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.
{
    String node_data;
    String entry_path = fs::path(queue_dir) / entry_name;
    auto task = std::make_unique<DDLTask>(entry_name, entry_path);
    if (!zookeeper->tryGet(entry_path, node_data))
    {
        
        out_reason = "The task was deleted";
        return {};
    }
    auto write_error_status = [&](const String & host_id, const String & error_message, const String & reason)
    {
        LOG_ERROR(log, "Cannot parse DDL task {}: {}. Will try to send error status: {}", entry_name, reason, error_message);
        createStatusDirs(entry_path, zookeeper);
        zookeeper->tryCreate(fs::path(entry_path) / "finished" / host_id, error_message, zkutil::CreateMode::Persistent);
    };
    try
    {
        
        task->entry.parse(node_data);
    }
    catch (...)
    {
        
        
        
        out_reason = "Incorrect task format";
        write_error_status(host_fqdn_id, ExecutionStatus::fromCurrentException().serializeText(), out_reason);
        return {};
    }