gh-137273: Fix debug assertion failure in locale.setlocale() on Windows by serhiy-storchaka · Pull Request #137300 · python/cpython · GitHub

It happened when there were at least 16 characters after dot in the locale name.

Issue: locale.setlocale() crashes on Windows for long locale name #137273

… Windows It happened when there were at least 16 characters after dot in the locale name.

This fix should probably be limited to just a few Windows releases, unless Win11 also shows the same behavior.

Where did you get the "16" from ?
Please include some comments to explain where you got it from and why this check is necessary. Thanks.

BTW: It's better to make this limit configurable via a constant.

Comes from https://github.com/huangqinjin/ucrt/blob/d6e817a4cc90f6f1fe54f8a0aa4af4fff0bb647d/inc/corecrt_internal.h#L401, though I don't think that constant is available for us at compile time (the internal headers for the CRT are, well, internal).

A constant at least gives it a name though, and using MAX_CP_LEN may help someone find an updated definition in the future.

Looks good now.

When you're done making the requested changes, leave the comment: I have made the requested changes; please review again.

How to check the Windows version? I do not have Windows 11, can you please test this PR on Windows 11?

How to check the Windows version?

I wouldn't bother, this limit has been around forever. The bigger challenge would be how to handle @<extras> if/when support is added, because right now there's no support for them in UCRT at all.

How to check the Windows version? I do not have Windows 11, can you please test this PR on Windows 11?

Sorry, but I don't have a Windows 11 build system available at the moment (not even a working Win10 system, after the VM crashed some time ago).

Let's not worry about this now. We can disable things again selectively, once the Windows SDK provides better support for handling locale names (perhaps in a few years).

The bigger challenge would be how to handle @<extras> if/when support is added, because right now there's no support for them in UCRT at all.

I am planning to handle this in Python code. Translate "zh_HK.UTF-8@hans" to "zh-Hans-HK.utf-8", etc.

I am planning to handle this in Python code. Translate "zh_HK.UTF-8@hans" to "zh-Hans-HK.utf-8", etc.

Is that how Windows handles modifiers ?

I have made the requested changes; please review again.

Thanks for making the requested changes!

@malemburg: please review the changes made to this pull request.

Looks good now.

Is that how Windows handles modifiers ?

No - Windows UCRT treats them as part of the code page name and overruns its own buffers, leading to a crash 😉

I don't think there's any support for modifiers in locales at all. The OS itself has always used a different system for those AFAICT, it's just the POSIX emulation in the CRT that's trying to fudge over it.

I don't think there's any support for modifiers in locales at all.

In that case, it's probably best to simply strip from them locale strings passed to setlocale() on Windows. Esp. since the meaning of those modifiers is not standardized anywhere, AFAIK.

Is that how Windows handles modifiers ?

Yes. It supports format language[-script][-region][.codepage] for BCP-47 locale names.

I seen also sr_Latn_RS instead of sr_RS@latin on some of Posix platforms, so we should support this format not only on Windows. setlocale() should try several variants.

Thanks @serhiy-storchaka for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14.
🐍🍒⛏🤖

… Windows (pythonGH-137300) It happened when there were at least 16 characters after dot in the locale name. (cherry picked from commit 718e0c8) Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

GH-137305 is a backport of this pull request to the 3.14 branch.

GH-137306 is a backport of this pull request to the 3.13 branch.

…n Windows (GH-137300) (GH-137306) It happened when there were at least 16 characters after dot in the locale name. (cherry picked from commit 718e0c8) Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Thank you for your review, @malemburg and @zooba.

… Windows (pythonGH-137300) It happened when there were at least 16 characters after dot in the locale name.

…n Windows (GH-137300) (#137305) gh-137273: Fix debug assertion failure in locale.setlocale() on Windows (GH-137300) It happened when there were at least 16 characters after dot in the locale name. (cherry picked from commit 718e0c8) Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

serhiy-storchaka requested review from malemburg and zooba August 1, 2025 12:13

serhiy-storchaka added needs backport to 3.13 only security fixes needs backport to 3.14 bugs and security fixes labels Aug 1, 2025

bedevere-app Bot added the awaiting core review label Aug 1, 2025

bedevere-app Bot mentioned this pull request Aug 1, 2025

locale.setlocale() crashes on Windows for long locale name #137273

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pythongh-137273 : Fix debug assertion failure in locale.setlocale() on…

c6dbeb8

… Windows It happened when there were at least 16 characters after dot in the locale name.

serhiy-storchaka force-pushed the locale-long-encoding branch from bbff466 to c6dbeb8 Compare August 1, 2025 12:22

malemburg requested changes Aug 1, 2025

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bedevere-app Bot added awaiting changes and removed awaiting core review labels Aug 1, 2025

Add more tests and comments.

3031948

MAX_ENCODING_SIZE -> MAX_CP_LEN

eae0bc4

bedevere-app Bot added awaiting change review and removed awaiting changes labels Aug 1, 2025

bedevere-app Bot requested a review from malemburg August 1, 2025 13:34

malemburg approved these changes Aug 1, 2025

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serhiy-storchaka merged commit 718e0c8 into python:main Aug 1, 2025

bedevere-app Bot removed the awaiting merge label Aug 1, 2025

serhiy-storchaka deleted the locale-long-encoding branch August 1, 2025 14:44

bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label Aug 1, 2025

bedevere-app Bot removed the needs backport to 3.13 only security fixes label Aug 1, 2025

Agent-Hellboy pushed a commit to Agent-Hellboy/cpython that referenced this pull request Aug 19, 2025

pythongh-137273 : Fix debug assertion failure in locale.setlocale() on…

9882a99

… Windows (pythonGH-137300) It happened when there were at least 16 characters after dot in the locale name.

+                  size_t len = end ? (size_t)(end - locale) : strlen(locale);
+                  const char *dot = memchr(locale, '.', len);
+                  if (dot && locale + len - dot > 16) {
+                      return -1;

+                  size_t len = end ? (size_t)(end - locale) : strlen(locale);
+                  const char *dot = memchr(locale, '.', len);
+                  if (dot && locale + len - dot > 16) {
+                      return -1;

gh-137273: Fix debug assertion failure in locale.setlocale() on Windows by serhiy-storchaka · Pull Request #137300 · python/cpython · GitHub

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gh-137273: Fix debug assertion failure in locale.setlocale() on Windows - #137300

gh-137273: Fix debug assertion failure in locale.setlocale() on Windows#137300serhiy-storchaka merged 3 commits intopython:mainpython/cpython:mainfrom serhiy-storchaka:locale-long-encodingserhiy-storchaka/cpython:locale-long-encodingCopy head branch name to clipboard

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gh-137273: Fix debug assertion failure in locale.setlocale() on Windows#137300
serhiy-storchaka merged 3 commits into
python:mainfrom
serhiy-storchaka:locale-long-encoding

serhiy-storchaka commented Aug 1, 2025 •
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