gh-154176: Fix locale.strxfrm() crash on DragonFly BSD (GH-154177)

https://github.com/python/cpython/commit/f389b03f9f2768342bfd49987a257460d4aa4faf
commit: f389b03f9f2768342bfd49987a257460d4aa4faf
branch: main
author: Serhiy Storchaka <[email protected]>
committer: serhiy-storchaka <[email protected]>
date: 2026-07-20T08:59:31+03:00
summary:

gh-154176: Fix locale.strxfrm() crash on DragonFly BSD (GH-154177)

Query the result size with a real one-element buffer instead of
wcsxfrm(NULL, s, 0): DragonFly BSD's wcsxfrm() crashes when the
destination is NULL or the size is 0.

Co-authored-by: Claude Fable 5 <[email protected]>

files:
A Misc/NEWS.d/next/Library/2026-07-19-21-30-00.gh-issue-154176.strxfrm.rst
M Modules/_localemodule.c

diff --git a/Misc/NEWS.d/next/Library/2026-07-19-21-30-00.gh-issue-154176.strxfrm.rst b/Misc/NEWS.d/next/Library/2026-07-19-21-30-00.gh-issue-154176.strxfrm.rst
new file mode 100644
index 00000000000000..9ffa5828000113
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-07-19-21-30-00.gh-issue-154176.strxfrm.rst
@@ -0,0 +1,2 @@
+Fix a crash in :func:`locale.strxfrm` on DragonFly BSD,
+whose ``wcsxfrm()`` does not support the zero size.
diff --git a/Modules/_localemodule.c b/Modules/_localemodule.c
index 0f71358a180dd2..20783dc00f90d3 100644
--- a/Modules/_localemodule.c
+++ b/Modules/_localemodule.c
@@ -443,6 +443,7 @@ _locale_strxfrm_impl(PyObject *module, PyObject *str)
 {
     Py_ssize_t n1;
     wchar_t *s = NULL, *buf = NULL;
+    wchar_t dummy[1];
     size_t n2;
     PyObject *result = NULL;
 
@@ -456,7 +457,9 @@ _locale_strxfrm_impl(PyObject *module, PyObject *str)
     }
 
     errno = 0;
-    n2 = wcsxfrm(NULL, s, 0);
+    /* Query the size with a real one-element buffer: DragonFly BSD's
+       wcsxfrm() crashes when the destination is NULL or the size is 0. */
+    n2 = wcsxfrm(dummy, s, 1);
     if (errno && errno != ERANGE) {
         PyErr_SetFromErrno(PyExc_OSError);
         goto exit;

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