Fix qsbr_per_thread::on_next_epoch_deallocate crash safety

Move current_interval_total_dealloc_size bump after emplacing the new element to
the deferred request list.

qsbr.hpp

@@ -848,15 +848,15 @@ inline void qsbr_per_thread::on_next_epoch_deallocate(
     return;
   }
 
-  current_interval_total_dealloc_size += size;
-
   current_interval_dealloc_requests.emplace_back(pointer
 #ifndef NDEBUG
                                                  ,
                                                  last_seen_epoch,
                                                  std::move(dealloc_callback)
 #endif
   );
+
+  current_interval_total_dealloc_size += size;
 }
 
 inline void qsbr_per_thread::advance_last_seen_epoch(

test/test_qsbr_oom.cpp

@@ -12,6 +12,7 @@
 #include "heap.hpp"
 #include "qsbr.hpp"
 #include "qsbr_gtest_utils.hpp"
+#include "thread_sync.hpp"
 
 namespace {
 
@@ -73,6 +74,38 @@ TEST_F(QSBROOMTest, StartThread) {
       [&second_thread] { join(second_thread); });
 }
 
+TEST_F(QSBROOMTest, DeferredDeallocation) {
+  auto *ptr = static_cast<char *>(allocate());
+  unodb::qsbr_thread second_thread;
+  const auto current_interval_total_dealloc_size_before =
+      unodb::this_thread().get_current_interval_total_dealloc_size();
+  oom_test(
+      2,
+      [&second_thread]() noexcept {
+        second_thread = unodb::qsbr_thread{[] {
+          unodb::detail::thread_syncs[0].notify();
+          unodb::detail::thread_syncs[1].wait();
+
+          quiescent();
+        }};
+
+        unodb::detail::thread_syncs[0].wait();
+      },
+      [ptr] { qsbr_deallocate(ptr); },
+      [ptr, current_interval_total_dealloc_size_before]() noexcept {
+        touch_memory(ptr);
+        const auto current_interval_total_dealloc_size_after =
+            unodb::this_thread().get_current_interval_total_dealloc_size();
+        UNODB_ASSERT_EQ(current_interval_total_dealloc_size_before,
+                        current_interval_total_dealloc_size_after);
+      },
+      [ptr] { qsbr_deallocate(ptr); },
+      [&second_thread] {
+        unodb::detail::thread_syncs[1].notify();
+        join(second_thread);
+      });
+}
+
 UNODB_END_TESTS()
 
 }  // namespace