gh-99275: Fix SystemError in ctypes during __initsubclass__ (GH-99283)

(cherry picked from commit 343eb0f) Co-authored-by: Nikita Sobolev <mail@sobolevn.me>

Author: miss-islington

Lib/ctypes/test/test_struct_fields.py

@@ -54,6 +54,15 @@ class X(Structure):
  x.char = b'a\0b\0'
  self.assertEqual(bytes(x), b'a\x00###')
 
+ def test_gh99275(self):
+ class BrokenStructure(Structure):
+ def __init_subclass__(cls, **kwargs):
+ cls._fields_ = [] # This line will fail, `stgdict` is not ready
+
+ with self.assertRaisesRegex(TypeError,
+ 'ctypes state is not initialized'):
+ class Subclass(BrokenStructure): ...
+
  # __set__ and __get__ should raise a TypeError in case their self
  # argument is not a ctype instance.
  def test___set__(self):

Misc/NEWS.d/next/Library/2022-11-09-12-16-35.gh-issue-99275.klOqoL.rst

@@ -0,0 +1,2 @@
+Fix ``SystemError`` in :mod:`ctypes` when exception was not set during
+``__initsubclass__``.

Modules/_ctypes/stgdict.c

@@ -430,8 +430,11 @@ PyCStructUnionType_update_stgdict(PyObject *type, PyObject *fields, int isStruct
  }
 
  stgdict = PyType_stgdict(type);
- if (!stgdict)
+ if (!stgdict) {
+ PyErr_SetString(PyExc_TypeError,
+ "ctypes state is not initialized");
  return -1;
+ }
  /* If this structure/union is already marked final we cannot assign
  _fields_ anymore. */