From 77f551548c888b576707ab2647478de00365a4fd Mon Sep 17 00:00:00 2001
From: Serhiy Storchaka <storchaka@gmail.com>
Date: Sun, 27 Jul 2025 13:17:59 +0300
Subject: [PATCH 1/2] Remove optimistic allocation in locale.strxfrm()

On modern systems, the result of wcsxfrm() is much larger the size of
the input string (from 4+2*n on Windows to 4+5*n on Linux for simple
ASCII strings), so optimistic allocation of the buffer of the same size
never works.
---
 Modules/_localemodule.c | 28 +++++++++-------------------
 1 file changed, 9 insertions(+), 19 deletions(-)

diff --git a/Modules/_localemodule.c b/Modules/_localemodule.c
index 41e6d48b1dbd9b..c0b63648676bfd 100644
--- a/Modules/_localemodule.c
+++ b/Modules/_localemodule.c
@@ -409,33 +409,23 @@ _locale_strxfrm_impl(PyObject *module, PyObject *str)
     }
 
     /* assume no change in size, first */
-    n1 = n1 + 1;
-    buf = PyMem_New(wchar_t, n1);
+    errno = 0;
+    n2 = wcsxfrm(NULL, s, 0);
+    if (errno && errno != ERANGE) {
+        PyErr_SetFromErrno(PyExc_OSError);
+        goto exit;
+    }
+    buf = PyMem_New(wchar_t, n2+1);
     if (!buf) {
         PyErr_NoMemory();
         goto exit;
     }
     errno = 0;
-    n2 = wcsxfrm(buf, s, n1);
-    if (errno && errno != ERANGE) {
+    n2 = wcsxfrm(buf, s, n2+1);
+    if (errno) {
         PyErr_SetFromErrno(PyExc_OSError);
         goto exit;
     }
-    if (n2 >= (size_t)n1) {
-        /* more space needed */
-        wchar_t * new_buf = PyMem_Realloc(buf, (n2+1)*sizeof(wchar_t));
-        if (!new_buf) {
-            PyErr_NoMemory();
-            goto exit;
-        }
-        buf = new_buf;
-        errno = 0;
-        n2 = wcsxfrm(buf, s, n2+1);
-        if (errno) {
-            PyErr_SetFromErrno(PyExc_OSError);
-            goto exit;
-        }
-    }
     result = PyUnicode_FromWideChar(buf, n2);
 exit:
     PyMem_Free(buf);

From fbf6e8e31c37f561142d4d7b89f1f242a80c68d4 Mon Sep 17 00:00:00 2001
From: Serhiy Storchaka <storchaka@gmail.com>
Date: Sun, 27 Jul 2025 14:14:11 +0300
Subject: [PATCH 2/2] Remove outdated comment.

---
 Modules/_localemodule.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Modules/_localemodule.c b/Modules/_localemodule.c
index c0b63648676bfd..a4c8dc22f91885 100644
--- a/Modules/_localemodule.c
+++ b/Modules/_localemodule.c
@@ -408,7 +408,6 @@ _locale_strxfrm_impl(PyObject *module, PyObject *str)
         goto exit;
     }
 
-    /* assume no change in size, first */
     errno = 0;
     n2 = wcsxfrm(NULL, s, 0);
     if (errno && errno != ERANGE) {
@@ -420,6 +419,7 @@ _locale_strxfrm_impl(PyObject *module, PyObject *str)
         PyErr_NoMemory();
         goto exit;
     }
+
     errno = 0;
     n2 = wcsxfrm(buf, s, n2+1);
     if (errno) {