[gcc r13-6282] [libstdc++] do not destruct mutex_pool mutexes

[Alexandre Oliva <aoliva@gcc.gnu.org>]

https://gcc.gnu.org/g:d70f49e98245f80fcf8b2e68a9f06955f52e1de7

commit r13-6282-gd70f49e98245f80fcf8b2e68a9f06955f52e1de7
Author: Alexandre Oliva <oliva@adacore.com>
Date:   Wed Feb 22 14:35:18 2023 -0300

    [libstdc++] do not destruct mutex_pool mutexes
    
    On vxworks, after destroying the semaphore used to implement a mutex,
    __gthread_mutex_lock fails and __gnu_cxx::__mutex::lock calls
    __throw_concurrence_lock_error.  Nothing ensures the mutex_pool
    mutexes survive init-once objects containing _Safe_sequence_base.  If
    such an object completes construction before mutex_pool
    initialization, it will be registered for atexit destruction after the
    mutex_pool mutexes, so the _M_detach_all() call in the
    _Safe_sequence_base dtor will use already-destructed mutexes, and
    basic_string/requirements/citerators_cc fails calling terminate.
    
    This patch fixes this problem by ensuring the mutex pool mutexes are
    constructed on demand, on a statically-allocated buffer, but never
    destructed.
    
    
    for  libstdc++-v3/ChangeLog
    
            * src/c++11/shared_ptr.cc (__gnu_internal::get_mutex):
            Avoid destruction of the mutex pool.

Diff:
---
 libstdc++-v3/src/c++11/shared_ptr.cc | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/src/c++11/shared_ptr.cc b/libstdc++-v3/src/c++11/shared_ptr.cc
index bc70134359c..74e879e5828 100644
--- a/libstdc++-v3/src/c++11/shared_ptr.cc
+++ b/libstdc++-v3/src/c++11/shared_ptr.cc
@@ -36,7 +36,11 @@ namespace __gnu_internal _GLIBCXX_VISIBILITY(hidden)
   {
     // increase alignment to put each lock on a separate cache line
     struct alignas(64) M : __gnu_cxx::__mutex { };
-    static M m[mask + 1];
+    // Use a static buffer, so that the mutexes are not destructed
+    // before potential users (or at all)
+    static __attribute__ ((aligned(__alignof__(M))))
+      char buffer[(sizeof (M)) * (mask + 1)];
+    static M *m = new (buffer) M[mask + 1];
     return m[i];
   }
 }