[PATCH] libstdc++: Fix unintended layout change to std::basic_filebuf [PR108331]

[Jonathan Wakely <jwakely@redhat.com>]

Tested x86_64-linux. Bootstrapped x86_64-w64-mingw32 with both
--enable-threads=posix and --enable-threads=win32.

I need a libgcc or mingw maintainer to approve the gthr-win32.h part.

OK for trunk?

-- >8 --

GCC 13 has a new implementation of gthr-win32.h which supports C++11
mutexes, threads etc. but this causes an unintended ABI break. The
__gthread_mutex_t type is always used in std::basic_filebuf even in
C++98, so independent of whether C++11 sync primitives work or not.
Because that type changed for the win32 thread model, we have a layout
change in std::basic_filebuf. The member is completely unused, it just
gets passed to the std::__basic_file constructor and ignored. So we
don't need that mutex to actually work, we just need its layout to not
change.

Introduce a new __gthr_win32_legacy_mutex_t struct in gthr-win32.h with
the old layout, and conditionally use that in std::basic_filebuf.

	PR libstdc++/108331

libgcc/ChangeLog:

	* config/i386/gthr-win32.h (__gthr_win32_legacy_mutex_t): New
	struct matching the previous __gthread_mutex_t struct.
	(__GTHREAD_LEGACY_MUTEX_T): Define.

libstdc++-v3/ChangeLog:

	* config/io/c_io_stdio.h (__c_lock): Define as a typedef for
	__GTHREAD_LEGACY_MUTEX_T if defined.
---
 libgcc/config/i386/gthr-win32.h     | 8 ++++++++
 libstdc++-v3/config/io/c_io_stdio.h | 7 +++++++
 2 files changed, 15 insertions(+)

diff --git a/libgcc/config/i386/gthr-win32.h b/libgcc/config/i386/gthr-win32.h
index 146357fa436..050c7a21fcc 100644
--- a/libgcc/config/i386/gthr-win32.h
+++ b/libgcc/config/i386/gthr-win32.h
@@ -381,6 +381,14 @@ typedef struct timespec __gthread_time_t;
 #define __GTHREAD_COND_INIT_FUNCTION __gthread_cond_init_function
 #define __GTHREAD_TIME_INIT {0, 0}
 
+// Libstdc++ std::basic_filebuf needs the old definition of __gthread_mutex_t
+// for layout purposes, but doesn't actually use it.
+typedef struct {
+  long __unused1;
+  void *__unused2;
+} __gthr_win32_legacy_mutex_t;
+#define __GTHREAD_LEGACY_MUTEX_T __gthr_win32_legacy_mutex_t
+
 #if defined (_WIN32) && !defined(__CYGWIN__)
 #define MINGW32_SUPPORTS_MT_EH 1
 /* Mingw runtime >= v0.3 provides a magic variable that is set to nonzero
diff --git a/libstdc++-v3/config/io/c_io_stdio.h b/libstdc++-v3/config/io/c_io_stdio.h
index 1a5e05a844a..e9e6e3ef4d8 100644
--- a/libstdc++-v3/config/io/c_io_stdio.h
+++ b/libstdc++-v3/config/io/c_io_stdio.h
@@ -39,7 +39,14 @@ namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
+#ifdef __GTHREAD_LEGACY_MUTEX_T
+  // The layout of __gthread_mutex_t changed in GCC 13, but libstdc++ doesn't
+  // actually use the basic_filebuf::_M_lock member, so define it consistently
+  // with the old __gthread_mutex_t to avoid an unnecessary layout change:
+  typedef __GTHREAD_LEGACY_MUTEX_T __c_lock;
+#else
   typedef __gthread_mutex_t __c_lock;
+#endif
 
   // for basic_file.h
   typedef FILE __c_file;
-- 
2.39.0