DDLWorker thread checks in a loop for new tasks by calling ZooKeeper GetChildren with a watch callback.

https://github.com/ClickHouse/ClickHouse/blob/5b0bc8a7fbf846cb45012e8009d58341656bf12c/src/Interpreters/DDLWorker.cpp#L374

Due to a recent change from #25373 instead of blocking on new events we retry the loop every 10 seconds instead of when watch callback is triggered.

https://github.com/ClickHouse/ClickHouse/blob/5b0bc8a7fbf846cb45012e8009d58341656bf12c/src/Interpreters/DDLWorker.cpp#L1140-L1142

This causes a zookeeper watch callback leak. It is probably not very important as all that happens is that a new std::function is added to the watches list for the ddl worker path every 10 seconds. What is 32 bytes (sizeof(std::function)?) every 10 seconds?

However, it is annoying to see that on metrics. The metric is somewhat useful for catching bugs and this isn't an important one.

I understand that we could probably fix this "temporary fix" in DDLWorker and remove the tryWait + retrying, but that pattern was seen in other places and is useful for implementing a non-blocking watch, also used in `waitForTableReplicaToProcessLogEntry`.

Is there a laconic fix for that? Maybe we could somehow remove the callback when not needed/before retry? Maybe we could change the behaviour slightly and add watchcallback only if it wasn't already added (this wouldn't work for getChildren which wraps a Poco Event in a watch)? Anything else beside just ignoring this?

---

I understand that all the callbacks will be cleared when the event is triggered.
LOG_DEBUG(log, "Started DDLWorker cleanup thread");
    Int64 last_cleanup_time_seconds = 0;
    while (!stop_flag)
    {
        try
        {
            cleanup_event->wait();
            if (stop_flag)
                break;
            Int64 current_time_seconds = Poco::Timestamp().epochTime();
            if (last_cleanup_time_seconds && current_time_seconds < last_cleanup_time_seconds + cleanup_delay_period)
            {
                LOG_TRACE(log, "Too early to clean queue, will do it later.");
                continue;
            }
            
            auto zookeeper = tryGetZooKeeper();
            if (zookeeper->expired())
                continue;
            cleanupQueue(current_time_seconds, zookeeper);
            last_cleanup_time_seconds = current_time_seconds;
        }
        catch (...)
        {
            tryLogCurrentException(log, __PRETTY_FUNCTION__);
        }
    }
}
}