Allow MR to finalize the approval state when merged

First the project rule is cloned if not overwritten in the MR.
The MR’s rule to approval mapping is then persisted.

ee/app/models/ee/merge_request.rb

@@ -81,5 +81,23 @@ module EE
 
       compare_reports(::Ci::CompareLicenseManagementReportsService)
     end
+
+    def finalize_approvals
+      return unless merged?
+
+      copy_project_approval_rules unless approval_rules.regular.exists?
+
+      approval_rules.each(&:sync_approvals)
+    end
+
+    private
+
+    def copy_project_approval_rules
+      target_project.approval_rules.each do |project_rule|
+        rule = approval_rules.create!(project_rule.attributes.slice('approvals_required', 'name'))
+        rule.users = project_rule.users
+        rule.groups = project_rule.groups
+      end
+    end
   end
 end

ee/spec/models/merge_request_spec.rb

@@ -37,6 +37,105 @@ describe MergeRequest do
     end
   end
 
+  describe '#finalize_approvals' do
+    let!(:member1) { create(:user) }
+    let!(:member2) { create(:user) }
+    let!(:member3) { create(:user) }
+    let!(:group1) { create(:group) }
+    let!(:group2) { create(:group) }
+    let!(:group1_member) { create(:user) }
+    let!(:group2_member) { create(:user) }
+    let!(:approval1) { create(:approval, merge_request: subject, user: member1) }
+    let!(:approval2) { create(:approval, merge_request: subject, user: member3) }
+    let!(:approval3) { create(:approval, merge_request: subject, user: group1_member) }
+    let!(:approval4) { create(:approval, merge_request: subject, user: group2_member) }
+
+    before do
+      group1.add_guest(group1_member)
+      group2.add_guest(group2_member)
+
+      rule = create(:approval_project_rule, project: project, name: 'foo', approvals_required: 12)
+
+      rule.users = [member1, member2]
+      rule.groups << group1
+    end
+
+    shared_examples 'skipping when unmerged' do
+      it 'does nothing if unmerged' do
+        expect do
+          subject.finalize_approvals
+        end.not_to change { ApprovalMergeRequestRule.count }
+
+        expect(approval1.approval_rules).to be_empty
+        expect(approval2.approval_rules).to be_empty
+        expect(approval3.approval_rules).to be_empty
+        expect(approval4.approval_rules).to be_empty
+      end
+    end
+
+    context 'when project rule is not overwritten' do
+      it_behaves_like 'skipping when unmerged'
+
+      context 'when merged' do
+        subject(:merge_request) { create(:merged_merge_request, source_project: project, target_project: project) }
+
+        before do
+          subject.approval_rules.code_owner.create(name: 'Code Owner')
+        end
+
+        it 'copies project rules to MR' do
+          expect do
+            subject.finalize_approvals
+          end.to change { ApprovalMergeRequestRule.count }.by(1)
+
+          rule = subject.approval_rules.regular.first
+
+          expect(rule.name).to eq('foo')
+          expect(rule.approvals_required).to eq(12)
+          expect(rule.users).to contain_exactly(member1, member2)
+          expect(rule.groups).to contain_exactly(group1)
+          expect(approval1.approval_rules).to contain_exactly(rule)
+          expect(approval2.approval_rules).to be_empty
+          expect(approval3.approval_rules).to contain_exactly(rule)
+          expect(approval4.approval_rules).to be_empty
+        end
+      end
+    end
+
+    context 'when project rule is overwritten' do
+      before do
+        rule = create(:approval_merge_request_rule, merge_request: subject, name: 'bar', approvals_required: 32)
+        rule.users = [member2, member3]
+        rule.groups << group2
+      end
+
+      it_behaves_like 'skipping when unmerged'
+
+      context 'when merged' do
+        subject(:merge_request) { create(:merged_merge_request, source_project: project, target_project: project) }
+
+        it 'does not copy project rules, and updates approval mapping with MR rules' do
+          expect(subject).not_to receive(:copy_project_approval_rules)
+
+          expect do
+            subject.finalize_approvals
+          end.not_to change { ApprovalMergeRequestRule.count }
+
+          rule = subject.approval_rules.regular.first
+
+          expect(rule.name).to eq('bar')
+          expect(rule.approvals_required).to eq(32)
+          expect(rule.users).to contain_exactly(member2, member3)
+          expect(rule.groups).to contain_exactly(group2)
+          expect(approval1.approval_rules).to be_empty
+          expect(approval2.approval_rules).to contain_exactly(rule)
+          expect(approval3.approval_rules).to be_empty
+          expect(approval4.approval_rules).to contain_exactly(rule)
+        end
+      end
+    end
+  end
+
   describe '#code_owners' do
     subject(:merge_request) { build(:merge_request) }
     let(:owners) { [double(:owner)] }