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.
const zkutil::ZooKeeperPtr & zookeeper, const String & lock_prefix, const String & lock_name, const String & lock_message)
{
    return std::make_unique<ZooKeeperLock>(zookeeper, lock_prefix, lock_name, lock_message);
}
}
DDLWorker::DDLWorker(
    int pool_size_,
    const std::string & zk_root_dir,
    ContextPtr context_,
    const Poco::Util::AbstractConfiguration * config,
    const String & prefix,
    const String & logger_name,
    const CurrentMetrics::Metric * max_entry_metric_,
    const CurrentMetrics::Metric * max_pushed_entry_metric_)
    : context(Context::createCopy(context_))
    , log(&Poco::Logger::get(logger_name))
    , pool_size(pool_size_)
    , max_entry_metric(max_entry_metric_)
    , max_pushed_entry_metric(max_pushed_entry_metric_)
{
    if (max_entry_metric)
        CurrentMetrics::set(*max_entry_metric, 0);
    if (max_pushed_entry_metric)
        CurrentMetrics::set(*max_pushed_entry_metric, 0);