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.
throw;
        task.execution_status = ExecutionStatus::fromCurrentException();
        tryLogCurrentException(log, "Query " + query + " wasn't finished successfully");
        
        
        
        
        
        bool no_sense_to_retry = e.code() != ErrorCodes::KEEPER_EXCEPTION &&
                                 e.code() != ErrorCodes::NOT_A_LEADER &&
                                 e.code() != ErrorCodes::CANNOT_ASSIGN_ALTER &&
                                 e.code() != ErrorCodes::CANNOT_ALLOCATE_MEMORY &&
                                 e.code() != ErrorCodes::MEMORY_LIMIT_EXCEEDED;
        return no_sense_to_retry;
    }
    catch (...)
    {
        if (task.is_initial_query)
            throw;
        task.execution_status = ExecutionStatus::fromCurrentException();
        tryLogCurrentException(log, "Query " + query + " wasn't finished successfully");
        
        
        return false;
    }
    task.execution_status = ExecutionStatus(0);
    LOG_DEBUG(log, "Executed query: {}", query);
    return true;
}
void DDLWorker::updateMaxDDLEntryID(const String & entry_name)