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, "Task {} has already been executed by replica ({}) of the same shard.", task.entry_name, zookeeper->get(is_executed_path));
            executed_by_other_leader = true;
            break;
        }
        else
        {
            String tries_count;
            zookeeper->tryGet(tries_to_execute_path, tries_count);
            if (parse<int>(tries_count) > MAX_TRIES_TO_EXECUTE)
            {
                
                LOG_WARNING(log, "Maximum retries count for task {} exceeded, cannot execute replicated DDL query", task.entry_name);
                break;
            }
            else
            {
                
                LOG_TRACE(log, "Task {} still not executed, will try to wait for it or execute ourselves, tries count {}", task.entry_name, tries_count);
            }
        }
    }
    assert(!(executed_by_us && executed_by_other_leader));
    
    if (!executed_by_us && !executed_by_other_leader)
    {
        
        if (stopwatch.elapsedSeconds() >= MAX_EXECUTION_TIMEOUT_SEC)
        {
            LOG_WARNING(log, "Task {} was not executed by anyone, maximum timeout {} seconds exceeded", task.entry_name, MAX_EXECUTION_TIMEOUT_SEC);
            task.execution_status = ExecutionStatus(ErrorCodes::TIMEOUT_EXCEEDED, "Cannot execute replicated DDL query, timeout exceeded");