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_DEBUG(log, "Processing task {} ({})", task.entry_name, task.entry.query);
    assert(!task.completely_processed);
    String active_node_path = task.getActiveNodePath();
    String finished_node_path = task.getFinishedNodePath();
    
    
    
    
    auto active_node = zkutil::EphemeralNodeHolder::existing(active_node_path, *zookeeper);
    
    auto create_active_res = zookeeper->tryCreate(active_node_path, {}, zkutil::CreateMode::Ephemeral);
    if (create_active_res != Coordination::Error::ZOK)
    {
        if (create_active_res != Coordination::Error::ZNONODE && create_active_res != Coordination::Error::ZNODEEXISTS)
        {
            assert(Coordination::isHardwareError(create_active_res));
            throw Coordination::Exception(create_active_res, active_node_path);
        }
        
        if (create_active_res == Coordination::Error::ZNONODE)
        {
            assert(dynamic_cast<DatabaseReplicatedTask *>(&task) == nullptr);
            if (task.was_executed)
            {