Hello,

so I'm working to integrate CatBoost model with our ClickHouse cluster. It appears that, whenever we create a new XML model file in the configured path, the **first** call to the new model never returns (this is true both of the HTTP interface as well as via clickhouse-client).

Independently from whether the locked statement gets cancelled, subsequent calls succeed immediately.

```
16e8bdb5bbb9 :) select version();

SELECT version()

┌─version()─┐
│ 20.6.3.28 │
└───────────┘

1 rows in set. Elapsed: 0.006 sec.
```

Is this known behavior? If it helps, my models look like this:

```
<models>
    <model>
        <type>catboost</type>
        <name>ctr_predictor</name>
        <path>/catboost/my_predictor</path>
        <lifetime>0</lifetime>
    </model>
</models>
```
: config_files_reader(config_files_reader_), loading_dispatcher(loading_dispatcher_)
    {
    }
    ~PeriodicUpdater() { enable(false); }
    void enable(bool enable_)
    {
        std::unique_lock lock{mutex};
        enabled = enable_;
        if (enable_)
        {
            if (!thread.joinable())
            {
                
                thread = ThreadFromGlobalPool{&PeriodicUpdater::doPeriodicUpdates, this};
            }
        }
        else
        {
            if (thread.joinable())
            {
                
                auto temp_thread = std::move(thread);
                lock.unlock();
                event.notify_one();
                temp_thread.join();
            }
        }
    }
private:
    void doPeriodicUpdates()