Mark ScheduleAggregationWorker as idempotent

This marks the Namespaces::ScheduleAggregationWorker as
idempotent. It was already idempotent, because it checks the existence
of the Namespace::AggregationSchedule before doing anything and it
uses `safe_find_or_create` to avoid race conditions.

This adds specs proving that it is idempotent.

It also adds the worker to the DROPPABLE_QUEUES so we can use it to
try out deduplicating jobs.

app/workers/all_queues.yml

@@ -919,7 +919,7 @@
   :urgency: :low
   :resource_boundary: :unknown
   :weight: 1
-  :idempotent: 
+  :idempotent: true
 - :name: authorized_keys
   :feature_category: :source_code_management
   :has_external_dependencies: 

app/workers/namespaces/schedule_aggregation_worker.rb

 # frozen_string_literal: true
 
 module Namespaces
-  class ScheduleAggregationWorker # rubocop:disable Scalability/IdempotentWorker
+  class ScheduleAggregationWorker
     include ApplicationWorker
 
     queue_namespace :update_namespace_statistics
     feature_category :source_code_management
+    idempotent!
 
     def perform(namespace_id)
       return unless aggregation_schedules_table_exists?

lib/gitlab/sidekiq_middleware/duplicate_jobs.rb

@@ -6,7 +6,8 @@ module Gitlab
   module SidekiqMiddleware
     module DuplicateJobs
       DROPPABLE_QUEUES = Set.new([
-        Namespaces::RootStatisticsWorker.queue
+        Namespaces::RootStatisticsWorker.queue,
+        Namespaces::ScheduleAggregationWorker.queue
       ]).freeze
 
       def self.drop_duplicates?(queue_name)

spec/workers/namespaces/schedule_aggregation_worker_spec.rb

@@ -54,6 +54,17 @@ describe Namespaces::ScheduleAggregationWorker, '#perform', :clean_gitlab_redis_
     end
   end
 
+  it_behaves_like 'an idempotent worker' do
+    let(:job_args) { [group.id] }
+
+    it 'creates a single aggregation schedule' do
+      expect { worker.perform(*job_args) }
+        .to change { Namespace::AggregationSchedule.count }.by(1)
+      expect { worker.perform(*job_args) }
+        .not_to change { Namespace::AggregationSchedule.count }
+    end
+  end
+
   def stub_aggregation_schedule_statistics
     # Namespace::Aggregations are deleted by
     # Namespace::AggregationSchedule::schedule_root_storage_statistics,