Add oncall_users to oncall schedule Graphql type

Changelog: added
EE: true

doc/api/graphql/reference/index.md

@@ -10161,6 +10161,7 @@ Describes an incident management on-call schedule.
 | <a id="incidentmanagementoncallscheduledescription"></a>`description` | [`String`](#string) | Description of the on-call schedule. |
 | <a id="incidentmanagementoncallscheduleiid"></a>`iid` | [`ID!`](#id) | Internal ID of the on-call schedule. |
 | <a id="incidentmanagementoncallschedulename"></a>`name` | [`String!`](#string) | Name of the on-call schedule. |
+| <a id="incidentmanagementoncallscheduleoncallusers"></a>`oncallUsers` | [`[UserCore!]!`](#usercore) |  |
 | <a id="incidentmanagementoncallschedulerotations"></a>`rotations` | [`IncidentManagementOncallRotationConnection!`](#incidentmanagementoncallrotationconnection) | On-call rotations for the on-call schedule. (see [Connections](#connections)) |
 | <a id="incidentmanagementoncallscheduletimezone"></a>`timezone` | [`String!`](#string) | Time zone of the on-call schedule. |
 

ee/app/graphql/resolvers/incident_management/oncall_users_resolver.rb

+# frozen_string_literal: true
+
+module Resolvers
+  module IncidentManagement
+    class OncallUsersResolver < BaseResolver
+      alias_method :schedule, :object
+
+      type [::Types::UserType], null: true
+
+      def resolve
+        ::IncidentManagement::OncallUsersFinder.new(schedule.project, schedule: schedule).execute
+      end
+    end
+  end
+end

ee/app/graphql/types/incident_management/oncall_schedule_type.rb

@@ -38,6 +38,11 @@ module Types
             null: true,
             description: 'On-call rotation for the on-call schedule.',
             resolver: ::Resolvers::IncidentManagement::OncallRotationsResolver.single
+
+      field :oncall_users,
+            [::Types::UserType],
+            null: false,
+            resolver: ::Resolvers::IncidentManagement::OncallUsersResolver
     end
   end
 end

ee/spec/graphql/resolvers/incident_management/oncall_users_resolver_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Resolvers::IncidentManagement::OncallUsersResolver do
+  include GraphqlHelpers
+
+  let_it_be(:current_user) { create(:user) }
+  let_it_be(:schedule) { create(:incident_management_oncall_schedule, :with_rotation, :utc) }
+  let_it_be(:project) { schedule.project }
+
+  let(:args) { {} }
+
+  subject(:users) { sync(resolve_oncall_users(args)) }
+
+  before do
+    stub_licensed_features(oncall_schedules: true)
+    project.add_reporter(current_user)
+  end
+
+  it 'returns on-call users' do
+    expect(users.length).to eq(1)
+    expect(users.first).to be_a(::User)
+    expect(schedule.participants.pluck(:user_id)).to include(users.first.id)
+  end
+
+  context 'when an error occurs while finding shifts' do
+    before do
+      stub_licensed_features(oncall_schedules: false)
+    end
+
+    it 'returns no users' do
+      expect(subject).to eq(::User.none)
+    end
+  end
+
+  private
+
+  def resolve_oncall_users(args = {}, context = { current_user: current_user })
+    resolve(described_class, obj: schedule, args: args, ctx: context)
+  end
+end

ee/spec/graphql/types/incident_management/oncall_schedule_type_spec.rb

@@ -15,6 +15,7 @@ RSpec.describe GitlabSchema.types['IncidentManagementOncallSchedule'] do
       timezone
       rotations
       rotation
+      oncallUsers
     ]
 
     expect(described_class).to have_graphql_fields(*expected_fields)