gh-124570: ctypes: Run some Structure tests on Union as well (GH-124976)

- Move some Structure tests to test_structunion; use a common base
  test class + two subclasses to run them on Union too
- Remove test_union for now as it's redundant

Note: `test_simple_structs` & `test_simple_unions` are in the common
file because they share `formats`.

Lib/test/test_ctypes/test_structunion.py

@@ -0,0 +1,353 @@
+"""Common tests for ctypes.Structure and ctypes.Union"""
+
+import unittest
+from ctypes import (Structure, Union, POINTER, sizeof, alignment,
+                    c_char, c_byte, c_ubyte,
+                    c_short, c_ushort, c_int, c_uint,
+                    c_long, c_ulong, c_longlong, c_ulonglong, c_float, c_double)
+from ._support import (_CData, PyCStructType, UnionType,
+                       Py_TPFLAGS_DISALLOW_INSTANTIATION,
+                       Py_TPFLAGS_IMMUTABLETYPE)
+from struct import calcsize
+
+
+class StructUnionTestBase:
+    formats = {"c": c_char,
+               "b": c_byte,
+               "B": c_ubyte,
+               "h": c_short,
+               "H": c_ushort,
+               "i": c_int,
+               "I": c_uint,
+               "l": c_long,
+               "L": c_ulong,
+               "q": c_longlong,
+               "Q": c_ulonglong,
+               "f": c_float,
+               "d": c_double,
+               }
+
+    def test_subclass(self):
+        class X(self.cls):
+            _fields_ = [("a", c_int)]
+
+        class Y(X):
+            _fields_ = [("b", c_int)]
+
+        class Z(X):
+            pass
+
+        self.assertEqual(sizeof(X), sizeof(c_int))
+        self.check_sizeof(Y,
+                          struct_size=sizeof(c_int)*2,
+                          union_size=sizeof(c_int))
+        self.assertEqual(sizeof(Z), sizeof(c_int))
+        self.assertEqual(X._fields_, [("a", c_int)])
+        self.assertEqual(Y._fields_, [("b", c_int)])
+        self.assertEqual(Z._fields_, [("a", c_int)])
+
+    def test_subclass_delayed(self):
+        class X(self.cls):
+            pass
+        self.assertEqual(sizeof(X), 0)
+        X._fields_ = [("a", c_int)]
+
+        class Y(X):
+            pass
+        self.assertEqual(sizeof(Y), sizeof(X))
+        Y._fields_ = [("b", c_int)]
+
+        class Z(X):
+            pass
+
+        self.assertEqual(sizeof(X), sizeof(c_int))
+        self.check_sizeof(Y,
+                          struct_size=sizeof(c_int)*2,
+                          union_size=sizeof(c_int))
+        self.assertEqual(sizeof(Z), sizeof(c_int))
+        self.assertEqual(X._fields_, [("a", c_int)])
+        self.assertEqual(Y._fields_, [("b", c_int)])
+        self.assertEqual(Z._fields_, [("a", c_int)])
+
+    def test_inheritance_hierarchy(self):
+        self.assertEqual(self.cls.mro(), [self.cls, _CData, object])
+        self.assertEqual(type(self.metacls), type)
+
+    def test_type_flags(self):
+        for cls in self.cls, self.metacls:
+            with self.subTest(cls=cls):
+                self.assertTrue(cls.__flags__ & Py_TPFLAGS_IMMUTABLETYPE)
+                self.assertFalse(cls.__flags__ & Py_TPFLAGS_DISALLOW_INSTANTIATION)
+
+    def test_metaclass_details(self):
+        # Abstract classes (whose metaclass __init__ was not called) can't be
+        # instantiated directly
+        NewClass = self.metacls.__new__(self.metacls, 'NewClass',
+                                        (self.cls,), {})
+        for cls in self.cls, NewClass:
+            with self.subTest(cls=cls):
+                with self.assertRaisesRegex(TypeError, "abstract class"):
+                    obj = cls()
+
+        # Cannot call the metaclass __init__ more than once
+        class T(self.cls):
+            _fields_ = [("x", c_char),
+                        ("y", c_char)]
+        with self.assertRaisesRegex(SystemError, "already initialized"):
+            self.metacls.__init__(T, 'ptr', (), {})
+
+    def test_alignment(self):
+        class X(self.cls):
+            _fields_ = [("x", c_char * 3)]
+        self.assertEqual(alignment(X), calcsize("s"))
+        self.assertEqual(sizeof(X), calcsize("3s"))
+
+        class Y(self.cls):
+            _fields_ = [("x", c_char * 3),
+                        ("y", c_int)]
+        self.assertEqual(alignment(Y), alignment(c_int))
+        self.check_sizeof(Y,
+                          struct_size=calcsize("3s i"),
+                          union_size=max(calcsize("3s"), calcsize("i")))
+
+        class SI(self.cls):
+            _fields_ = [("a", X),
+                        ("b", Y)]
+        self.assertEqual(alignment(SI), max(alignment(Y), alignment(X)))
+        self.check_sizeof(SI,
+                          struct_size=calcsize("3s0i 3si 0i"),
+                          union_size=max(calcsize("3s"), calcsize("i")))
+
+        class IS(self.cls):
+            _fields_ = [("b", Y),
+                        ("a", X)]
+
+        self.assertEqual(alignment(SI), max(alignment(X), alignment(Y)))
+        self.check_sizeof(IS,
+                          struct_size=calcsize("3si 3s 0i"),
+                          union_size=max(calcsize("3s"), calcsize("i")))
+
+        class XX(self.cls):
+            _fields_ = [("a", X),
+                        ("b", X)]
+        self.assertEqual(alignment(XX), alignment(X))
+        self.check_sizeof(XX,
+                          struct_size=calcsize("3s 3s 0s"),
+                          union_size=calcsize("3s"))
+
+    def test_empty(self):
+        # I had problems with these
+        #
+        # Although these are pathological cases: Empty Structures!
+        class X(self.cls):
+            _fields_ = []
+
+        # Is this really the correct alignment, or should it be 0?
+        self.assertTrue(alignment(X) == 1)
+        self.assertTrue(sizeof(X) == 0)
+
+        class XX(self.cls):
+            _fields_ = [("a", X),
+                        ("b", X)]
+
+        self.assertEqual(alignment(XX), 1)
+        self.assertEqual(sizeof(XX), 0)
+
+    def test_fields(self):
+        # test the offset and size attributes of Structure/Union fields.
+        class X(self.cls):
+            _fields_ = [("x", c_int),
+                        ("y", c_char)]
+
+        self.assertEqual(X.x.offset, 0)
+        self.assertEqual(X.x.size, sizeof(c_int))
+
+        if self.cls == Structure:
+            self.assertEqual(X.y.offset, sizeof(c_int))
+        else:
+            self.assertEqual(X.y.offset, 0)
+        self.assertEqual(X.y.size, sizeof(c_char))
+
+        # readonly
+        self.assertRaises((TypeError, AttributeError), setattr, X.x, "offset", 92)
+        self.assertRaises((TypeError, AttributeError), setattr, X.x, "size", 92)
+
+        # XXX Should we check nested data types also?
+        # offset is always relative to the class...
+
+    def test_invalid_field_types(self):
+        class POINT(self.cls):
+            pass
+        self.assertRaises(TypeError, setattr, POINT, "_fields_", [("x", 1), ("y", 2)])
+
+    def test_invalid_name(self):
+        # field name must be string
+        def declare_with_name(name):
+            class S(self.cls):
+                _fields_ = [(name, c_int)]
+
+        self.assertRaises(TypeError, declare_with_name, b"x")
+
+    def test_intarray_fields(self):
+        class SomeInts(self.cls):
+            _fields_ = [("a", c_int * 4)]
+
+        # can use tuple to initialize array (but not list!)
+        self.assertEqual(SomeInts((1, 2)).a[:], [1, 2, 0, 0])
+        self.assertEqual(SomeInts((1, 2)).a[::], [1, 2, 0, 0])
+        self.assertEqual(SomeInts((1, 2)).a[::-1], [0, 0, 2, 1])
+        self.assertEqual(SomeInts((1, 2)).a[::2], [1, 0])
+        self.assertEqual(SomeInts((1, 2)).a[1:5:6], [2])
+        self.assertEqual(SomeInts((1, 2)).a[6:4:-1], [])
+        self.assertEqual(SomeInts((1, 2, 3, 4)).a[:], [1, 2, 3, 4])
+        self.assertEqual(SomeInts((1, 2, 3, 4)).a[::], [1, 2, 3, 4])
+        # too long
+        # XXX Should raise ValueError?, not RuntimeError
+        self.assertRaises(RuntimeError, SomeInts, (1, 2, 3, 4, 5))
+
+    def test_huge_field_name(self):
+        # issue12881: segfault with large structure field names
+        def create_class(length):
+            class S(self.cls):
+                _fields_ = [('x' * length, c_int)]
+
+        for length in [10 ** i for i in range(0, 8)]:
+            try:
+                create_class(length)
+            except MemoryError:
+                # MemoryErrors are OK, we just don't want to segfault
+                pass
+
+    def test_abstract_class(self):
+        class X(self.cls):
+            _abstract_ = "something"
+        with self.assertRaisesRegex(TypeError, r"^abstract class$"):
+            X()
+
+    def test_methods(self):
+        self.assertIn("in_dll", dir(type(self.cls)))
+        self.assertIn("from_address", dir(type(self.cls)))
+        self.assertIn("in_dll", dir(type(self.cls)))
+
+
+class StructureTestCase(unittest.TestCase, StructUnionTestBase):
+    cls = Structure
+    metacls = PyCStructType
+
+    def test_metaclass_name(self):
+        self.assertEqual(self.metacls.__name__, "PyCStructType")
+
+    def check_sizeof(self, cls, *, struct_size, union_size):
+        self.assertEqual(sizeof(cls), struct_size)
+
+    def test_simple_structs(self):
+        for code, tp in self.formats.items():
+            class X(Structure):
+                _fields_ = [("x", c_char),
+                            ("y", tp)]
+            self.assertEqual((sizeof(X), code),
+                                 (calcsize("c%c0%c" % (code, code)), code))
+
+
+class UnionTestCase(unittest.TestCase, StructUnionTestBase):
+    cls = Union
+    metacls = UnionType
+
+    def test_metaclass_name(self):
+        self.assertEqual(self.metacls.__name__, "UnionType")
+
+    def check_sizeof(self, cls, *, struct_size, union_size):
+        self.assertEqual(sizeof(cls), union_size)
+
+    def test_simple_unions(self):
+        for code, tp in self.formats.items():
+            class X(Union):
+                _fields_ = [("x", c_char),
+                            ("y", tp)]
+            self.assertEqual((sizeof(X), code),
+                             (calcsize("%c" % (code)), code))
+
+
+class PointerMemberTestBase:
+    def test(self):
+        # a Structure/Union with a POINTER field
+        class S(self.cls):
+            _fields_ = [("array", POINTER(c_int))]
+
+        s = S()
+        # We can assign arrays of the correct type
+        s.array = (c_int * 3)(1, 2, 3)
+        items = [s.array[i] for i in range(3)]
+        self.assertEqual(items, [1, 2, 3])
+
+        s.array[0] = 42
+
+        items = [s.array[i] for i in range(3)]
+        self.assertEqual(items, [42, 2, 3])
+
+        s.array[0] = 1
+
+        items = [s.array[i] for i in range(3)]
+        self.assertEqual(items, [1, 2, 3])
+
+class PointerMemberTestCase_Struct(unittest.TestCase, PointerMemberTestBase):
+    cls = Structure
+
+    def test_none_to_pointer_fields(self):
+        class S(self.cls):
+            _fields_ = [("x", c_int),
+                        ("p", POINTER(c_int))]
+
+        s = S()
+        s.x = 12345678
+        s.p = None
+        self.assertEqual(s.x, 12345678)
+
+class PointerMemberTestCase_Union(unittest.TestCase, PointerMemberTestBase):
+    cls = Union
+
+    def test_none_to_pointer_fields(self):
+        class S(self.cls):
+            _fields_ = [("x", c_int),
+                        ("p", POINTER(c_int))]
+
+        s = S()
+        s.x = 12345678
+        s.p = None
+        self.assertFalse(s.p)  # NULL pointers are falsy
+
+
+class TestRecursiveBase:
+    def test_contains_itself(self):
+        class Recursive(self.cls):
+            pass
+
+        try:
+            Recursive._fields_ = [("next", Recursive)]
+        except AttributeError as details:
+            self.assertIn("Structure or union cannot contain itself",
+                          str(details))
+        else:
+            self.fail("Structure or union cannot contain itself")
+
+
+    def test_vice_versa(self):
+        class First(self.cls):
+            pass
+        class Second(self.cls):
+            pass
+
+        First._fields_ = [("second", Second)]
+
+        try:
+            Second._fields_ = [("first", First)]
+        except AttributeError as details:
+            self.assertIn("_fields_ is final", str(details))
+        else:
+            self.fail("AttributeError not raised")
+
+class TestRecursiveStructure(unittest.TestCase, TestRecursiveBase):
+    cls = Structure
+
+class TestRecursiveUnion(unittest.TestCase, TestRecursiveBase):
+    cls = Union