Check participant limit in service

ee/app/services/incident_management/oncall_rotations/create_service.rb

@@ -3,6 +3,8 @@
 module IncidentManagement
   module OncallRotations
     class CreateService
+      MAXIMUM_PARTICIPANTS = 100
+
       # @param schedule [IncidentManagement::OncallSchedule]
       # @param project [Project]
       # @param current_user [User]
@@ -26,15 +28,19 @@ module IncidentManagement
         return error_no_license unless available?
         return error_no_permissions unless allowed?
 
+        participant_params = Array(params[:participants])
+
+        return error_too_many_participants if participant_params.size > MAXIMUM_PARTICIPANTS
+
         oncall_rotation = schedule.rotations.create(params.except(:participants))
 
         return error_in_create(oncall_rotation) unless oncall_rotation.persisted?
 
-        new_participants = Array(params[:participants]).map do |participant|
+        new_participants = Array(participant_params).map do |participant|
           OncallParticipant.new(
             rotation: oncall_rotation,
             user: participant[:user],
-            color_palette:  participant[:color_palette],
+            color_palette: participant[:color_palette],
             color_weight: participant[:color_weight]
           )
         end
@@ -64,6 +70,10 @@ module IncidentManagement
         ServiceResponse.success(payload: { oncall_rotation: oncall_rotation })
       end
 
+      def error_too_many_participants
+        error("A maximum of #{MAXIMUM_PARTICIPANTS} participants can be added")
+      end
+
       def error_no_permissions
         error('You have insufficient permissions to create an on-call rotation for this project')
       end

ee/spec/services/incident_management/oncall_rotations/create_service_spec.rb

@@ -74,6 +74,17 @@ RSpec.describe IncidentManagement::OncallRotations::CreateService do
       it_behaves_like 'error response', 'Name has already been taken'
     end
 
+    context 'when too many participants' do
+      before do
+        stub_const('IncidentManagement::OncallRotations::CreateService::MAXIMUM_PARTICIPANTS', 0)
+      end
+
+      it 'has an informative error message' do
+        expect(execute).to be_error
+        expect(execute.message).to eq("A maximum of #{IncidentManagement::OncallRotations::CreateService::MAXIMUM_PARTICIPANTS} participants can be added")
+      end
+    end
+
     context 'with valid params' do
       it 'successfully creates an on-call rotation' do
         expect(execute).to be_success