Make VirusScanner available as a service c.f. S3Storage

Now that the VirusScanner class is a stateless service, we can include
it in the standard Services module to bring this app more into line with
other GOV.UK apps.

app/workers/virus_scan_worker.rb

@@ -1,12 +1,11 @@
-require 'virus_scanner'
+require 'services'
 
 class VirusScanWorker
   include Sidekiq::Worker
 
   def perform(asset_id)
     asset = Asset.find(asset_id)
-    scanner = VirusScanner.new
-    scanner.scan(asset.file.path)
+    Services.virus_scanner.scan(asset.file.path)
     asset.scanned_clean
   rescue VirusScanner::InfectedFile => e
     GovukError.notify(e, extra: { id: asset.id, filename: asset.filename })

lib/services.rb

@@ -1,7 +1,12 @@
 require 's3_storage'
+require 'virus_scanner'
 
 module Services
   def self.cloud_storage
     @cloud_storage ||= S3Storage.build
   end
+
+  def self.virus_scanner
+    @virus_scanner ||= VirusScanner.new
+  end
 end

spec/workers/virus_scan_worker_spec.rb

@@ -1,12 +1,13 @@
 require 'rails_helper'
+require 'services'
 
 RSpec.describe VirusScanWorker do
   let(:worker) { described_class.new }
   let(:asset) { FactoryBot.create(:asset) }
   let(:scanner) { instance_double('VirusScanner') }
 
   before do
-    allow(VirusScanner).to receive(:new).and_return(scanner)
+    allow(Services).to receive(:virus_scanner).and_return(scanner)
   end
 
   it "calls out to the VirusScanner to scan the file" do