Make ProjectCacheWorker idempotent

The job can be run multiple times with the same params without
side effects

app/workers/all_queues.yml

@@ -1820,7 +1820,7 @@
   :urgency: :high
   :resource_boundary: :unknown
   :weight: 1
-  :idempotent:
+  :idempotent: true
   :tags: []
 - :name: project_daily_statistics
   :feature_category: :source_code_management

app/workers/project_cache_worker.rb

 # frozen_string_literal: true
 
 # Worker for updating any project specific caches.
-class ProjectCacheWorker # rubocop:disable Scalability/IdempotentWorker
+class ProjectCacheWorker
   include ApplicationWorker
 
   LEASE_TIMEOUT = 15.minutes.to_i
@@ -9,6 +9,7 @@ class ProjectCacheWorker # rubocop:disable Scalability/IdempotentWorker
   feature_category :source_code_management
   urgency :high
   loggable_arguments 1, 2, 3
+  idempotent!
 
   # project_id - The ID of the project for which to flush the cache.
   # files - An Array containing extra types of files to refresh such as

ee/spec/workers/project_cache_worker_spec.rb

@@ -3,8 +3,9 @@
 require 'spec_helper'
 
 RSpec.describe ProjectCacheWorker do
+  let_it_be(:project) { create(:project, :repository) }
+
   let(:worker) { described_class.new }
-  let(:project) { create(:project, :repository) }
 
   describe '#perform' do
     context 'with an existing project' do
@@ -23,6 +24,10 @@ RSpec.describe ProjectCacheWorker do
 
           worker.perform(project.id, %w(readme))
         end
+
+        it 'is idempotent' do
+          expect { perform_multiple([project.id, %w(readme)]) }.not_to raise_error
+        end
       end
     end
   end

spec/workers/project_cache_worker_spec.rb

@@ -5,8 +5,9 @@ require 'spec_helper'
 RSpec.describe ProjectCacheWorker do
   include ExclusiveLeaseHelpers
 
+  let_it_be(:project) { create(:project, :repository) }
+
   let(:worker) { described_class.new }
-  let(:project) { create(:project, :repository) }
   let(:lease_key) { ["project_cache_worker", project.id, *statistics.sort].join(":") }
   let(:lease_timeout) { ProjectCacheWorker::LEASE_TIMEOUT }
   let(:statistics) { [] }
@@ -126,4 +127,15 @@ RSpec.describe ProjectCacheWorker do
       end
     end
   end
+
+  it_behaves_like 'an idempotent worker' do
+    let(:job_args) { [project.id, %w(readme), %w(repository_size)] }
+
+    it 'calls Projects::UpdateStatisticsService service twice', :clean_gitlab_redis_shared_state do
+      expect(Projects::UpdateStatisticsService).to receive(:new).once.and_return(double(execute: true))
+      expect(UpdateProjectStatisticsWorker).to receive(:perform_in).once
+
+      subject
+    end
+  end
 end