Allow multiple reviewers on Starter+

- One reviewer is allowed on CE
- Multiple reviewer is allowed on Starter and up

app/models/merge_request.rb

@@ -1690,6 +1690,10 @@ class MergeRequest < ApplicationRecord
     Feature.enabled?(:merge_request_reviewers, project)
   end
 
+  def allows_multiple_reviewers?
+    false
+  end
+
   private
 
   def with_rebase_lock

app/services/merge_requests/base_service.rb

@@ -110,6 +110,10 @@ module MergeRequests
         return
       end
 
+      unless merge_request.allows_multiple_reviewers?
+        params[:reviewer_ids] = params[:reviewer_ids].first(1)
+      end
+
       reviewer_ids = params[:reviewer_ids].select { |reviewer_id| user_can_read?(merge_request, reviewer_id) }
 
       if params[:reviewer_ids].map(&:to_s) == [IssuableFinder::Params::NONE]

ee/app/models/ee/merge_request.rb

@@ -120,6 +120,10 @@ module EE
       project.feature_available?(:multiple_merge_request_assignees)
     end
 
+    def allows_multiple_reviewers?
+      project.feature_available?(:multiple_merge_request_reviewers)
+    end
+
     def visible_blocking_merge_requests(user)
       Ability.merge_requests_readable_by_user(blocking_merge_requests, user)
     end

ee/app/models/license.rb

@@ -29,6 +29,7 @@ class License < ApplicationRecord
     multiple_issue_assignees
     multiple_ldap_servers
     multiple_merge_request_assignees
+    multiple_merge_request_reviewers
     project_merge_request_analytics
     protected_refs_for_users
     push_rules

ee/spec/models/merge_request_spec.rb

@@ -118,6 +118,24 @@ RSpec.describe MergeRequest do
     end
   end
 
+  describe '#allows_multiple_reviewers?' do
+    it 'returns false without license' do
+      stub_licensed_features(multiple_merge_request_reviewers: false)
+
+      merge_request = build_stubbed(:merge_request)
+
+      expect(merge_request.allows_multiple_reviewers?).to be(false)
+    end
+
+    it 'returns true when licensed' do
+      stub_licensed_features(multiple_merge_request_reviewers: true)
+
+      merge_request = build(:merge_request)
+
+      expect(merge_request.allows_multiple_reviewers?).to be(true)
+    end
+  end
+
   describe '#participants' do
     context 'with approval rule' do
       before do

ee/spec/services/ee/merge_requests/create_service_spec.rb

@@ -51,6 +51,10 @@ RSpec.describe MergeRequests::CreateService do
     it_behaves_like 'new issuable with scoped labels' do
       let(:parent) { project }
     end
+
+    it_behaves_like 'service with multiple reviewers' do
+      let(:execute) { service.execute }
+    end
   end
 
   describe '#execute with blocking merge requests', :clean_gitlab_redis_shared_state do

ee/spec/services/ee/merge_requests/update_service_spec.rb

@@ -36,6 +36,11 @@ RSpec.describe MergeRequests::UpdateService, :mailer do
       let(:parent) { project }
     end
 
+    it_behaves_like 'service with multiple reviewers' do
+      let(:opts) { {} }
+      let(:execute) { update_merge_request(opts) }
+    end
+
     def update_merge_request(opts)
       described_class.new(project, user, opts).execute(merge_request)
     end

ee/spec/support/shared_examples/services/merge_merge_requests_shared_examples.rb

@@ -96,3 +96,38 @@ RSpec.shared_examples 'merge validation hooks' do |args|
     end
   end
 end
+
+RSpec.shared_examples 'service with multiple reviewers' do
+  context 'with multiple reviewer assignments' do
+    let(:opts) { super().merge(reviewer_ids_param) }
+    let(:reviewer_ids_param) { { reviewer_ids: [reviewer1.id, reviewer2.id] } }
+    let(:reviewer1) { create(:user) }
+    let(:reviewer2) { create(:user) }
+
+    before do
+      stub_feature_flags(merge_request_reviewer: true)
+      project.add_developer(reviewer1)
+      project.add_developer(reviewer2)
+    end
+
+    context 'with multiple_merge_request_reviewers feature on' do
+      before do
+        stub_licensed_features(multiple_merge_request_reviewers: true)
+      end
+
+      it 'allows multiple reviewers' do
+        expect(execute.reviewers).to contain_exactly(reviewer1, reviewer2)
+      end
+    end
+
+    context 'with multiple_merge_request_reviewers feature off' do
+      before do
+        stub_licensed_features(multiple_merge_request_reviewers: false)
+      end
+
+      it 'only allows one reviewer' do
+        expect(execute.reviewers).to contain_exactly(reviewer1)
+      end
+    end
+  end
+end

spec/support/shared_examples/services/merge_request_shared_examples.rb

@@ -13,11 +13,10 @@ RSpec.shared_examples 'reviewer_ids filter' do
     end
 
     context 'with reviewer_ids' do
-      let(:reviewer_ids_param) { { reviewer_ids: [reviewer1.id, reviewer2.id, reviewer3.id] } }
+      let(:reviewer_ids_param) { { reviewer_ids: [reviewer1.id, reviewer2.id] } }
 
       let(:reviewer1) { create(:user) }
       let(:reviewer2) { create(:user) }
-      let(:reviewer3) { create(:user) }
 
       context 'when the current user can admin the merge_request' do
         context 'when merge_request_reviewer feature is enabled' do
@@ -25,14 +24,13 @@ RSpec.shared_examples 'reviewer_ids filter' do
             stub_feature_flags(merge_request_reviewer: true)
           end
 
-          context 'with reviewers who can read the merge_request' do
+          context 'with a reviewer who can read the merge_request' do
             before do
               project.add_developer(reviewer1)
-              project.add_developer(reviewer2)
             end
 
             it 'contains reviewers who can read the merge_request' do
-              expect(execute.reviewers).to contain_exactly(reviewer1, reviewer2)
+              expect(execute.reviewers).to contain_exactly(reviewer1)
             end
           end
         end