foo.rb

@cached_value = nil
@value_cached = false
@mutex = Mutex.new

def expensive_calculation
  sleep 5

  42
end

def foo
  return @cached_value if @value_cached

  @mutex.synchronize do
    return @cached_value if @value_cached

    @cached_value = expensive_calculation
    @value_cached = true

    return @cached_value
  end
end