gh-97588: Move ctypes struct/union layout logic to Python (GH-123352)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Lib/test/test_ctypes/test_bitfields.py

@@ -5,7 +5,9 @@ from ctypes import (CDLL, Structure, sizeof, POINTER, byref, alignment,
                     LittleEndianStructure, BigEndianStructure,
                     c_byte, c_ubyte, c_char, c_char_p, c_void_p, c_wchar,
                     c_uint8, c_uint16, c_uint32, c_uint64,
-                    c_short, c_ushort, c_int, c_uint, c_long, c_ulong, c_longlong, c_ulonglong)
+                    c_short, c_ushort, c_int, c_uint, c_long, c_ulong,
+                    c_longlong, c_ulonglong,
+                    Union)
 from test import support
 from test.support import import_helper
 _ctypes_test = import_helper.import_module("_ctypes_test")
@@ -186,8 +188,10 @@ class BitFieldTest(unittest.TestCase):
                 self.assertEqual((c_typ, x.a, x.b, x.c), (c_typ, 0, 7, 0))
 
     def fail_fields(self, *fields):
-        return self.get_except(type(Structure), "X", (),
-                               {"_fields_": fields})
+        for layout in "ms", "gcc-sysv":
+            with self.subTest(layout=layout):
+                return self.get_except(type(Structure), "X", (),
+                                       {"_fields_": fields, "layout": layout})
 
     def test_nonint_types(self):
         # bit fields are not allowed on non-integer types.
@@ -204,9 +208,15 @@ class BitFieldTest(unittest.TestCase):
         result = self.fail_fields(("a", c_char, 1))
         self.assertEqual(result, (TypeError, 'bit fields not allowed for type c_char'))
 
-        class Dummy(Structure):
+        class Empty(Structure):
             _fields_ = []
 
+        result = self.fail_fields(("a", Empty, 1))
+        self.assertEqual(result, (ValueError, "number of bits invalid for bit field 'a'"))
+
+        class Dummy(Structure):
+            _fields_ = [("x", c_int)]
+
         result = self.fail_fields(("a", Dummy, 1))
         self.assertEqual(result, (TypeError, 'bit fields not allowed for type Dummy'))
 
@@ -518,6 +528,21 @@ class BitFieldTest(unittest.TestCase):
         x.c = 2
         self.assertEqual(b, b'\xab\xcd\xef\x12')
 
+    def test_union_bitfield(self):
+        class BitfieldUnion(Union):
+            _fields_ = [("a", c_uint32, 1),
+                        ("b", c_uint32, 2),
+                        ("c", c_uint32, 3)]
+        self.assertEqual(sizeof(BitfieldUnion), 4)
+        b = bytearray(4)
+        x = BitfieldUnion.from_buffer(b)
+        x.a = 1
+        self.assertEqual(int.from_bytes(b).bit_count(), 1)
+        x.b = 3
+        self.assertEqual(int.from_bytes(b).bit_count(), 2)
+        x.c = 7
+        self.assertEqual(int.from_bytes(b).bit_count(), 3)
+
 
 if __name__ == "__main__":
     unittest.main()

Lib/test/test_ctypes/test_struct_fields.py

@@ -60,7 +60,6 @@ class StructFieldsTestCase(unittest.TestCase):
         self.assertRaises(TypeError, CField)
 
     def test_cfield_type_flags(self):
-        self.assertTrue(CField.__flags__ & Py_TPFLAGS_DISALLOW_INSTANTIATION)
         self.assertTrue(CField.__flags__ & Py_TPFLAGS_IMMUTABLETYPE)
 
     def test_cfield_inheritance_hierarchy(self):