Add read service and specs

Adjust factory

ee/app/services/incident_management/oncall_shifts/read_service.rb

+# frozen_string_literal: true
+
+module IncidentManagement
+  module OncallShifts
+    class ReadService
+      # @param rotation [IncidentManagement::OncallRotation]
+      # @param current_user [User]
+      # @param params [Hash<Symbol,Any>]
+      # @option params - starts_at [Time]
+      # @option params - ends_at [Time]
+      def initialize(rotation, current_user, starts_at:, ends_at:)
+        @rotation = rotation
+        @current_user = current_user
+        @starts_at = starts_at
+        @ends_at = ends_at
+      end
+
+      def execute
+        return error_no_license unless available?
+        return error_no_permissions unless allowed?
+
+        success(
+          ::IncidentManagement::OncallShiftGenerator
+          .new(rotation)
+          .for_timeframe(starts_at: starts_at, ends_at: ends_at)
+        )
+      end
+
+      private
+
+      attr_reader :rotation, :current_user, :starts_at, :ends_at
+
+      def available?
+        ::Gitlab::IncidentManagement.oncall_schedules_available?(rotation.project)
+      end
+
+      def allowed?
+        Ability.allowed?(current_user, :read_incident_management_oncall_schedule, rotation)
+      end
+
+      def error(message)
+        ServiceResponse.error(message: message)
+      end
+
+      def success(shifts)
+        ServiceResponse.success(payload: { shifts: shifts })
+      end
+
+      def error_no_permissions
+        error(_('You have insufficient permissions to view shifts for this rotation'))
+      end
+
+      def error_no_license
+        error(_('Your license does not support on-call rotations'))
+      end
+    end
+  end
+end

ee/spec/factories/incident_management/oncall_rotations.rb

@@ -7,5 +7,13 @@ FactoryBot.define do
     starts_at { Time.current }
     length { 5 }
     length_unit { :days }
+
+    trait :with_participant do
+      after(:create) do |rotation|
+        user = create(:user)
+        rotation.project.add_reporter(user)
+        create(:incident_management_oncall_participant, rotation: rotation, user: user)
+      end
+    end
   end
 end

ee/spec/services/incident_management/oncall_shifts/read_service_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe ::IncidentManagement::OncallShifts::ReadService do
+  let_it_be_with_refind(:rotation) { create(:incident_management_oncall_rotation, :with_participant) }
+  let_it_be(:project) { rotation.project }
+  let_it_be(:user_with_permissions) { create(:user) }
+  let_it_be(:user_without_permissions) { create(:user) }
+  let_it_be(:current_user) { user_with_permissions }
+
+  let(:params) { { starts_at: rotation.starts_at + 15.minutes, ends_at: rotation.starts_at + 3.weeks } }
+  let(:service) { described_class.new(rotation, current_user, params) }
+
+  before_all do
+    project.add_reporter(user_with_permissions)
+  end
+
+  before do
+    stub_licensed_features(oncall_schedules: true)
+  end
+
+  describe '#execute' do
+    shared_examples 'error response' do |message|
+      it 'has an informative message' do
+        expect(execute).to be_error
+        expect(execute.message).to eq(message)
+      end
+    end
+
+    subject(:execute) { service.execute }
+
+    context 'when the current_user is anonymous' do
+      let(:current_user) { nil }
+
+      it_behaves_like 'error response', 'You have insufficient permissions to view shifts for this rotation'
+    end
+
+    context 'when the current_user does not have permissions to create on-call schedules' do
+      let(:current_user) { user_without_permissions }
+
+      it_behaves_like 'error response', 'You have insufficient permissions to view shifts for this rotation'
+    end
+
+    context 'when feature is not available' do
+      before do
+        stub_licensed_features(oncall_schedules: false)
+      end
+
+      it_behaves_like 'error response', 'Your license does not support on-call rotations'
+    end
+
+    context 'when feature flag is disabled' do
+      before do
+        stub_feature_flags(oncall_schedules_mvc: false)
+      end
+
+      it_behaves_like 'error response', 'Your license does not support on-call rotations'
+    end
+
+    context 'with valid params' do
+      it 'successfully returns a sorted collection of IncidentManagement::OncallShifts' do
+        expect(execute).to be_success
+        shifts = execute.payload[:shifts]
+
+        expect(shifts).to all( be_a(::IncidentManagement::OncallShift) )
+        expect(shifts).to all( be_valid )
+        expect(shifts.sort_by(&:starts_at)).to eq(shifts)
+        expect(shifts.first.starts_at).to be <= params[:starts_at]
+        expect(shifts.last.ends_at).to be >= params[:ends_at]
+      end
+    end
+  end
+end