[WIP] bpo-42671: Make Python finalization deterministic by vstinner · Pull Request #23826 · python/cpython

Make the Python finalization more deterministic:

* First, clear the __main__ module.
* Then, clear modules from the most recently imported to the least
  recently imported: reversed(sys.modules).
* builtins and sys modumes are always cleared last.
* Module attributes are set to None from the most recently defined to the
  least recently defined: reversed(module.__dict__).

Changes:

* finalize_modules() no longer uses a list of weak references to
  modules while clearing sys.modules dict.
* When -vv command line option is used, the module name is now also
  logged, not only the attribute name.
* test_module.test_module_finalization_at_shutdown(): final_a.x is
  now None when final_a.c is cleared.
* test_sys.test_sys_ignores_cleaning_up_user_data(): the exception is
  no longer silently ignored. Rename the test to
  test_sys_cleaning_up_user_data().
* test_threading.test_main_thread_during_shutdown() keeps a reference
  to threading functions since threading module variables are cleared
  before RefCycle object is deleted by the garbage collector.