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.
LOG_WARNING(log, "Task {} is executed, but looks like entry {} was deleted, cannot write status", task.entry_name, task.entry_path);
                task.completely_processed = true;
                return;
            }
            createStatusDirs(task.entry_path, zookeeper);
        }
        if (create_active_res == Coordination::Error::ZNODEEXISTS)
        {
            
            
            zkutil::EventPtr eph_node_disappeared = std::make_shared<Poco::Event>();
            String dummy;
            if (zookeeper->tryGet(active_node_path, dummy, nullptr, eph_node_disappeared))
            {
                constexpr int timeout_ms = 60 * 1000;
                if (!eph_node_disappeared->tryWait(timeout_ms))
                    throw Exception(ErrorCodes::LOGICAL_ERROR, "Ephemeral node {} still exists, "
                                    "probably it's owned by someone else", active_node_path);
            }
        }
        zookeeper->create(active_node_path, {}, zkutil::CreateMode::Ephemeral);
    }
    
    if (!task.was_executed)
    {