gh-104690 Disallow thread creation and fork at interpreter finalization (#104826)

Disallow thread creation and fork at interpreter finalization.

in the following functions, check if interpreter is finalizing and raise `RuntimeError` with appropriate message:
* `_thread.start_new_thread` and thus `threading`
* `posix.fork`
* `posix.fork1`
* `posix.forkpty`
* `_posixsubprocess.fork_exec` when a `preexec_fn=` is supplied.

---------

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>

1 files changed, 10 insertions, 0 deletions

diff --git a/Doc/library/atexit.rst b/Doc/library/atexit.rst
index a2bd85b..3dbef69 100644
--- a/Doc/library/atexit.rst
+++ b/Doc/library/atexit.rst
@@ -48,6 +48,16 @@ a cleanup function is undefined.
    This function returns *func*, which makes it possible to use it as a
    decorator.
 
+   .. warning::
+       Starting new threads or calling :func:`os.fork` from a registered
+       function can lead to race condition between the main Python
+       runtime thread freeing thread states while internal :mod:`threading`
+       routines or the new process try to use that state. This can lead to
+       crashes rather than clean shutdown.
+
+   .. versionchanged:: 3.12
+       Attempts to start a new thread or :func:`os.fork` a new process
+       in a registered function now leads to :exc:`RuntimeError`.
 
 .. function:: unregister(func)