Handle rotation start time within active period

- Add specs
- Add comment for upcoming feature

ee/app/graphql/mutations/incident_management/oncall_rotation/create.rb

@@ -122,6 +122,8 @@ module Mutations
           parsed_from = Time.parse(start_time)
           parsed_to = Time.parse(end_time)
 
+          # Overnight shift times will be supported via
+          # https://gitlab.com/gitlab-org/gitlab/-/issues/322079
           if parsed_to < parsed_from
             raise ::Gitlab::Graphql::Errors::ArgumentError, "'start_time' time must be before 'end_time' time"
           end

ee/lib/incident_management/oncall_shift_generator.rb

@@ -132,7 +132,7 @@ module IncidentManagement
 
           starts_at, ends_at = rotation.active_period.for_date(shift_cycle_starts_at + shift_count.days)
 
-          shift_for(participant, starts_at, limit_end_time(ends_at))
+          shift_for(participant, [rotation.starts_at, starts_at].max, limit_end_time(ends_at))
         end
       else
         # This is the normal shift start/end times

ee/spec/lib/incident_management/oncall_shift_generator_spec.rb

@@ -146,6 +146,25 @@ RSpec.describe IncidentManagement::OncallShiftGenerator do
              [:participant2, '2020-12-20 08:00:00 UTC', '2020-12-20 17:00:00 UTC'],
              [:participant2, '2020-12-21 08:00:00 UTC', '2020-12-21 17:00:00 UTC']]
         end
+
+        context 'rotation start time is in middle of active period' do
+          before do
+            rotation.update!(starts_at: rotation_start_time.change(hour: 10))
+          end
+
+          it_behaves_like 'unsaved shifts',
+            '5 shifts for each participant split by each day',
+            [[:participant1, '2020-12-08 10:00:00 UTC', '2020-12-08 17:00:00 UTC'],
+             [:participant1, '2020-12-09 08:00:00 UTC', '2020-12-09 17:00:00 UTC'],
+             [:participant1, '2020-12-10 08:00:00 UTC', '2020-12-10 17:00:00 UTC'],
+             [:participant1, '2020-12-11 08:00:00 UTC', '2020-12-11 17:00:00 UTC'],
+             [:participant1, '2020-12-12 08:00:00 UTC', '2020-12-12 17:00:00 UTC'],
+             [:participant2, '2020-12-13 08:00:00 UTC', '2020-12-13 17:00:00 UTC'],
+             [:participant2, '2020-12-14 08:00:00 UTC', '2020-12-14 17:00:00 UTC'],
+             [:participant2, '2020-12-15 08:00:00 UTC', '2020-12-15 17:00:00 UTC'],
+             [:participant2, '2020-12-16 08:00:00 UTC', '2020-12-16 17:00:00 UTC'],
+             [:participant2, '2020-12-17 08:00:00 UTC', '2020-12-17 17:00:00 UTC']]
+        end
       end
 
       context 'when end time is earlier than start time' do