[libcxx-commits] [libcxx] Avoid calling `setlocale` in `do_unshift` when unnecessary (PR #117153)

[via libcxx-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libcxx

Author: Michael Maltsev (m417z)

<details>
<summary>Changes</summary>

This is an attempt to mitigate #<!-- -->110954.

As part of the libc++.dll initialization, `static DoIOSInit` is initialized:
https://github.com/llvm/llvm-project/blob/5bdee35544eb21762857390014598748c64ad485/libcxx/src/iostream.cpp#L165-L167

When the dll is unloaded or on process shutdown, `DoIOSInit::~DoIOSInit` is called. It ends up calling `flush`:
https://github.com/llvm/llvm-project/blob/5bdee35544eb21762857390014598748c64ad485/libcxx/src/iostream.cpp#L159

Which calls `pubsync`:
https://github.com/llvm/llvm-project/blob/5bdee35544eb21762857390014598748c64ad485/libcxx/include/__ostream/basic_ostream.h#L659

Which ends up calling `do_unshift`:
https://github.com/llvm/llvm-project/blob/5bdee35544eb21762857390014598748c64ad485/libcxx/src/locale.cpp#L1475-L1479

Which, as can be seen, unconditionally calls `__locale::__wcrtomb`, which ends up calling `setlocale` via `__libcpp_locale_guard`.

All this means that `setlocale` is called on process shutdown even if `wcout` is never used, or even if nothing stream-related is used. Calling `setlocale` on process shutdown causes problems, as described in the mentioned issue.

This PR is an attempt to avoid calling `setlocale` in the vast majority of cases, when there's no output to be flushed. It's not a complete fix to the issue, but it will make it much less common, and it will at least allow to flush output manually to avoid the issue if streams are used.

---
Full diff: https://github.com/llvm/llvm-project/pull/117153.diff


1 Files Affected:

- (modified) libcxx/src/locale.cpp (+2) 


``````````diff
diff --git a/libcxx/src/locale.cpp b/libcxx/src/locale.cpp
index a1e10401f0b299..5ecd99c53cd516 100644
--- a/libcxx/src/locale.cpp
+++ b/libcxx/src/locale.cpp
@@ -1475,6 +1475,8 @@ codecvt<wchar_t, char, mbstate_t>::result codecvt<wchar_t, char, mbstate_t>::do_
 codecvt<wchar_t, char, mbstate_t>::result codecvt<wchar_t, char, mbstate_t>::do_unshift(
     state_type& st, extern_type* to, extern_type* to_end, extern_type*& to_nxt) const {
   to_nxt = to;
+  if (std::mbsinit(&st))
+    return ok;
   extern_type tmp[MB_LEN_MAX];
   size_t n = __locale::__wcrtomb(tmp, intern_type(), &st, __l_);
   if (n == size_t(-1) || n == 0) // on error

``````````

</details>


https://github.com/llvm/llvm-project/pull/117153