Update libcxx to 2 Apr 2024 239236b8c2154aa49e98bc7ed774a7d2712edf50

```
[libc++] Remove pair dependency (#85621)
[libc++] Simplify the implementation of <stddef.h> (#86843)
Update the "Current Status" section of the website to be current. (#84507)
[libc++] Optimize the two range overload of mismatch (#86853)
[libc++] Refactor __tuple_like and __pair_like (#85206)
[libc++][test] Improves src include paths. (#86864)
[libc++] Document guidelines for applying [[nodiscard]] (#84000)
[libc++] Optimize the std::mismatch tail (#83440)
[libc++] Remove <queue> and <stack> includes from <format> (#85520)
[libc++] Switch FreeBSD to C++26 (#86658)
[libc++] Fix one case in saturate_cast.pass.cpp for 64-bit on z/OS (#86724)
[libc++][test] Remove tests that testing std::variant<T&> (#84222)
[libc++] <experimental/simd> Reduce types for template parameter U in tests (#83781)
[libc++] Switch FreeBSD CI job to Clang 17 (#86320)
[libc++][CI] Updates the documentation. (#85264)
[libc++][format] Adds ABI tags to inline constexpr variables. (#86293)
[libc++][test] Fix race condition in condition_variable_any tests (#84788)
[libc++] Vectorize mismatch (#73255)
[libc++] Don't push and pop extensions diagnostics when using clang modules (#85917)
[libc++][ranges] use `static operator()` for C++23 ranges (#86052)
[libc++] Implement LWG3528 (`make_from_tuple` can perform (the equivalent of) a C-style cast) (#85263)
[libc++] Remove macros for keeping std::allocator members and void specialization after C++20 (#85806)
[libc++] Deprecates std::errc constants. (#80542)
[libcxx] [cmake] Fix cmake_path(ABSOLUTE_PATH) for empty CMAKE_INSTALL_PREFIX
[libc++][CI] Reenables the module tests. (#85799)
[libc++][ranges] remove `__workaround_52970` (#85683)
[libc++][ranges] LWG3715: `view_interface::empty` is overconstrained (#85004)
[libcxx] [modules] Fix relative paths with absolute LIBCXX_INSTALL_MODULES_DIR (#85756)
[libc++][format] Fixes nested concept evaluation. (#85548)
[libc++][CMake] Removes LIBCXX_ENABLE_CLANG_TIDY. (#85262)
[libc++] Remove __unconstrained_reverse_iterator (#85582)
[libc++] Simplify the implementation of remove_reference (#85207)
[libcxx][test] Create feature host-can-create-symlinks (#82204)
[libc++] Use __is_scoped_enum for the implementation of is_scoped_enum if it's available (#85580)
[libc++] Add missing hide-from-ABI annotations on virtual functions (#85439)
[libc++] Optimize ranges::fill{,_n} for vector<bool>::iterator (#84642)
[libc++] Add <locale> include in <chrono> (#85521)
[libc++] Remove <locale> includes from <format> (#85478)
[libc++] P2602R2 Poison Pills are Too Toxic (#74534)
[libc++] Fixes run-buildbot-container. (#84644)
[libc++] Add [[nodiscard]] to static numeric limit functions (#83748)
 Rework Modules CMake to be (more) idiomatic. (#84936)
[libc++] Enables TZDB tests. (#82108)
[libc++] Remove a few includes from <__functional/hash.h> (#83254)
[libc++] Implements filebuf unbuffered. (#76629)
[libc++][CI] Updates the version number of the latest release.
[libc++] Remove <tuple> include from <__format/concepts.h> (#80214)
[runtimes] Prefer -fvisibility-global-new-delete=force-hidden (#84917)
[libc++] Fix tests on musl (#85085)
[libc++] Remove _LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT (#83928)
[libcxx] Fix incorrect type in the has-1024-bit-atomics feature test (#84904)
[libc++] Improves UB handling in ios_base destructor. (#76525)
[libc++][CI] Installs tzdata package in Docker. (#84643)
[libc++][TZDB] Fixes parsing interleaved rules. (#84808)
[libc++][hardening] Check bounds on arithmetic in __bounded_iter (#78876)
[libc++][hardening] Add iterator validity checks on unordered containers (#80230)
[libc++][hardening] Reclassify string_view(ptr, len)'s size assertion (#79297)
[libc++] Re-enable the clang_modules_include test for Objective-C++ (#66801)
[libc++] Remove XFAIL for SIMD in optimized build (#84767)
[libcxx] Update 128-bit-atomics feature test (#83841)
[libc++] Only forward-declare ABI-functions in exception_ptr.h if they are meant to be used (#84707)
[libc++][test] Don't include `test_format_context.h` in `parse.pass.cpp` (#83734)
[libc++] Add missing include in test (#84579)
[libc++] Remove <tuple> from <variant> (#83183)
[RFC][libc++] Reworks clang-tidy selection. (#81362)
[libc++] Remove <array> include from <span> (#83742)
[libc++][format] Updates LWG3462 status. (#80550)
[libc++][format] Update LWG3701 status. (#80545)
[libc++][test] Fix MSVC warning C4127 in `array.cons/initialization.pass.cpp` (#79793)
[libc++] Simplify the std::pair constructor overload set (#81448)
[libc++] Allow the use of extensions in the implementation (#79532)
[libc++] Don't build against libcxxrt by default on FreeBSD (#84484)
Actually disable the module generation tests.
[libc++] Disable module_std and module_std_compat tests
[libc++] Enable availability based on the compiler instead of __has_extension (#84065)
[libc++] Fixes time formatter test output for Linux on PowerPC (#75526)
[libc++] Remove unused includes from __type_traits/is_convertible.h (#83747)
```
commit_hash:171258d93ab92f516dfb1789300677a164e63219

1 files changed, 59 insertions, 1 deletions

diff --git a/contrib/libs/cxxsupp/libcxx/include/fstream b/contrib/libs/cxxsupp/libcxx/include/fstream
index 776641b347e..7a084d114b1 100644
--- a/contrib/libs/cxxsupp/libcxx/include/fstream
+++ b/contrib/libs/cxxsupp/libcxx/include/fstream
@@ -308,6 +308,43 @@ private:
   state_type __st_;
   state_type __st_last_;
   ios_base::openmode __om_;
+  // There have been no file operations yet, which allows setting unbuffered
+  // I/O mode.
+  static const ios_base::openmode __no_io_operations = ios_base::trunc;
+  // Unbuffered I/O mode has been requested.
+  static const ios_base::openmode __use_unbuffered_io = ios_base::ate;
+  // Used to track the currently used mode and track whether the output should
+  // be unbuffered.
+  // [filebuf.virtuals]/12
+  //   If setbuf(0, 0) is called on a stream before any I/O has occurred on
+  //   that stream, the stream becomes unbuffered. Otherwise the results are
+  //   implementation-defined.
+  // This allows calling setbuf(0, 0)
+  // - before opening a file,
+  // - after opening a file, before
+  //   - a read
+  //   - a write
+  //   - a seek.
+  // Note that opening a file with ios_base::ate does a seek operation.
+  // Normally underflow, overflow, and sync change this flag to ios_base::in,
+  // ios_base_out, or 0.
+  //
+  // The ios_base::trunc and ios_base::ate flags are not used in __cm_. They
+  // are used to track the state of the unbuffered request. For readability
+  // they have the aliases __no_io_operations and __use_unbuffered_io
+  // respectively.
+  //
+  // The __no_io_operations and __use_unbuffered_io flags are used in the
+  // following way:
+  // - __no_io_operations is set upon construction to indicate the unbuffered
+  //   state can be set.
+  // - When requesting unbuffered output:
+  //   - If the file is open it sets the mode.
+  //   - Else places a request by adding the __use_unbuffered_io flag.
+  // - When a file is opened it checks whether both __no_io_operations and
+  //   __use_unbuffered_io are set. If so switches to unbuffered mode.
+  // - All file I/O operations change the mode effectively clearing the
+  //   __no_io_operations and __use_unbuffered_io flags.
   ios_base::openmode __cm_;
   bool __owns_eb_;
   bool __owns_ib_;
@@ -327,7 +364,13 @@ private:
       return nullptr;
 
     __om_ = __mode;
+    if (__cm_ == (__no_io_operations | __use_unbuffered_io)) {
+      std::setbuf(__file_, nullptr);
+      __cm_ = 0;
+    }
+
     if (__mode & ios_base::ate) {
+      __cm_ = 0;
       if (fseek(__file_, 0, SEEK_END)) {
         fclose(__file_);
         __file_ = nullptr;
@@ -337,6 +380,20 @@ private:
 
     return this;
   }
+
+  // If the file is already open, switch to unbuffered mode. Otherwise, record
+  // the request to use unbuffered mode so that we use that mode when we
+  // eventually open the file.
+  _LIBCPP_HIDE_FROM_ABI void __request_unbuffered_mode(char_type* __s, streamsize __n) {
+    if (__cm_ == __no_io_operations && __s == nullptr && __n == 0) {
+      if (__file_) {
+        std::setbuf(__file_, nullptr);
+        __cm_ = 0;
+      } else {
+        __cm_ = __no_io_operations | __use_unbuffered_io;
+      }
+    }
+  }
 };
 
 template <class _CharT, class _Traits>
@@ -352,7 +409,7 @@ basic_filebuf<_CharT, _Traits>::basic_filebuf()
       __st_(),
       __st_last_(),
       __om_(0),
-      __cm_(0),
+      __cm_(__no_io_operations),
       __owns_eb_(false),
       __owns_ib_(false),
       __always_noconv_(false) {
@@ -810,6 +867,7 @@ template <class _CharT, class _Traits>
 basic_streambuf<_CharT, _Traits>* basic_filebuf<_CharT, _Traits>::setbuf(char_type* __s, streamsize __n) {
   this->setg(nullptr, nullptr, nullptr);
   this->setp(nullptr, nullptr);
+  __request_unbuffered_mode(__s, __n);
   if (__owns_eb_)
     delete[] __extbuf_;
   if (__owns_ib_)