[libc++] Make sure the test for compare-and-wait bug doesn't hang forever (#97907)

This patch adds an explicit timeout mechanism in the compare-and-wait
test for std::atomic, ensuring that it doesn't run forever when the bug
is present. This is not an issue when we run inside the CI because we
specify a timeout manually, but it can be a problem when running
locally, for example.

Author: ldionne

libcxx/test/libcxx/atomics/atomics.syn/wait.pass.cpp renamed to libcxx/test/libcxx/atomics/atomics.syn/wait.issue_85107.pass.cpp

@@ -15,22 +15,40 @@
 
 // XFAIL: availability-synchronization_library-missing
 
+// This is a regression test for https://github.com/llvm/llvm-project/issues/85107, which describes
+// how we were using UL_COMPARE_AND_WAIT instead of UL_COMPARE_AND_WAIT64 in the implementation of
+// atomic::wait, leading to potential infinite hangs.
+
 #include <atomic>
 #include <cassert>
+#include <chrono>
+
+#include "make_test_thread.h"
 
-void test_85107() {
+int main(int, char**) {
   if constexpr (sizeof(std::__cxx_contention_t) == 8 && sizeof(long) > 4) {
+    std::atomic<bool> done = false;
+    auto const timeout     = std::chrono::system_clock::now() + std::chrono::seconds(600); // fail after 10 minutes
+
+    auto timeout_thread = support::make_test_thread([&] {
+      while (!done) {
+        assert(std::chrono::system_clock::now() < timeout); // make sure we don't hang forever
+      }
+    });
+
     // https://github.com/llvm/llvm-project/issues/85107
     // [libc++] atomic_wait uses UL_COMPARE_AND_WAIT when it should use UL_COMPARE_AND_WAIT64 on Darwin
     constexpr std::__cxx_contention_t old_val = 0;
     constexpr std::__cxx_contention_t new_val = old_val + (1ll << 32);
     std::__cxx_atomic_contention_t ct(new_val);
-    std::__libcpp_atomic_wait(&ct, old_val); // this will hang forever if the bug is present
-  }
-}
 
-int main(int, char**) {
-  test_85107();
+    // This would hang forever if the bug is present, but the test will fail in a bounded amount of
+    // time due to the timeout above.
+    std::__libcpp_atomic_wait(&ct, old_val);
+
+    done = true;
+    timeout_thread.join();
+  }
 
   return 0;
 }