Demonstrates how Ruby's idiom "my_var ||= MyObject.new" is NOT thread safe, as part of discussion at https://github.com/svenfuchs/i18n/pull/352#pullrequestreview-24816351. The script returns when a race condition is detected.

assign-race-condition.rb

class MyObject
  def initialize
    @data = 'x' * 1_000_000
  end

  def inspect
    object_id
  end
end

# Unsafe function being tested
def init_once
  $init_once_var ||= MyObject.new
end

loop do
  $init_once_var = nil
  $new_objects = []
  $lock = Mutex.new

  threads = (1..10).map do
    Thread.new do
      obj = init_once
      $lock.synchronize do
        $new_objects << obj unless $new_objects.include?(obj)
      end
    end
  end.each { |t| t.join }

  if $new_objects.length != 1
    puts $new_objects.inspect
    break
  end
end