[Buildroot] [PATCH] package/gcc: fix gcc12 with uClibc

[Gleb Mazovetskiy <glex.spb@gmail.com>]

GCC 12 produces broken binaries when used with uClibc.
E.g. `gdb` crashes on startup.

GCC HEAD fixes this in:
https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;h=ee4af2ed0b7322884ec4ff537564683c3749b813

Adds the commit as patch for GCC 12.

Signed-off-by: Gleb Mazovetskiy <glex.spb@gmail.com>
---
 package/gcc/12.2.0/0002-fix-condvar.patch | 80 +++++++++++++++++++++++
 1 file changed, 80 insertions(+)
 create mode 100644 package/gcc/12.2.0/0002-fix-condvar.patch

diff --git a/package/gcc/12.2.0/0002-fix-condvar.patch b/package/gcc/12.2.0/0002-fix-condvar.patch
new file mode 100644
index 0000000000..b5575e5df2
--- /dev/null
+++ b/package/gcc/12.2.0/0002-fix-condvar.patch
@@ -0,0 +1,80 @@
+From ee4af2ed0b7322884ec4ff537564683c3749b813 Mon Sep 17 00:00:00 2001
+From: Jonathan Wakely <jwakely@redhat.com>
+Date: Thu, 22 Dec 2022 09:56:47 +0000
+Subject: [PATCH] libstdc++: Avoid recursion in __nothrow_wait_cv::wait
+ [PR105730]
+
+The commit r12-5877-g9e18a25331fa25 removed the incorrect
+noexcept-specifier from std::condition_variable::wait and gave the new
+symbol version @@GLIBCXX_3.4.30. It also redefined the original symbol
+std::condition_variable::wait(unique_lock<mutex>&)@GLIBCXX_3.4.11 as an
+alias for a new symbol, __gnu_cxx::__nothrow_wait_cv::wait, which still
+has the incorrect noexcept guarantee. That __nothrow_wait_cv::wait is
+just a wrapper around the real condition_variable::wait which adds
+noexcept and so terminates on a __forced_unwind exception.
+
+This doesn't work on uclibc, possibly due to a dynamic linker bug. When
+__nothrow_wait_cv::wait calls the condition_variable::wait function it
+binds to the alias symbol, which means it just calls itself recursively
+until the stack overflows.
+
+This change avoids the possibility of a recursive call by changing the
+__nothrow_wait_cv::wait function so that instead of calling
+condition_variable::wait it re-implements it. This requires accessing
+the private _M_cond member of condition_variable, so we need to use the
+trick of instantiating a template with the member-pointer of the private
+member.
+
+libstdc++-v3/ChangeLog:
+
+	PR libstdc++/105730
+	* src/c++11/compatibility-condvar.cc (__nothrow_wait_cv::wait):
+	Access private data member of base class and call its wait
+	member.
+---
+ .../src/c++11/compatibility-condvar.cc        | 22 ++++++++++++++++++-
+ 1 file changed, 21 insertions(+), 1 deletion(-)
+
+diff --git a/libstdc++-v3/src/c++11/compatibility-condvar.cc b/libstdc++-v3/src/c++11/compatibility-condvar.cc
+index e3a8b8403ca..3cef3bc0714 100644
+--- a/libstdc++-v3/src/c++11/compatibility-condvar.cc
++++ b/libstdc++-v3/src/c++11/compatibility-condvar.cc
+@@ -67,6 +67,24 @@ _GLIBCXX_END_NAMESPACE_VERSION
+     && defined(_GLIBCXX_HAVE_SYMVER_SYMBOL_RENAMING_RUNTIME_SUPPORT)
+ namespace __gnu_cxx _GLIBCXX_VISIBILITY(default)
+ {
++namespace
++{
++  // Pointer-to-member for private std::condition_variable::_M_cond member.
++  std::__condvar std::condition_variable::* __base_member;
++
++  template<std::__condvar std::condition_variable::*X>
++    struct cracker
++    { static std::__condvar std::condition_variable::* value; };
++
++  // Initializer for this static member also initializes __base_member.
++  template<std::__condvar std::condition_variable::*X>
++    std::__condvar std::condition_variable::*
++      cracker<X>::value = __base_member = X;
++
++  // Explicit instantiation is allowed to access the private member.
++  template class cracker<&std::condition_variable::_M_cond>;
++}
++
+ struct __nothrow_wait_cv : std::condition_variable
+ {
+   void wait(std::unique_lock<std::mutex>&) noexcept;
+@@ -76,7 +94,9 @@ __attribute__((used))
+ void
+ __nothrow_wait_cv::wait(std::unique_lock<std::mutex>& lock) noexcept
+ {
+-  this->condition_variable::wait(lock);
++  // In theory this could be simply this->std::condition_variable::wait(lock)
++  // but with uclibc that binds to the @GLIBCXX_3.4.11 symbol, see PR 105730.
++  (this->*__base_member).wait(*lock.mutex());
+ }
+ } // namespace __gnu_cxx
+ 
+-- 
+2.31.1
+
-- 
2.37.2

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