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.
{
    assert(!initialized);
    setThreadName("DDLWorker");
    LOG_DEBUG(log, "Initializing DDLWorker thread");
    while (!stop_flag)
    {
        try
        {
            auto zookeeper = getAndSetZooKeeper();
            zookeeper->createAncestors(fs::path(queue_dir) / "");
            initialized = true;
            return true;
        }
        catch (const Coordination::Exception & e)
        {
            if (!Coordination::isHardwareError(e.code))
            {
                
                LOG_ERROR(log, "ZooKeeper error: {}. Failed to start DDLWorker.", getCurrentExceptionMessage(true));
                assert(false);  
            }
            tryLogCurrentException(__PRETTY_FUNCTION__);
        }
        catch (...)
        {
            tryLogCurrentException(log, "Cannot initialize DDL queue.");
        }