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.
{
    
    String query_prefix = "/* ddl_entry=" + task.entry_name + " */ ";
    String query_to_execute = query_prefix + query;
    ReadBufferFromString istr(query_to_execute);
    String dummy_string;
    WriteBufferFromString ostr(dummy_string);
    std::optional<CurrentThread::QueryScope> query_scope;
    try
    {
        auto query_context = task.makeQueryContext(context, zookeeper);
        if (!task.is_initial_query)
            query_scope.emplace(query_context);
        executeQuery(istr, ostr, !task.is_initial_query, query_context, {});
        if (auto txn = query_context->getZooKeeperMetadataTransaction())
        {
            
            
            if (!txn->isExecuted())
                txn->commit();
        }
    }
    catch (const DB::Exception & e)
    {
        if (task.is_initial_query)