status, MergeTrain.state_machines[:status].states[:merged].value)
+            allow_any_instance_of(described_class).to receive(:unsafe_refresh).with(merge_request_2) do
+              service.execute(merge_request_1)
+            end
           end
 
-          it 'does not refresh the merge request 1' do
-            expect(AutoMergeProcessWorker).not_to receive(:perform_async).with(merge_request_1.id)
+          it 'refreshes the merge request 1 later with AutoMergeProcessWorker' do
+            expect(AutoMergeProcessWorker).to receive(:perform_async).with(merge_request_1.id).once
 
             subject
           end
 
-          it_behaves_like 'logging results', 1
+          context 'when ci_always_refresh_merge_requests_from_beginning is disabled' do
+            before do
+              stub_feature_flags(ci_always_refresh_merge_requests_from_beginning: false)
+            end
+
+            it 'refreshes the merge request 1 later with AutoMergeProcessWorker' do
+              expect(AutoMergeProcessWorker).to receive(:perform_async).with(merge_request_1.id).once
+
+              subject
+            end
+          end
+
+          it_behaves_like 'logging results', 4
+
+          context 'when merge request 1 has already been merged' do
+            before do
+              allow(merge_request_1.merge_train).to receive(:cleanup_ref)
+              merge_request_1.merge_train.update_column(:status, MergeTrain.state_machines[:status].states[:merged].value)
+            end
+
+            it 'does not refresh the merge request 1' do
+              expect(AutoMergeProcessWorker).not_to receive(:perform_async).with(merge_request_1.id)
+
+              subject
+            end
+
+            it_behaves_like 'logging results', 1
+          end
         end
       end
+
+      it_behaves_like 'logging results', 3
     end
   end
 end