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.
bool status_written = task.ops.empty();
    if (!status_written)
    {
        zookeeper->multi(task.ops);
        task.ops.clear();
    }
    
    active_node->setAlreadyRemoved();
    task.completely_processed = true;
}
bool DDLWorker::taskShouldBeExecutedOnLeader(const ASTPtr & ast_ddl, const StoragePtr storage)
{
    
    if (auto * query = ast_ddl->as<ASTDropQuery>(); query && query->kind != ASTDropQuery::Kind::Truncate)
        return false;
    if (!ast_ddl->as<ASTAlterQuery>() && !ast_ddl->as<ASTOptimizeQuery>() && !ast_ddl->as<ASTDropQuery>())
        return false;
    if (auto * alter = ast_ddl->as<ASTAlterQuery>())
    {
        
        if (alter->isSettingsAlter())
            return false;
        if (alter->isFreezeAlter())
            return false;
    }
    return storage->supportsReplication();
}
bool DDLWorker::tryExecuteQueryOnLeaderReplica(