ctypes: crash when deleting Pointer.contents on the free-threaded build

[serhiy-storchaka]

Crash report

del ptr.contents crashes the free-threaded build.

>>> from ctypes import POINTER, c_int
>>> ptr = POINTER(c_int)(c_int(42))
>>> del ptr.contents
Segmentation fault

Pointer_set_contents() passes the new value to Py_BEGIN_CRITICAL_SECTION2(), which dereferences it to get the mutex, but the value is NULL when the attribute is deleted. The check for NULL is in Pointer_set_contents_lock_held(), which is only called after the critical section is entered.

With the GIL it raises TypeError: Pointer does not support item deletion.

The critical section was added in gh-128182, so this affects 3.14 and newer.

Linked PRs

• gh-156124: Fix a crash when deleting ctypes Pointer.contents #156125
• [3.15] gh-156124: Fix a crash when deleting ctypes Pointer.contents (GH-156125) #156153
• [3.14] gh-156124: Fix a crash when deleting ctypes Pointer.contents (GH-156125) #156154