Mailchimp integration with locking

gistfile1.rb

 def sync
    loop do
      User.transaction do
        @supporter_batch = supporter_batch_relation.lock
        @supporters = @supporter_batch.to_a
        break if @supporters.size <= MIN_BATCH_SIZE
        @supporter_batch.update_all(mail_service_identifier: 'pending')
      end
      @supporters.map do |supporter|
        sync_supporter(supporter)
      end
    end
  rescue Exception => e
    User.where(id: @supporters.map(&:id), mail_service_identifier: 'pending')
      .update_all(mail_service_identifier: nil)
    raise e
  end

  def supporter_batch_relation
    User.where(mail_service_identifier: nil).limit(BATCH_SIZE)
  end

  def sync_supporter(supporter)
    supporter.language ||= supporter.movement.default_language
    result = call_api(:subscribe, { email: { email: supporter.email },
                                      merge_vars: {
                                        COUNTRY: supporter.language.iso_code,
                                        SOURCE: 'Purpose'},
                                        double_optin: false,
                                        update_existing: true})
    supporter.mail_service_identifier = result['euid']
    supporter.save!
    result
  rescue Gibbon::MailChimpError => e
    if e.message.match(/(Signup disabled|invalid)/)
      supporter.update_attribute(:mail_service_identifier, 'invalid')
    else
      Honeybadger.notify(e)
    end
  end


    fit 'locks pending supporters to avoid deadlocks', type: :concurrent do
        expect(User.where(mail_service_identifier: nil).count).to eq(3)
        ActiveRecord::Base.connection.disconnect!
        pid = Process.fork do
          ActiveRecord::Base.establish_connection
          Apartment::Tenant.switch 'test'
          original_update = User::ActiveRecord_Relation.instance_method(:update_all)
          expect_any_instance_of(User::ActiveRecord_Relation)
            .to receive(:update_all) do |relation, conditions|
              puts 'first'
              sleep 2
              original_update.bind(relation).call(conditions)
            end
          mailchimp.sync
        end
        sleep 1
        puts 'second'
        ActiveRecord::Base.establish_connection
        Apartment::Tenant.switch 'test'
        expect(User.update_all(mail_service_identifier: 'pending')).to eq(1)
        sleep 5
      end