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.
worker_pool->scheduleOrThrowOnError([this, &saved_task, zookeeper]()
            {
                setThreadName("DDLWorkerExec");
                processTask(saved_task, zookeeper);
            });
        }
        else
        {
            processTask(saved_task, zookeeper);
        }
    }
}
DDLTaskBase & DDLWorker::saveTask(DDLTaskPtr && task)
{
    current_tasks.remove_if([](const DDLTaskPtr & t) { return t->completely_processed.load(); });
    
    assert((worker_pool != nullptr) == (1 < pool_size));
    
    
    assert(worker_pool || current_tasks.empty());
    
    
    
    assert(!worker_pool || current_tasks.size() <= pool_size);
    current_tasks.emplace_back(std::move(task));
    if (first_failed_task_name && *first_failed_task_name == current_tasks.back()->entry_name)
        first_failed_task_name.reset();
    return *current_tasks.back();
}
bool DDLWorker::tryExecuteQuery(const String & query, DDLTaskBase & task, const ZooKeeperPtr & zookeeper)