Constrain VirusScanWorker to a single job per asset

This will prevent multiple jobs becoming enqueued for a single asset
until the original job has finished executing.

We are currently seeing an issue where a user can replace the file after
uploading but before the virus scan has completed. This results in a
second virus scan being enqueued, which subsequently fails as the asset
has been deleted from the filesystem. This will prevent that from
happening.

app/workers/virus_scan_worker.rb

@@ -3,6 +3,8 @@
 class VirusScanWorker
   include Sidekiq::Worker
 
+  sidekiq_options lock: :until_and_while_executing
+
   def perform(asset_id)
     asset = Asset.find(asset_id)
     if asset.unscanned?

spec/workers/virus_scan_worker_spec.rb

@@ -1,5 +1,6 @@
 require "rails_helper"
 require "services"
+require "sidekiq_unique_jobs/testing"
 
 RSpec.describe VirusScanWorker do
   let(:worker) { described_class.new }
@@ -10,6 +11,15 @@
     allow(Services).to receive(:virus_scanner).and_return(scanner)
   end
 
+  specify { expect(described_class).to have_valid_sidekiq_options }
+
+  it "does not permit multiple jobs to be enqueued for the same asset" do
+    SidekiqUniqueJobs.use_config(enabled: true) do
+      expect { described_class.perform_in(1.minute, asset.id.to_s) }.to enqueue_sidekiq_job(described_class)
+      expect { described_class.perform_in(1.minute, asset.id.to_s) }.not_to enqueue_sidekiq_job(described_class)
+    end
+  end
+
   it "calls out to the VirusScanner to scan the file" do
     expect(scanner).to receive(:scan).with(asset.file.path)