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.
{
    all_nodes.erase(std::remove_if(all_nodes.begin(), all_nodes.end(), [] (const String & s) { return !startsWith(s, "query-"); }), all_nodes.end());
    std::sort(all_nodes.begin(), all_nodes.end());
}
void DDLWorker::scheduleTasks(bool reinitialized)
{
    LOG_DEBUG(log, "Scheduling tasks");
    auto zookeeper = tryGetZooKeeper();
    
    
    
    
    
    if (reinitialized)
    {
        if (current_tasks.empty())
            LOG_TRACE(log, "Don't have unfinished tasks after restarting");
        else
            LOG_INFO(log, "Have {} unfinished tasks, will check them", current_tasks.size());
        assert(current_tasks.size() <= pool_size + (worker_pool != nullptr));
        auto task_it = current_tasks.begin();
        while (task_it != current_tasks.end())
        {
            auto & task = *task_it;
            if (task->completely_processed)
            {