coroutine: exception: workaround broken destroy coroutine handle in await_suspend

gcc 15.2 and above crash if a coroutine handle is destroyed when
await_suspend for that coroutine is called [1]. An internal reference
counter is accessed after the coroutine is destroyed.

Work around this by scheduling the work to a task. This is slower,
but doesn't crash, and exceptions are supposed to be somewhat rare.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121961

Author: avikivity

include/seastar/core/coroutine.hh

@@ -32,6 +32,11 @@
 #include <coroutine>
 #endif
 
+#if defined(__GNUC__) && !defined(__clang__) && (__GNUC__ > 15 || (__GNUC__ == 15 && __GNUC_MINOR__ >= 2))
+// See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121961
+#define SEATAR_INTERNAL_DESTROY_HANDLE_IN_SUSPEND_BROKEN 1
+#endif
+
 namespace seastar {
 
 namespace internal {

include/seastar/coroutine/exception.hh

@@ -43,8 +43,14 @@ struct exception_awaiter {
 
     template<typename U>
     void await_suspend(std::coroutine_handle<U> hndl) noexcept {
+#ifdef SEATAR_INTERNAL_DESTROY_HANDLE_IN_SUSPEND_BROKEN
+      schedule(new lambda_task([hndl, eptr = std::move(eptr)] () mutable {
+#endif
         hndl.promise().set_exception(std::move(eptr));
         hndl.destroy();
+#ifdef SEATAR_INTERNAL_DESTROY_HANDLE_IN_SUSPEND_BROKEN
+      }));
+#endif
     }
 
     void await_resume() noexcept {}