bpo-29753: revert 0d7ad9f (GH-19850) (GH-27085)

This reverts commit 0d7ad9f as it has a regression.

See https://github.com/python/cpython/pull/19850GH-issuecomment-869410686
(cherry picked from commit e14d5ae)

Co-authored-by: Filipe Laíns <[email protected]>

Author: miss-islington

Lib/ctypes/test/test_bitfields.py

@@ -233,69 +233,6 @@ class X(Structure):
         else:
             self.assertEqual(sizeof(X), sizeof(c_int) * 2)
 
-    @unittest.skipIf(os.name == 'nt', reason='Posix only')
-    def test_packed_posix(self):
-        test_cases = {
-            (
-                ("a", c_uint8, 4),
-                ("b", c_uint8, 4),
-            ): 1,
-            (
-                ("a", c_uint8, 1),
-                ("b", c_uint16, 1),
-                ("c", c_uint32, 1),
-                ("d", c_uint64, 1),
-            ): 1,
-            (
-                ("a", c_uint8, 8),
-                ("b", c_uint16, 1),
-                ("c", c_uint32, 1),
-                ("d", c_uint64, 1),
-            ): 2,
-            (
-                ("a", c_uint32, 9),
-                ("b", c_uint16, 10),
-                ("c", c_uint32, 25),
-                ("d", c_uint64, 1),
-            ): 6,
-            (
-                ("a", c_uint32, 9),
-                ("b", c_uint16, 10),
-                ("c", c_uint32, 25),
-                ("d", c_uint64, 5),
-            ): 7,
-            (
-                ("a", c_uint16),
-                ("b", c_uint16, 9),
-                ("c", c_uint16, 1),
-                ("d", c_uint16, 1),
-                ("e", c_uint16, 1),
-                ("f", c_uint16, 1),
-                ("g", c_uint16, 3),
-                ("h", c_uint32, 10),
-                ("i", c_uint32, 20),
-                ("j", c_uint32, 2),
-            ): 8,
-            (
-                ("a", c_uint16, 9),
-                ("b", c_uint16, 10),
-                ("d", c_uint16),
-                ("c", c_uint8, 8),
-            ): 6,
-            (
-                ("a", c_uint32, 9),
-                ("b", c_uint32),
-                ("c", c_uint32, 8),
-            ): 7,
-        }
-
-        for fields, size in test_cases.items():
-            with self.subTest(fields=fields):
-                class X(Structure):
-                    _pack_ = 1
-                    _fields_ = list(fields)
-                self.assertEqual(sizeof(X), size)
-
     def test_anon_bitfields(self):
         # anonymous bit-fields gave a strange error message
         class X(Structure):

Modules/_ctypes/cfield.c

@@ -71,18 +71,6 @@ PyCField_FromDesc(PyObject *desc, Py_ssize_t index,
         Py_DECREF(self);
         return NULL;
     }
-
-#ifndef MS_WIN32
-    /* if we have a packed bitfield, calculate the minimum number of bytes we
-    need to fit it. otherwise use the specified size. */
-    if (pack && bitsize) {
-        size = (bitsize - 1) / 8 + 1;
-    } else
-#endif
-        size = dict->size;
-
-    proto = desc;
-
     if (bitsize /* this is a bitfield request */
         && *pfield_size /* we have a bitfield open */
 #ifdef MS_WIN32
@@ -99,26 +87,25 @@ PyCField_FromDesc(PyObject *desc, Py_ssize_t index,
     } else if (bitsize /* this is a bitfield request */
         && *pfield_size /* we have a bitfield open */
         && dict->size * 8 >= *pfield_size
-        /* if this is a packed bitfield, always expand it.
-           otherwise calculate if we need to expand it. */
-        && (((*pbitofs + bitsize) <= dict->size * 8) || pack)) {
+        && (*pbitofs + bitsize) <= dict->size * 8) {
         /* expand bit field */
         fieldtype = EXPAND_BITFIELD;
 #endif
     } else if (bitsize) {
         /* start new bitfield */
         fieldtype = NEW_BITFIELD;
         *pbitofs = 0;
-        /* use our calculated size (size) instead of type size (dict->size),
-           which can be different for packed bitfields */
-        *pfield_size = size * 8;
+        *pfield_size = dict->size * 8;
     } else {
         /* not a bit field */
         fieldtype = NO_BITFIELD;
         *pbitofs = 0;
         *pfield_size = 0;
     }
 
+    size = dict->size;
+    proto = desc;
+
     /*  Field descriptors for 'c_char * n' are be scpecial cased to
         return a Python string instead of an Array object instance...
     */
@@ -183,16 +170,10 @@ PyCField_FromDesc(PyObject *desc, Py_ssize_t index,
         break;
 
     case EXPAND_BITFIELD:
-        /* increase the size if it is a packed bitfield.
-           EXPAND_BITFIELD should not be selected for non-packed fields if the
-           current size isn't already enough. */
-        if (pack)
-            size = (*pbitofs + bitsize - 1) / 8 + 1;
-
-        *poffset += size - *pfield_size/8;
-        *psize += size - *pfield_size/8;
+        *poffset += dict->size - *pfield_size/8;
+        *psize += dict->size - *pfield_size/8;
 
-        *pfield_size = size * 8;
+        *pfield_size = dict->size * 8;
 
         if (big_endian)
             self->size = (bitsize << 16) + *pfield_size - *pbitofs - bitsize;