1 files changed, 139 insertions, 3 deletions
diff --git a/spec/services/merge_requests/post_merge_service_spec.rb b/spec/services/merge_requests/post_merge_service_spec.rb
index 6523b5a158c..71329905558 100644
--- a/spec/services/merge_requests/post_merge_service_spec.rb
+++ b/spec/services/merge_requests/post_merge_service_spec.rb
@@ -3,9 +3,11 @@
 require 'spec_helper'
 
 RSpec.describe MergeRequests::PostMergeService do
-  let(:user) { create(:user) }
-  let(:merge_request) { create(:merge_request, assignees: [user]) }
-  let(:project) { merge_request.project }
+  include ProjectForksHelper
+
+  let_it_be(:user) { create(:user) }
+  let_it_be(:merge_request, reload: true) { create(:merge_request, assignees: [user]) }
+  let_it_be(:project) { merge_request.project }
 
   subject { described_class.new(project, user).execute(merge_request) }
 
@@ -128,5 +130,139 @@ RSpec.describe MergeRequests::PostMergeService do
         expect(deploy_job.reload.canceled?).to be false
       end
     end
+
+    context 'for a merge request chain' do
+      before do
+        ::MergeRequests::UpdateService
+          .new(project, user, force_remove_source_branch: '1')
+          .execute(merge_request)
+      end
+
+      context 'when there is another MR' do
+        let!(:another_merge_request) do
+          create(:merge_request,
+            source_project: source_project,
+            source_branch: 'my-awesome-feature',
+            target_project: merge_request.source_project,
+            target_branch: merge_request.source_branch
+          )
+        end
+
+        shared_examples 'does not retarget merge request' do
+          it 'another merge request is unchanged' do
+            expect { subject }.not_to change { another_merge_request.reload.target_branch }
+              .from(merge_request.source_branch)
+          end
+        end
+
+        shared_examples 'retargets merge request' do
+          it 'another merge request is retargeted' do
+            expect(SystemNoteService)
+              .to receive(:change_branch).once
+              .with(another_merge_request, another_merge_request.project, user,
+                'target', 'delete',
+                merge_request.source_branch, merge_request.target_branch)
+
+            expect { subject }.to change { another_merge_request.reload.target_branch }
+              .from(merge_request.source_branch)
+              .to(merge_request.target_branch)
+          end
+
+          context 'when FF retarget_merge_requests is disabled' do
+            before do
+              stub_feature_flags(retarget_merge_requests: false)
+            end
+
+            include_examples 'does not retarget merge request'
+          end
+
+          context 'when source branch is to be kept' do
+            before do
+              ::MergeRequests::UpdateService
+                .new(project, user, force_remove_source_branch: false)
+                .execute(merge_request)
+            end
+
+            include_examples 'does not retarget merge request'
+          end
+        end
+
+        context 'in the same project' do
+          let(:source_project) { project }
+
+          it_behaves_like 'retargets merge request'
+
+          context 'and is closed' do
+            before do
+              another_merge_request.close
+            end
+
+            it_behaves_like 'does not retarget merge request'
+          end
+
+          context 'and is merged' do
+            before do
+              another_merge_request.mark_as_merged
+            end
+
+            it_behaves_like 'does not retarget merge request'
+          end
+        end
+
+        context 'in forked project' do
+          let!(:source_project) { fork_project(project) }
+
+          context 'when user has access to source project' do
+            before do
+              source_project.add_developer(user)
+            end
+
+            it_behaves_like 'retargets merge request'
+          end
+
+          context 'when user does not have access to source project' do
+            it_behaves_like 'does not retarget merge request'
+          end
+        end
+
+        context 'and current and another MR is from a fork' do
+          let(:project) { create(:project) }
+          let(:source_project) { fork_project(project) }
+
+          let(:merge_request) do
+            create(:merge_request,
+              source_project: source_project,
+              target_project: project
+            )
+          end
+
+          before do
+            source_project.add_developer(user)
+          end
+
+          it_behaves_like 'does not retarget merge request'
+        end
+      end
+
+      context 'when many merge requests are to be retargeted' do
+        let!(:many_merge_requests) do
+          create_list(:merge_request, 10, :unique_branches,
+            source_project: merge_request.source_project,
+            target_project: merge_request.source_project,
+            target_branch: merge_request.source_branch
+          )
+        end
+
+        it 'retargets only 4 of them' do
+          subject
+
+          expect(many_merge_requests.each(&:reload).pluck(:target_branch).tally)
+            .to eq(
+              merge_request.source_branch => 6,
+              merge_request.target_branch => 4
+            )
+        end
+      end
+    end
   end
 end