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.
}
        }
        catch (const Coordination::Exception & e)
        {
            if (Coordination::isHardwareError(e.code))
            {
                initialized = false;
                
                if (1 < pool_size)
                    worker_pool = std::make_unique<ThreadPool>(pool_size);
                LOG_INFO(log, "Lost ZooKeeper connection, will try to connect again: {}", getCurrentExceptionMessage(true));
            }
            else
            {
                LOG_ERROR(log, "Unexpected ZooKeeper error, will try to restart main thread: {}", getCurrentExceptionMessage(true));
                reset_state();
            }
            sleepForSeconds(1);
        }
        catch (...)
        {
            tryLogCurrentException(log, "Unexpected error, will try to restart main thread:");
            reset_state();
            sleepForSeconds(5);
        }
    }
}
void DDLWorker::runCleanupThread()
{
    setThreadName("DDLWorkerClnr");