test connection pool v.s. thread v.s. `with_connection`

test-conn.rake

# with pool size = 2

def query thread_idx
  p "#{thread_idx}: in thread"
  p "#{thread_idx}: conn id: #{ActiveRecord::Base.connection.object_id}"
  p "#{thread_idx}: #{User.count}"
  puts "#{thread_idx}: finish query"
  sleep 0.5
  puts "#{thread_idx}: end thread"
end

task :test_conn => :environment do
  require 'user'

  threads = 3.times.map do |i|
    Thread.new(i) do |idx|
      # not ok
      # query(idx)

      # ok
      ActiveRecord::Base.connection_pool.with_connection do
        query(idx)
      end
    end
  end
  threads.each(&:join)
  p 'done1'

  th = Thread.new do
    query('main')
  end

  th.join

  p 'ok!'
end

task :test_conn2 => :environment do
  # ref: https://github.com/puma/puma/issues/1004#issuecomment-227242301
  # ref: https://github.com/rails/rails/blob/578f2c87d004a0afb1a2fc74c25dc457288c41ac/activerecord/test/cases/reaper_test.rb#L61-L82
  pool = ActiveRecord::Base.connection_pool

  conn = nil
  child = Thread.new do
    conn = pool.checkout

    Thread.stop
  end
  while conn.nil?
    Thread.pass
  end

  assert conn.in_use?

  child.terminate

  while conn.in_use?
    Thread.pass
  end
  # this line will never be reached
  assert !conn.in_use?
end