Fix merge train is not refreshed on abort

This commit fixes the merge train bug that
it's not refreshed on abort

changelogs/unreleased/fix-merge-train-is-not-refreshed-when-aborted.yml

+---
+title: Fix merge train is not refreshed when the system aborts/drops a merge request
+merge_request: 19763
+author:
+type: fixed

ee/app/services/auto_merge/merge_train_service.rb

@@ -31,10 +31,15 @@ module AutoMerge
       end
     end
 
-    def abort(merge_request, reason)
-      super do
+    def abort(merge_request, reason, process_next: true)
+      # Before dropping a merge request from a merge train, get the next
+      # merge request in order to refresh it later.
+      next_merge_request = merge_request.merge_train&.next
+
+      super(merge_request, reason) do
         if merge_request.merge_train&.destroy
           SystemNoteService.abort_merge_train(merge_request, project, current_user, reason)
+          AutoMergeProcessWorker.perform_async(next_merge_request.id) if next_merge_request && process_next
         end
       end
     end

ee/app/services/merge_trains/refresh_merge_request_service.rb

@@ -19,7 +19,7 @@ module MergeTrains
 
       success(pipeline_created: pipeline_created.present?)
     rescue ProcessError => e
-      drop(e)
+      abort(e)
     end
 
     private
@@ -159,9 +159,9 @@ module MergeTrains
       params[:require_recreate]
     end
 
-    def drop(error)
+    def abort(error)
       AutoMerge::MergeTrainService.new(project, merge_user)
-        .abort(merge_request, error.message)
+        .abort(merge_request, error.message, process_next: false)
 
       error(error.message)
     end

ee/spec/features/merge_trains/two_merge_requests_on_train_spec.rb

@@ -174,6 +174,29 @@ describe 'Two merge requests on a merge train' do
     end
   end
 
+  context 'when merge request 1 got a new commit' do
+    before do
+      oldrev = project.repository.commit('feature').sha
+      create_file_in_repo(project, 'refs/heads/feature', 'refs/heads/feature', 'test.txt', 'This is test')
+      newrev = project.repository.commit('feature').sha
+      MergeRequests::RefreshService.new(project, maintainer_1)
+        .execute(oldrev, newrev, 'refs/heads/feature')
+
+      merge_request_1.reload
+      merge_request_2.reload
+    end
+
+    it_behaves_like 'drops merge request 1 from the merge train' do
+      let(:system_note) do
+        'removed this merge request from the merge train because source branch was updated'
+      end
+    end
+
+    it_behaves_like 're-creates a pipeline for merge request 2' do
+      let(:target_branch_sha) { project.repository.commit('refs/heads/master').sha }
+    end
+  end
+
   context 'when merge request 1 is not mergeable' do
     before do
       merge_request_1.update!(title: merge_request_1.wip_title)

ee/spec/services/auto_merge/merge_train_service_spec.rb

@@ -159,7 +159,7 @@ describe AutoMerge::MergeTrainService do
   end
 
   describe '#abort' do
-    subject { service.abort(merge_request, 'an error') }
+    subject { service.abort(merge_request, 'an error', **args) }
 
     let!(:merge_request) do
       create(:merge_request, :on_train,
@@ -167,6 +167,8 @@ describe AutoMerge::MergeTrainService do
         target_project: project, target_branch: 'master')
     end
 
+    let(:args) { {} }
+
     it 'aborts auto merge on the merge request' do
       subject
 
@@ -186,6 +188,30 @@ describe AutoMerge::MergeTrainService do
 
       subject
     end
+
+    context 'when the other merge request is following the merge request' do
+      let!(:merge_request_2) do
+        create(:merge_request, :on_train,
+          source_project: project, source_branch: 'signed-commits',
+          target_project: project, target_branch: 'master')
+      end
+
+      it 'processes the next merge request on the train' do
+        expect(AutoMergeProcessWorker).to receive(:perform_async).with(merge_request_2.id)
+
+        subject
+      end
+
+      context 'when process_next is false' do
+        let(:args) { { process_next: false } }
+
+        it 'does not process the next merge request on the train' do
+          expect(AutoMergeProcessWorker).not_to receive(:perform_async)
+
+          subject
+        end
+      end
+    end
   end
 
   describe '#available_for?' do

ee/spec/services/merge_trains/refresh_merge_request_service_spec.rb

@@ -28,7 +28,7 @@ describe MergeTrains::RefreshMergeRequestService do
 
       it do
         expect_next_instance_of(AutoMerge::MergeTrainService) do |service|
-          expect(service).to receive(:abort).with(merge_request, kind_of(String))
+          expect(service).to receive(:abort).with(merge_request, kind_of(String), hash_including(process_next: false))
         end
 
         subject