Always set a _MainThread as a main thread after os.fork() is called from
a thread started not by the threading module.
A new _MainThread was already set as a new main thread after fork if
threading.current_thread() was not called for a foreign thread before fork.
Now, if it was called before fork, the implicitly created _DummyThread will
be turned into _MainThread after fork.
It fixes, in particularly, an incompatibility of _DummyThread with
the threading shutdown logic which relies on the main thread
having tstate_lock.
(cherry picked from commit 49785b06de)
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Miss Islington (bot)
1bd2c93474