[3.15] gh-159739: Preserve ctypes base metadata when cloning fails (GH-160174) (#160293)

When creating a ctypes structure subclass, cloning its layout initially copied pointers to buffers owned by the base class. If a later allocation failed, cleaning up the subclass could free those buffers, leaving the base with invalid metadata and potentially causing a crash. Detach the owned pointers until their copies have been allocated. Backport to 3.15 and 3.14.
(cherry picked from commit 7c87319)

Co-authored-by: Jelle Zijlstra <jelle@openai.com>

Author: miss-islington

Misc/NEWS.d/next/Library/2026-10-22-03-55-33.gh-issue-159739.b4c371.rst

@@ -0,0 +1,2 @@
+Fix a crash when a :mod:`ctypes` structure subclass cannot be created due
+to an allocation failure.

Modules/_ctypes/stgdict.c

@@ -28,6 +28,10 @@ PyCStgInfo_clone(StgInfo *dst_info, StgInfo *src_info)
     ctype_free_stginfo_members(dst_info);
 
     memcpy(dst_info, src_info, sizeof(StgInfo));
+    /* Owned buffers must not be shared if a subsequent allocation fails. */
+    dst_info->format = NULL;
+    dst_info->shape = NULL;
+    dst_info->ffi_type_pointer.elements = NULL;
 #ifdef Py_GIL_DISABLED
     dst_info->mutex = (PyMutex){0};
 #endif