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.
}
        else 
        {
            LOG_WARNING(log, "Task {} was not executed by anyone, maximum number of retries exceeded", task.entry_name);
            task.execution_status = ExecutionStatus(ErrorCodes::UNFINISHED, "Cannot execute replicated DDL query, maximum retries exceeded");
        }
        return false;
    }
    if (executed_by_us)
        LOG_DEBUG(log, "Task {} executed by current replica", task.entry_name);
    else 
        LOG_DEBUG(log, "Task {} has already been executed by replica ({}) of the same shard.", task.entry_name, zookeeper->get(is_executed_path));
    return true;
}
void DDLWorker::cleanupQueue(Int64, const ZooKeeperPtr & zookeeper)
{
    LOG_DEBUG(log, "Cleaning queue");
    Strings queue_nodes = zookeeper->getChildren(queue_dir);
    filterAndSortQueueNodes(queue_nodes);
    for (auto it = queue_nodes.cbegin(); it < queue_nodes.cend(); ++it)
    {
        if (stop_flag)
            return;
        String node_name = *it;
        String node_path = fs::path(queue_dir) / node_name;
        Coordination::Stat stat;
        String dummy;
        try
        {