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>

Lib/test/test_threading.py

@@ -531,34 +531,6 @@ class ThreadTests(BaseTestCase):
         t = threading.Thread(daemon=True)
         self.assertTrue(t.daemon)
 
-    @support.requires_fork()
-    def test_fork_at_exit(self):
-        # bpo-42350: Calling os.fork() after threading._shutdown() must
-        # not log an error.
-        code = textwrap.dedent("""
-            import atexit
-            import os
-            import sys
-            from test.support import wait_process
-
-            # Import the threading module to register its "at fork" callback
-            import threading
-
-            def exit_handler():
-                pid = os.fork()
-                if not pid:
-                    print("child process ok", file=sys.stderr, flush=True)
-                    # child process
-                else:
-                    wait_process(pid, exitcode=0)
-
-            # exit_handler() will be called after threading._shutdown()
-            atexit.register(exit_handler)
-        """)
-        _, out, err = assert_python_ok("-c", code)
-        self.assertEqual(out, b'')
-        self.assertEqual(err.rstrip(), b'child process ok')
-
     @support.requires_fork()
     def test_dummy_thread_after_fork(self):
         # Issue #14308: a dummy thread in the active list doesn't mess up
@@ -1048,6 +1020,22 @@ class ThreadTests(BaseTestCase):
         self.assertEqual(out, b'')
         self.assertEqual(err, b'')
 
+    def test_start_new_thread_at_exit(self):
+        code = """if 1:
+            import atexit
+            import _thread
+
+            def f():
+                print("shouldn't be printed")
+
+            def exit_handler():
+                _thread.start_new_thread(f, ())
+
+            atexit.register(exit_handler)
+        """
+        _, out, err = assert_python_ok("-c", code)
+        self.assertEqual(out, b'')
+        self.assertIn(b"can't create new thread at interpreter shutdown", err)
 
 class ThreadJoinOnShutdown(BaseTestCase):