[3.13] gh-127563: use `dk_log2_index_bytes=3` in empty dicts (GH-127568) (GH-127798)

This fixes a UBSan failure (unaligned zero-size memcpy) in `dictobject.c`.
(cherry picked from commit 9af96f440618304e7cc609c246e1f8c8b2d7a119)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index e82cd2d308ca7839073a388d52a185296f021850..ff16a41b9a5d4e2127d0ab4d49a185ba916f6222 100644 (file)
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -624,11 +624,14 @@ estimate_log2_keysize(Py_ssize_t n)
 
 /* This immutable, empty PyDictKeysObject is used for PyDict_Clear()
  * (which cannot fail and thus can do no allocation).
+ *
+ * See https://github.com/python/cpython/pull/127568#discussion_r1868070614
+ * for the rationale of using dk_log2_index_bytes=3 instead of 0.
  */
 static PyDictKeysObject empty_keys_struct = {
         _Py_IMMORTAL_REFCNT, /* dk_refcnt */
         0, /* dk_log2_size */
-        0, /* dk_log2_index_bytes */
+        3, /* dk_log2_index_bytes */
         DICT_KEYS_UNICODE, /* dk_kind */
 #ifdef Py_GIL_DISABLED
         {0}, /* dk_mutex */