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.
#include <Storages/StorageReplicatedMergeTree.h>
#include <Poco/Timestamp.h>
#include <common/sleep.h>
#include <common/getFQDNOrHostName.h>
#include <common/logger_useful.h>
#include <random>
#include <pcg_random.hpp>
#include <common/scope_guard_safe.h>
#include <Interpreters/ZooKeeperLog.h>
namespace fs = std::filesystem;
namespace DB
{
namespace ErrorCodes
{
    extern const int LOGICAL_ERROR;
    extern const int TIMEOUT_EXCEEDED;
    extern const int UNFINISHED;
    extern const int NOT_A_LEADER;
    extern const int KEEPER_EXCEPTION;
    extern const int CANNOT_ASSIGN_ALTER;
    extern const int CANNOT_ALLOCATE_MEMORY;
    extern const int MEMORY_LIMIT_EXCEEDED;
}
constexpr const char * TASK_PROCESSED_OUT_REASON = "Task has been already processed";
namespace
{
/** Caveats: usage of locks in ZooKeeper is incorrect in 99% of cases,
  *  and highlights your poor understanding of distributed systems.
  *
  * It's only correct if all the operations that are performed under lock