gh-156124: Fix a crash when deleting ctypes Pointer.contents

In the free-threaded build the setter passed the value to
Py_BEGIN_CRITICAL_SECTION2() before checking it for NULL.

Author: serhiy-storchaka

Lib/test/test_ctypes/test_delattr.py

@@ -1,5 +1,5 @@
 import unittest
-from ctypes import Structure, c_char, c_int
+from ctypes import POINTER, Structure, c_char, c_int
 
 
 class X(Structure):
@@ -16,6 +16,11 @@ def test_chararray(self):
         with self.assertRaises(TypeError):
             del chararray.value
 
+    def test_pointer_contents(self):
+        ptr = POINTER(c_int)(c_int(42))
+        with self.assertRaises(TypeError):
+            del ptr.contents
+
     def test_struct(self):
         struct = X()
         with self.assertRaises(TypeError):

Misc/NEWS.d/next/Library/2026-08-20-16-30-00.gh-issue-156124.Kq3vXz.rst

@@ -0,0 +1,2 @@
+Fix a crash in the free-threaded build when deleting the :attr:`!contents`
+attribute of a :mod:`ctypes` pointer.

Modules/_ctypes/_ctypes.c

@@ -5713,11 +5713,6 @@ Pointer_set_contents_lock_held(PyObject *op, PyObject *value, void *closure)
     PyObject *keep;
     CDataObject *self = _CDataObject_CAST(op);
 
-    if (value == NULL) {
-        PyErr_SetString(PyExc_TypeError,
-                        "Pointer does not support item deletion");
-        return -1;
-    }
     ctypes_state *st = get_module_state_by_def(Py_TYPE(Py_TYPE(self)));
     StgInfo *stginfo;
     if (PyStgInfo_FromObject(st, op, &stginfo) < 0) {
@@ -5761,6 +5756,11 @@ Pointer_set_contents_lock_held(PyObject *op, PyObject *value, void *closure)
 static int
 Pointer_set_contents(PyObject *op, PyObject *value, void *closure)
 {
+    if (value == NULL) {
+        PyErr_SetString(PyExc_TypeError,
+                        "Pointer does not support item deletion");
+        return -1;
+    }
     int res;
     Py_BEGIN_CRITICAL_SECTION2(op, value);
     res = Pointer_set_contents_lock_held(op, value, closure);