Add tests of buffer pointer alignment

Author: jakelishman

Lib/test/support/__init__.py

@@ -5,6 +5,7 @@
 
 import annotationlib
 import contextlib
+import ctypes
 import functools
 import inspect
 import logging
@@ -69,6 +70,7 @@
     "in_systemd_nspawn_sync_suppressed",
     "run_no_yield_async_fn", "run_yielding_async_fn", "async_yield",
     "reset_code",
+    "ctypes_py_buffer",
     ]
 
 
@@ -3183,3 +3185,37 @@ def linked_to_musl():
         return _linked_to_musl
     _linked_to_musl = tuple(map(int, version.split('.')))
     return _linked_to_musl
+
+
+class _py_buffer(ctypes.Structure):
+    _fields_ = [
+        ("buf", ctypes.c_void_p),
+        ("obj", ctypes.py_object),
+        ("len", ctypes.c_ssize_t),
+        ("itemsize", ctypes.c_ssize_t),
+        ("readonly", ctypes.c_int),
+        ("ndim", ctypes.c_int),
+        ("format", ctypes.c_char_p),
+        ("shape", ctypes.POINTER(ctypes.c_ssize_t)),
+        ("strides", ctypes.POINTER(ctypes.c_ssize_t)),
+        ("suboffsets", ctypes.POINTER(ctypes.c_ssize_t)),
+        ("internal", ctypes.c_void_p),
+    ]
+
+
+@contextlib.contextmanager
+def ctypes_py_buffer(ob, flags=inspect.BufferFlags.SIMPLE):
+    """
+    Safely acquire a `Py_buffer` as a ctypes struct.
+
+    `ob` must implement the buffer protocol, and the retrieved buffer is
+    released on exit of the context manager.
+    """
+    buf = _py_buffer()
+    ctypes.pythonapi.PyObject_GetBuffer(ctypes.py_object(ob),
+                                        ctypes.byref(buf),
+                                        ctypes.c_int(flags))
+    try:
+        yield buf
+    finally:
+        ctypes.pythonapi.PyBuffer_Release(ctypes.byref(buf))

Lib/test/test_array.py

@@ -8,6 +8,8 @@
 from test.support import import_helper
 from test.support import os_helper
 from test.support import _2G
+from test.support import ctypes_py_buffer
+import ctypes
 import weakref
 import pickle
 import operator
@@ -67,6 +69,20 @@ def test_empty(self):
         a += a
         self.assertEqual(len(a), 0)
 
+    def test_empty_alignment(self):
+        # gh-140557: pointer alignment of empty allocation
+        self.enterContext(warnings.catch_warnings())
+        warnings.filterwarnings(
+            "ignore",
+            message="The 'u' type code is deprecated and "
+                    "will be removed in Python 3.16",
+            category=DeprecationWarning)
+        max_align = ctypes.alignment(ctypes.c_longdouble)
+        for typecode in typecodes:
+            a = array.array(typecode)
+            with ctypes_py_buffer(a) as buf:
+                self.assertEqual(buf.buf % max_align, 0)
+
 
 # Machine format codes.
 #

Lib/test/test_builtin.py

@@ -4,6 +4,7 @@
 import builtins
 import collections
 import contextlib
+import ctypes
 import decimal
 import fractions
 import gc
@@ -38,6 +39,7 @@
 from test.support.testcase import ComplexesAreIdenticalMixin
 from test.support.warnings_helper import check_warnings
 from test.support import requires_IEEE_754
+from test.support import ctypes_py_buffer
 from unittest.mock import MagicMock, patch
 try:
     import pty, signal
@@ -2404,6 +2406,13 @@ def iterator():
             yield b'B'
         self.assertEqual(bytearray(b'A,B'), array.join(iterator()))
 
+    def test_bytearray_empty_alignment(self):
+        # gh-140557: alignment of pointer in empty allocation
+        max_align = ctypes.alignment(ctypes.c_longdouble)
+        array = bytearray()
+        with ctypes_py_buffer(array) as buf:
+            self.assertEqual(buf.buf % max_align, 0)
+
     def test_construct_singletons(self):
         for const in None, Ellipsis, NotImplemented:
             tp = type(const)