class Concurrent::Map

def [](key)
  if value = super # non-falsy value is an existing mapping, return it right away
    value
    # re-check is done with get_or_default(key, NULL) instead of a simple !key?(key) in order to avoid a race condition, whereby by the time the current thread gets to the key?(key) call
    # a key => value mapping might have already been created by a different thread (key?(key) would then return true, this elsif branch wouldn't be taken and an incorrect +nil+ value
    # would be returned)
    # note: nil == value check is not technically necessary
  elsif @default_proc && nil == value && NULL == (value = get_or_default(key, NULL))
    @default_proc.call(self, key)
  else
    value
  end
end