gh-113964: Don't prevent new threads until all non-daemon threads exit (#116677)

Starting in Python 3.12, we prevented calling fork() and starting new threads
during interpreter finalization (shutdown). This has led to a number of
regressions and flaky tests. We should not prevent starting new threads
(or `fork()`) until all non-daemon threads exit and finalization starts in
earnest.

This changes the checks to use `_PyInterpreterState_GetFinalizing(interp)`,
which is set immediately before terminating non-daemon threads.

Lib/test/test_os.py

@@ -5357,20 +5357,21 @@ def test_fork_warns_when_non_python_thread_exists(self):
         self.assertEqual(err.decode("utf-8"), "")
         self.assertEqual(out.decode("utf-8"), "")
 
-    def test_fork_at_exit(self):
+    def test_fork_at_finalization(self):
         code = """if 1:
             import atexit
             import os
 
-            def exit_handler():
-                pid = os.fork()
-                if pid != 0:
-                    print("shouldn't be printed")
-
-            atexit.register(exit_handler)
+            class AtFinalization:
+                def __del__(self):
+                    print("OK")
+                    pid = os.fork()
+                    if pid != 0:
+                        print("shouldn't be printed")
+            at_finalization = AtFinalization()
         """
         _, out, err = assert_python_ok("-c", code)
-        self.assertEqual(b"", out)
+        self.assertEqual(b"OK\n", out)
         self.assertIn(b"can't fork at interpreter shutdown", err)
 
 

Lib/test/test_subprocess.py

@@ -3398,14 +3398,15 @@ def test_preexec_at_exit(self):
         def dummy():
             pass
 
-        def exit_handler():
-            subprocess.Popen({ZERO_RETURN_CMD}, preexec_fn=dummy)
-            print("shouldn't be printed")
-
-        atexit.register(exit_handler)
+        class AtFinalization:
+            def __del__(self):
+                print("OK")
+                subprocess.Popen({ZERO_RETURN_CMD}, preexec_fn=dummy)
+                print("shouldn't be printed")
+        at_finalization = AtFinalization()
         """
         _, out, err = assert_python_ok("-c", code)
-        self.assertEqual(out, b'')
+        self.assertEqual(out.strip(), b"OK")
         self.assertIn(b"preexec_fn not supported at interpreter shutdown", err)
 
     @unittest.skipIf(not sysconfig.get_config_var("HAVE_VFORK"),

Lib/test/test_threading.py

@@ -1154,21 +1154,21 @@ def import_threading():
         self.assertEqual(out, b'')
         self.assertEqual(err, b'')
 
-    def test_start_new_thread_at_exit(self):
+    def test_start_new_thread_at_finalization(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)
+            class AtFinalization:
+                def __del__(self):
+                    print("OK")
+                    _thread.start_new_thread(f, ())
+            at_finalization = AtFinalization()
         """
         _, out, err = assert_python_ok("-c", code)
-        self.assertEqual(out, b'')
+        self.assertEqual(out.strip(), b"OK")
         self.assertIn(b"can't create new thread at interpreter shutdown", err)
 
 class ThreadJoinOnShutdown(BaseTestCase):
@@ -1297,6 +1297,30 @@ def main():
         rc, out, err = assert_python_ok('-c', script)
         self.assertFalse(err)
 
+    def test_thread_from_thread(self):
+        script = """if True:
+            import threading
+            import time
+
+            def thread2():
+                time.sleep(0.05)
+                print("OK")
+
+            def thread1():
+                time.sleep(0.05)
+                t2 = threading.Thread(target=thread2)
+                t2.start()
+
+            t = threading.Thread(target=thread1)
+            t.start()
+            # do not join() -- the interpreter waits for non-daemon threads to
+            # finish.
+            """
+        rc, out, err = assert_python_ok('-c', script)
+        self.assertEqual(err, b"")
+        self.assertEqual(out.strip(), b"OK")
+        self.assertEqual(rc, 0)
+
     @skip_unless_reliable_fork
     def test_reinit_tls_after_fork(self):
         # Issue #13817: fork() would deadlock in a multithreaded program with

Misc/NEWS.d/next/Core and Builtins/2024-03-12-20-31-57.gh-issue-113964.bJppzg.rst

@@ -0,0 +1,2 @@
+Starting new threads and process creation through :func:`os.fork` are now
+only prevented once all non-daemon threads exit.

Modules/_posixsubprocess.c

@@ -1031,7 +1031,9 @@ subprocess_fork_exec_impl(PyObject *module, PyObject *process_args,
     Py_ssize_t fds_to_keep_len = PyTuple_GET_SIZE(py_fds_to_keep);
 
     PyInterpreterState *interp = _PyInterpreterState_GET();
-    if ((preexec_fn != Py_None) && interp->finalizing) {
+    if ((preexec_fn != Py_None) &&
+        _PyInterpreterState_GetFinalizing(interp) != NULL)
+    {
         PyErr_SetString(PyExc_PythonFinalizationError,
                         "preexec_fn not supported at interpreter shutdown");
         return NULL;

Modules/_threadmodule.c

@@ -1729,7 +1729,7 @@ do_start_new_thread(thread_module_state *state, PyObject *func, PyObject *args,
                         "thread is not supported for isolated subinterpreters");
         return -1;
     }
-    if (interp->finalizing) {
+    if (_PyInterpreterState_GetFinalizing(interp) != NULL) {
         PyErr_SetString(PyExc_PythonFinalizationError,
                         "can't create new thread at interpreter shutdown");
         return -1;

Modules/posixmodule.c

@@ -7842,7 +7842,7 @@ os_fork1_impl(PyObject *module)
     pid_t pid;
 
     PyInterpreterState *interp = _PyInterpreterState_GET();
-    if (interp->finalizing) {
+    if (_PyInterpreterState_GetFinalizing(interp) != NULL) {
         PyErr_SetString(PyExc_PythonFinalizationError,
                         "can't fork at interpreter shutdown");
         return NULL;
@@ -7886,7 +7886,7 @@ os_fork_impl(PyObject *module)
 {
     pid_t pid;
     PyInterpreterState *interp = _PyInterpreterState_GET();
-    if (interp->finalizing) {
+    if (_PyInterpreterState_GetFinalizing(interp) != NULL) {
         PyErr_SetString(PyExc_PythonFinalizationError,
                         "can't fork at interpreter shutdown");
         return NULL;
@@ -8719,7 +8719,7 @@ os_forkpty_impl(PyObject *module)
     pid_t pid;
 
     PyInterpreterState *interp = _PyInterpreterState_GET();
-    if (interp->finalizing) {
+    if (_PyInterpreterState_GetFinalizing(interp) != NULL) {
         PyErr_SetString(PyExc_PythonFinalizationError,
                         "can't fork at interpreter shutdown");
         return NULL;

Objects/unicodeobject.c

@@ -505,7 +505,7 @@ unicode_check_encoding_errors(const char *encoding, const char *errors)
 
     /* Disable checks during Python finalization. For example, it allows to
        call _PyObject_Dump() during finalization for debugging purpose. */
-    if (interp->finalizing) {
+    if (_PyInterpreterState_GetFinalizing(interp) != NULL) {
         return 0;
     }