Steal stages statuses migration

db/migrate/20170912113435_clean_stages_statuses_migration.rb

+class CleanStagesStatusesMigration < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    Gitlab::BackgroundMigration.steal('MigrateStageStatus')
+  end
+
+  def down
+    # noop
+  end
+end

spec/migrations/clean_stages_statuses_migration_spec.rb

+require 'spec_helper'
+require Rails.root.join('db', 'migrate', '20170912113435_clean_stages_statuses_migration.rb')
+
+describe CleanStagesStatusesMigration, :migration, :sidekiq, :redis do
+  let(:migration) { spy('migration') }
+
+  before do
+    allow(Gitlab::BackgroundMigration::MigrateStageStatus)
+      .to receive(:new).and_return(migration)
+  end
+
+  context 'when there are pending background migrations' do
+    it 'processes pending jobs synchronously' do
+      Sidekiq::Testing.disable! do
+        BackgroundMigrationWorker
+          .perform_in(2.minutes, 'MigrateStageStatus', [1, 1])
+        BackgroundMigrationWorker
+          .perform_async('MigrateStageStatus', [1, 1])
+
+        migrate!
+
+        expect(migration).to have_received(:perform).with(1, 1).twice
+      end
+    end
+  end
+  context 'when there are no background migrations pending' do
+    it 'does nothing' do
+      Sidekiq::Testing.disable! do
+        migrate!
+
+        expect(migration).not_to have_received(:perform)
+      end
+    end
+  end
+end