template <typename Pool, typename Scope>

        static auto async_mysql_stmt_prepare(service_context<Pool, Scope> &ctx,

                                             char *sql_ptr, std::size_t size)

            -> ::mcs::execution::task<int>

        {

            int ret{0};

            auto *stmt = ctx.key.connect.raw_stmt();

            auto status = ::mysql_stmt_prepare_start(&ret, stmt, sql_ptr, size);

            auto h = co_await utils::current_coroutine{};

            auto call_back_impl = [&](int new_status) noexcept {

                status = new_status;

                h.resume();

            };

            using call_back_impl_type = decltype(call_back_impl);

            mariadb_poll_callback_object<call_back_impl_type> callback{call_back_impl};

            auto *mysql = ctx.key.connect.data();

            auto &loop = ctx.key.loop;

            auto &pool = ctx.pool;

            auto &counting_scope = ctx.counting_scope;

            while (status != 0)

            {

                loop.register_event(::mysql_get_socket(mysql), status, &callback);

                co_await std::suspend_always{};

#if 0

//NOTE: get stuck here. resume from a thread different from the thread of mysql_stmt_prepare_start

                ex::spawn(ex::starts_on(

                              pool.get_scheduler(),

                              ex::just() | ex::then([&]() noexcept {

                                  status = ::mysql_stmt_prepare_cont(&ret, stmt, status);

                                  h.resume();

                              })),

                          counting_scope.get_token());

                co_await std::suspend_always{};

#else

                // NOTE: OK。The threads of mysql_stmt_prepare_start and mysql_stmt_prepare_cont are the same.

                status = ::mysql_stmt_prepare_cont(&ret, stmt, status);

#endif

            }

            co_return ret;

        }