[Bug libstdc++/105880] eh_globals_init destructor not setting _M_init to false

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105880

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
As Andrew says, the problem here is that __cxa_get_globals is being used after
the global has been destroyed. Nothing done to the _M_init member in the
destructor can be changed to fix that, accessing an object after its destructor
runs is undefined, period.

What might "work" is to make the _M_init member a static data member, which
outlives the singleton instance of the class.

Something like this:

--- a/libstdc++-v3/libsupc++/eh_globals.cc
+++ b/libstdc++-v3/libsupc++/eh_globals.cc
@@ -90,29 +90,34 @@ eh_globals_dtor(void* ptr)
 struct __eh_globals_init
 {
   __gthread_key_t      _M_key;
-  bool                         _M_init;
+  static bool          _S_init;

-  __eh_globals_init() : _M_init(false)
-  { 
+  __eh_globals_init()
+  {
     if (__gthread_active_p())
-      _M_init = __gthread_key_create(&_M_key, eh_globals_dtor) == 0; 
+      _S_init = __gthread_key_create(&_M_key, eh_globals_dtor) == 0;
   }

   ~__eh_globals_init()
   {
-    if (_M_init)
+    if (_S_init)
       __gthread_key_delete(_M_key);
-    _M_init = false;
+    _S_init = false;
   }
+
+  __eh_globals_init(const __eh_globals_init&) = delete;
+  __eh_globals_init& operator=(const __eh_globals_init&) = delete;
 };

+bool __eh_globals_init::_S_init = false;
+
 static __eh_globals_init init;

 extern "C" __cxa_eh_globals*
 __cxxabiv1::__cxa_get_globals_fast() _GLIBCXX_NOTHROW
 {
   __cxa_eh_globals* g;
-  if (init._M_init)
+  if (init._S_init)
     g = static_cast<__cxa_eh_globals*>(__gthread_getspecific(init._M_key));
   else
     g = &eh_globals;
@@ -123,7 +128,7 @@ extern "C" __cxa_eh_globals*
 __cxxabiv1::__cxa_get_globals() _GLIBCXX_NOTHROW
 {
   __cxa_eh_globals* g;
-  if (init._M_init)
+  if (init._S_init)
     {
       g = static_cast<__cxa_eh_globals*>(__gthread_getspecific(init._M_key));
       if (!g)


This doesn't really seem correct though, as it just means we revert to the
eh_globals single-threaded fallback buffer after _S_init becomes false. But
that fallback buffer has also been destroyed at that point. So maybe we need to
make eh_globals immortal.