test_transaction_deadlock.rb

require 'action_controller/railtie'
require 'active_record/railtie'

require 'active_record'

ActiveRecord::Base.logger = Logger.new(STDOUT)

class TestApp < Rails::Application
  config.root = File.dirname(__FILE__)
  config.session_store :cookie_store, key: 'cookie_store_key'
  secrets.secret_token    = 'secret_token'
  secrets.secret_key_base = 'secret_key_base'

  config.logger = Logger.new($stdout)
  Rails.logger  = config.logger

  routes.draw do
    get '/' => 'test#index'
  end
end

class Post < ActiveRecord::Base
  has_many :comments
end

class TestController < ActionController::Base
  include Rails.application.routes.url_helpers

  def index
    Post.create!
    render plain: 'Home'
  end
end

TestApp.configure do
  config.eager_load = true
  def config.database_configuration
    {'development' => {'adapter' => 'postgresql', 'pool' => 4, 'host' => 'localhost', 'username' => ENV["USER"], 'database' => "test_tran_deadlock"}}
  end
end

TestApp.initialize!

ActiveRecord::Tasks::DatabaseTasks.create_current

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
  end
end

require 'minitest/autorun'
require 'rack/test'

class BugTest < ActionDispatch::IntegrationTest

  def test_returns_success
    q,r = Queue.new,Queue.new

    Post.create!
    ActiveRecord::Base.transaction do
      Post.destroy_all
      puts "[main] pid: #{ActiveRecord::Base.connection.instance_variable_get("@connection").backend_pid}"
      get '/'
      # db connection placed back to pool with open transaction
      assert_response :success #last_response.ok?
      # start a new thread grabing connection
      th = Thread.new do
        conn = ActiveRecord::Base.connection
        q.push true
        puts "[thread] PID: #{conn.instance_variable_get("@connection").backend_pid}"
        r.pop
      end
      q.pop
      puts "[main] PID: #{ActiveRecord::Base.connection.instance_variable_get("@connection").backend_pid}"
      r.push true
      Post.destroy_all
      th.join
    end
  end

  private
    def app
      Rails.application
    end
end