> The function must take care of fork() in worker threads, too. The
> isinstance(current_thread(), _MainThread) trick may not work.

Well, there are two possibilities:
- main_thread() returns the original _MainThread instance, even if it's
  dead in the child process
- main_thread() returns the main thread of the current process

Both are reasonable, but we must settle for one :-)

(also, the use case of forking from a thread is really obscure, I don't
think we should worry too much about it)