Fix data race in freeing control block

tests/test_reference.cpp

@@ -1,3 +1,4 @@
+#include <barrier>
 #include <doctest/doctest.h>
 #include <latch>
 #include <thread>
@@ -156,6 +157,63 @@ TEST_CASE("Ref thread safety") {
   }
 }
 
+TEST_CASE("Control block cleanup race condition test") {
+  // This test specifically targets the race condition where both
+  // the last strong reference and last weak reference are destroyed
+  // simultaneously, potentially causing double-free of control block
+
+  const int test_iterations = 10000;
+
+  // Shared state for passing references between threads
+  Ref<TestObject> ptr1;
+  WeakRef<TestObject> ptr2;
+  auto setup = [&]() {
+    ptr1 = make_ref<TestObject>(0);
+    ptr2 = ptr1;
+  };
+
+  // Barrier for synchronization - 2 participants (main thread + worker thread)
+  std::barrier sync_barrier{2};
+
+  std::thread worker_thread([&]() {
+    for (int iter = 0; iter < test_iterations; ++iter) {
+      // Wait for main thread to create the references
+      sync_barrier.arrive_and_wait();
+
+      // Worker thread destroys the weak reference simultaneously with main
+      // thread
+      ptr2.reset();
+
+      // Wait for next iteration
+      sync_barrier.arrive_and_wait();
+    }
+  });
+
+  for (int iter = 0; iter < test_iterations; ++iter) {
+    // Create references
+    setup();
+
+    // Both threads are ready - synchronize for simultaneous destruction
+    sync_barrier.arrive_and_wait();
+
+    // Main thread destroys the strong reference at the same time
+    // as worker thread destroys the weak reference
+    ptr1.reset();
+
+    // Wait for both destructions to complete
+    sync_barrier.arrive_and_wait();
+
+    // Clean up for next iteration
+    ptr1.reset();
+    ptr2.reset();
+  }
+
+  worker_thread.join();
+
+  // If we reach here without segfault/double-free, the test passes
+  // The bug would manifest as a crash or memory corruption
+}
+
 TEST_CASE("WeakRef prevents circular references") {
   SUBCASE("simple weak reference lifecycle") {
     WeakRef<TestObject> weak_ref;