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 (status.is_leader && lock->tryLock())
        {
            
            
            
            if (zookeeper->tryGet(is_executed_path, executed_by))
            {
                LOG_DEBUG(log, "Task {} has already been executed by replica ({}) of the same shard.", task.entry_name, executed_by);
                executed_by_other_leader = true;
                break;
            }
            
            size_t counter = parse<int>(zookeeper->get(tries_to_execute_path));
            if (counter > MAX_TRIES_TO_EXECUTE)
                break;
            zookeeper->set(tries_to_execute_path, toString(counter + 1));
            task.ops.push_back(create_shard_flag);
            SCOPE_EXIT_MEMORY({ if (!executed_by_us && !task.ops.empty()) task.ops.pop_back(); });
            
            
            if (tryExecuteQuery(rewritten_query, task, zookeeper))
            {
                executed_by_us = true;
                break;
            }
            lock->unlock();
        }
        
        if (event->tryWait(std::uniform_int_distribution<int>(0, 1000)(rng)))
        {