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.
if (!task->findCurrentHostID(context, log))
    {
        out_reason = "There is no a local address in host list";
        return {};
    }
    try
    {
        
        task->parseQueryFromEntry(context);
        
        task->setClusterInfo(context, log);
    }
    catch (...)
    {
        out_reason = "Cannot parse query or obtain cluster info";
        write_error_status(task->host_id_str, ExecutionStatus::fromCurrentException().serializeText(), out_reason);
        return {};
    }
    if (zookeeper->exists(task->getFinishedNodePath()))
    {
        out_reason = TASK_PROCESSED_OUT_REASON;
        return {};
    }
    
    return task;
}
static void filterAndSortQueueNodes(Strings & all_nodes)