[Bug libstdc++/98108] Broken Schwarz counter for iostreams initialization

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98108

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think it's QoI whether this works.

"The objects are constructed and the associations are established at some time
prior to or during the first time an object of class ios_base::Init is
constructed, ..."

"The results of including <iostream> in a translation unit shall be as if
<iostream> defined an instance of ios_base::Init with static storage duration."

So there's no guarantee that the global iostreams will be usable during the
static initialization phase, because there's no guarantee that a given
constructor will be before the first ios_base::Init constructor.

The problem is *not* that the counters use atomics rather than waiting.

Even if the initialization is done as in the patch below, it still crashes:

--- a/libstdc++-v3/src/c++98/ios_init.cc
+++ b/libstdc++-v3/src/c++98/ios_init.cc
@@ -77,7 +77,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION

   ios_base::Init::Init()
   {
-    if (__gnu_cxx::__exchange_and_add_dispatch(&_S_refcount, 1) == 0)
+    struct do_init
+    {
+      do_init()
       {
        // Standard streams default to synced with "C" operations.
        _S_synced_with_stdio = true;
@@ -116,8 +118,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        // streams are not re-initialized with uses of ios_base::Init
        // besides <iostream> static object, ie just using <ios> with
        // ios_base::Init objects.
-       __gnu_cxx::__atomic_add_dispatch(&_S_refcount, 1);
+       _S_refcount = 1;
       }
+    };
+#ifndef __cpp_threadsafe_static_init
+# warning "ios_base::Init::Init() initialization is not thread-safe"
+#endif
+    static const do_init once __attribute__((unused)) = do_init();
+
+    __gnu_cxx::__exchange_and_add_dispatch(&_S_refcount, 1);
   }

   ios_base::Init::~Init()

This doesn't help, because the thread1 and thread2 objects in file1.cc are
still constructed before the static ios_base::Init object in file2.cc and so
they still try to use std::cout before the first _S_refcount increment has
happened.

Waiting for the iostream initialization to finish doesn't help if it hasn't
started yet. And isn't relevant in your program, because only one
ios_base::Init constructor ever runs in the whole program. Making the second
one wait for the first doesn't help if there is only one.

What does make your program work is:

--- a/libstdc++-v3/include/std/iostream
+++ b/libstdc++-v3/include/std/iostream
@@ -71,7 +71,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   //@}

   // For construction of filebuffers for cout, cin, cerr, clog et. al.
-  static ios_base::Init __ioinit;
+  static ios_base::Init __ioinit __attribute__((__init_priority__(50)));

 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace

And this works with or without changing the Schwarz counter.