Testing concurrency with Thread.new in RSpec

Question

I'm trying to set up a test around concurrency. The end goal is to test that a service using ActiveRecord skips locked records in PostgreSQL.

This works great in two consoles:

# in console 1
queue = MyFancyQueue.first
item_1 = queue.items.first
item_1.with_lock { sleep 30 } # locks item_1 for 30 seconds

# in console 2, while item_1 is locked
queue = MyFancyQueue.first
queue.items.first # => item_1
queue.items.lock('FOR UPDATE SKIP LOCKED').first # => item_2

However, with RSpec, any code I run in a Thread.new can't find any record, something like:

let(:queue) { MyFancyQueue.create }
let!(:items) { create_list(:item, 2, queue: queue) }

context 'with first item locked' do
  it 'returns the second item' do
    Thread.new { queue.items.first.with_lock { sleep 30 } }

    expect(queue.items.lock('FOR UPDATE SKIP LOCKED').first).to eq items.last
  end
end

would throw an error saying queue.items.first can't be found, and when using pry to look into the thread, I can't see any items.

How do I effectively test something like this with RSpec?

Answer 1

If you use DatabaseCleaner make sure DatabaseCleaner.strategy = :truncation for this test.