gh-136003: Execute pre-finalization callbacks in a loop

Include/cpython/pystate.h

@@ -107,6 +107,7 @@ struct _ts {
 #  define _PyThreadState_WHENCE_THREADING 3
 #  define _PyThreadState_WHENCE_GILSTATE 4
 #  define _PyThreadState_WHENCE_EXEC 5
+#  define _PyThreadState_WHENCE_THREADING_DAEMON 6
 #endif
 
     /* Currently holds the GIL. Must be its own field to avoid data races */

Lib/test/test_atexit.py

@@ -79,6 +79,62 @@ def thready():
         # want them to affect the rest of the tests.
         script_helper.assert_python_ok("-c", textwrap.dedent(source))
 
+    @threading_helper.requires_working_threading()
+    def test_thread_created_in_atexit(self):
+        source =  """if True:
+        import atexit
+        import threading
+        import time
+
+
+        def run():
+            print(24)
+            time.sleep(1)
+            print(42)
+
+        @atexit.register
+        def start_thread():
+            threading.Thread(target=run).start()
+        """
+        return_code, stdout, stderr = script_helper.assert_python_ok("-c", source)
+        self.assertEqual(return_code, 0)
+        self.assertEqual(stdout, f"24{os.linesep}42{os.linesep}".encode("utf-8"))
+        self.assertEqual(stderr, b"")
+
+    @threading_helper.requires_working_threading()
+    @unittest.skipUnless(hasattr(os, "pipe"), "requires os.pipe()")
+    def test_thread_created_in_atexit_subinterpreter(self):
+        try:
+            from concurrent import interpreters
+        except ImportError:
+            self.skipTest("subinterpreters are not available")
+
+        read, write = os.pipe()
+        source = f"""if True:
+        import atexit
+        import threading
+        import time
+        import os
+
+        def run():
+            os.write({write}, b'spanish')
+            time.sleep(1)
+            os.write({write}, b'inquisition')
+
+        @atexit.register
+        def start_thread():
+            threading.Thread(target=run).start()
+        """
+        interp = interpreters.create()
+        try:
+            interp.exec(source)
+
+            # Close the interpreter to invoke atexit callbacks
+            interp.close()
+            self.assertEqual(os.read(read, 100), b"spanishinquisition")
+        finally:
+            os.close(read)
+            os.close(write)
 
 @support.cpython_only
 class SubinterpreterTest(unittest.TestCase):

Lib/test/test_capi/test_misc.py

@@ -22,6 +22,7 @@
 from test import support
 from test.support import MISSING_C_DOCSTRINGS
 from test.support import import_helper
+from test.support import script_helper
 from test.support import threading_helper
 from test.support import warnings_helper
 from test.support import requires_limited_api
@@ -1641,6 +1642,36 @@ def subthread():
 
             self.assertEqual(actual, int(interpid))
 
+    @threading_helper.requires_working_threading()
+    def test_pending_call_creates_thread(self):
+        source = """
+        import _testcapi
+        import threading
+        import time
+
+
+        def output():
+            print(24)
+            time.sleep(1)
+            print(42)
+
+
+        def callback():
+            threading.Thread(target=output).start()
+
+
+        def create_pending_call():
+            time.sleep(1)
+            _testcapi.simple_pending_call(callback)
+
+
+        threading.Thread(target=create_pending_call).start()
+        """
+        return_code, stdout, stderr = script_helper.assert_python_ok('-c', textwrap.dedent(source))
+        self.assertEqual(return_code, 0)
+        self.assertEqual(stdout, f"24{os.linesep}42{os.linesep}".encode("utf-8"))
+        self.assertEqual(stderr, b"")
+
 
 class SubinterpreterTest(unittest.TestCase):
 

Lib/threading.py

@@ -1557,8 +1557,9 @@ def _shutdown():
     # normally - that won't happen until the interpreter is nearly dead. So
     # mark it done here.
     if _main_thread._os_thread_handle.is_done() and _is_main_interpreter():
-        # _shutdown() was already called
-        return
+        # _shutdown() was already called, but threads might have started
+        # in the meantime.
+        return _thread_shutdown()
 
     global _SHUTTING_DOWN
     _SHUTTING_DOWN = True

Misc/NEWS.d/next/Core_and_Builtins/2025-06-26-18-44-34.gh-issue-136003.sln51d.rst

@@ -0,0 +1,3 @@
+Fix :class:`threading.Thread` objects becoming incorrectly daemon when
+created from an :mod:`atexit` callback or a pending call
+(:c:func:`Py_AddPendingCall`).

Modules/_testcapimodule.c

@@ -2546,6 +2546,16 @@ toggle_reftrace_printer(PyObject *ob, PyObject *arg)
     Py_RETURN_NONE;
 }
 
+static PyObject *
+simple_pending_call(PyObject *self, PyObject *callable)
+{
+    if (Py_AddPendingCall(_pending_callback, Py_NewRef(callable)) < 0) {
+        return NULL;
+    }
+
+    Py_RETURN_NONE;
+}
+
 static PyMethodDef TestMethods[] = {
     {"set_errno",               set_errno,                       METH_VARARGS},
     {"test_config",             test_config,                     METH_NOARGS},
@@ -2640,6 +2650,7 @@ static PyMethodDef TestMethods[] = {
     {"test_atexit", test_atexit, METH_NOARGS},
     {"code_offset_to_line", _PyCFunction_CAST(code_offset_to_line), METH_FASTCALL},
     {"toggle_reftrace_printer", toggle_reftrace_printer, METH_O},
+    {"simple_pending_call", simple_pending_call, METH_O},
     {NULL, NULL} /* sentinel */
 };
 

Modules/_threadmodule.c

@@ -415,7 +415,7 @@ force_done(void *arg)
 
 static int
 ThreadHandle_start(ThreadHandle *self, PyObject *func, PyObject *args,
-                   PyObject *kwargs)
+                   PyObject *kwargs, int daemon)
 {
     // Mark the handle as starting to prevent any other threads from doing so
     PyMutex_Lock(&self->mutex);
@@ -439,7 +439,8 @@ ThreadHandle_start(ThreadHandle *self, PyObject *func, PyObject *args,
         goto start_failed;
     }
     PyInterpreterState *interp = _PyInterpreterState_GET();
-    boot->tstate = _PyThreadState_New(interp, _PyThreadState_WHENCE_THREADING);
+    uint8_t whence = daemon ? _PyThreadState_WHENCE_THREADING_DAEMON : _PyThreadState_WHENCE_THREADING;
+    boot->tstate = _PyThreadState_New(interp, whence);
     if (boot->tstate == NULL) {
         PyMem_RawFree(boot);
         if (!PyErr_Occurred()) {
@@ -1874,7 +1875,7 @@ do_start_new_thread(thread_module_state *state, PyObject *func, PyObject *args,
         add_to_shutdown_handles(state, handle);
     }
 
-    if (ThreadHandle_start(handle, func, args, kwargs) < 0) {
+    if (ThreadHandle_start(handle, func, args, kwargs, daemon) < 0) {
         if (!daemon) {
             remove_from_shutdown_handles(handle);
         }

Python/pylifecycle.c

@@ -2003,6 +2003,99 @@ resolve_final_tstate(_PyRuntimeState *runtime)
     return main_tstate;
 }
 
+static int
+interp_has_threads(PyInterpreterState *interp)
+{
+    /* This needs to check for non-daemon threads only, otherwise we get stuck
+     * in an infinite loop. */
+    assert(interp != NULL);
+    assert(interp->runtime->stoptheworld.world_stopped);
+    assert(interp->threads.head != NULL);
+    if (interp->threads.head->next == NULL) {
+        // No other threads active, easy way out.
+        return 0;
+    }
+
+    // We don't have to worry about locking this because the
+    // world is stopped.
+    _Py_FOR_EACH_TSTATE_UNLOCKED(interp, tstate) {
+        if (tstate->_whence == _PyThreadState_WHENCE_THREADING) {
+            return 1;
+        }
+    }
+
+    return 0;
+}
+
+static int
+interp_has_pending_calls(PyInterpreterState *interp)
+{
+    assert(interp != NULL);
+    assert(interp->runtime->stoptheworld.world_stopped);
+    return interp->ceval.pending.npending != 0;
+}
+
+static int
+interp_has_atexit_callbacks(PyInterpreterState *interp)
+{
+    assert(interp != NULL);
+    assert(interp->atexit.callbacks != NULL);
+    assert(interp->runtime->stoptheworld.world_stopped);
+    assert(PyList_CheckExact(interp->atexit.callbacks));
+    return PyList_GET_SIZE(interp->atexit.callbacks) != 0;
+}
+
+static void
+make_pre_finalization_calls(PyThreadState *tstate)
+{
+    assert(tstate != NULL);
+    PyInterpreterState *interp = tstate->interp;
+    /* Each of these functions can start one another, e.g. a pending call
+     * could start a thread or vice versa. To ensure that we properly clean
+     * call everything, we run these in a loop until none of them run anything. */
+    for (;;) {
+        assert(!interp->runtime->stoptheworld.world_stopped);
+
+        // Wrap up existing "threading"-module-created, non-daemon threads.
+        wait_for_thread_shutdown(tstate);
+
+        // Make any remaining pending calls.
+        _Py_FinishPendingCalls(tstate);
+
+        /* The interpreter is still entirely intact at this point, and the
+        * exit funcs may be relying on that.  In particular, if some thread
+        * or exit func is still waiting to do an import, the import machinery
+        * expects Py_IsInitialized() to return true.  So don't say the
+        * runtime is uninitialized until after the exit funcs have run.
+        * Note that Threading.py uses an exit func to do a join on all the
+        * threads created thru it, so this also protects pending imports in
+        * the threads created via Threading.
+        */
+
+        _PyAtExit_Call(tstate->interp);
+
+        /* Stop the world to prevent other threads from creating threads or
+         * atexit callbacks. On the default build, this is simply locked by
+         * the GIL. For pending calls, we acquire the dedicated mutex, because
+         * Py_AddPendingCall() can be called without an attached thread state.
+         */
+
+        // XXX Why does _PyThreadState_DeleteList() rely on all interpreters
+        // being stopped?
+        PyMutex_Lock(&interp->ceval.pending.mutex);
+        _PyEval_StopTheWorldAll(interp->runtime);
+        int should_continue = (interp_has_threads(interp)
+                              || interp_has_atexit_callbacks(interp)
+                              || interp_has_pending_calls(interp));
+        if (!should_continue) {
+            break;
+        }
+        _PyEval_StartTheWorldAll(interp->runtime);
+        PyMutex_Unlock(&interp->ceval.pending.mutex);
+    }
+    assert(interp->runtime->stoptheworld.world_stopped);
+}
+
 static int
 _Py_Finalize(_PyRuntimeState *runtime)
 {
@@ -2019,23 +2112,7 @@ _Py_Finalize(_PyRuntimeState *runtime)
     // Block some operations.
     tstate->interp->finalizing = 1;
 
-    // Wrap up existing "threading"-module-created, non-daemon threads.
-    wait_for_thread_shutdown(tstate);
-
-    // Make any remaining pending calls.
-    _Py_FinishPendingCalls(tstate);
-
-    /* The interpreter is still entirely intact at this point, and the
-     * exit funcs may be relying on that.  In particular, if some thread
-     * or exit func is still waiting to do an import, the import machinery
-     * expects Py_IsInitialized() to return true.  So don't say the
-     * runtime is uninitialized until after the exit funcs have run.
-     * Note that Threading.py uses an exit func to do a join on all the
-     * threads created thru it, so this also protects pending imports in
-     * the threads created via Threading.
-     */
-
-    _PyAtExit_Call(tstate->interp);
+    make_pre_finalization_calls(tstate);
 
     assert(_PyThreadState_GET() == tstate);
 
@@ -2053,7 +2130,7 @@ _Py_Finalize(_PyRuntimeState *runtime)
 #endif
 
     /* Ensure that remaining threads are detached */
-    _PyEval_StopTheWorldAll(runtime);
+    assert(tstate->interp->runtime->stoptheworld.world_stopped);
 
     /* Remaining daemon threads will be trapped in PyThread_hang_thread
        when they attempt to take the GIL (ex: PyEval_RestoreThread()). */
@@ -2074,6 +2151,7 @@ _Py_Finalize(_PyRuntimeState *runtime)
         _PyThreadState_SetShuttingDown(p);
     }
     _PyEval_StartTheWorldAll(runtime);
+    PyMutex_Unlock(&tstate->interp->ceval.pending.mutex);
 
     /* Clear frames of other threads to call objects destructors. Destructors
        will be called in the current Python thread. Since
@@ -2435,13 +2513,7 @@ Py_EndInterpreter(PyThreadState *tstate)
     }
     interp->finalizing = 1;
 
-    // Wrap up existing "threading"-module-created, non-daemon threads.
-    wait_for_thread_shutdown(tstate);
-
-    // Make any remaining pending calls.
-    _Py_FinishPendingCalls(tstate);
-
-    _PyAtExit_Call(tstate->interp);
+    make_pre_finalization_calls(tstate);
 
     if (tstate != interp->threads.head || tstate->next != NULL) {
         Py_FatalError("not the last thread");
@@ -2450,6 +2522,8 @@ Py_EndInterpreter(PyThreadState *tstate)
     /* Remaining daemon threads will automatically exit
        when they attempt to take the GIL (ex: PyEval_RestoreThread()). */
     _PyInterpreterState_SetFinalizing(interp, tstate);
+    _PyEval_StartTheWorldAll(interp->runtime);
+    PyMutex_Unlock(&interp->ceval.pending.mutex);
 
     // XXX Call something like _PyImport_Disable() here?
 

Python/pystate.c

@@ -1447,7 +1447,7 @@ init_threadstate(_PyThreadStateImpl *_tstate,
     assert(tstate->prev == NULL);
 
     assert(tstate->_whence == _PyThreadState_WHENCE_NOTSET);
-    assert(whence >= 0 && whence <= _PyThreadState_WHENCE_EXEC);
+    assert(whence >= 0 && whence <= _PyThreadState_WHENCE_THREADING_DAEMON);
     tstate->_whence = whence;
 
     assert(id > 0);