Setting a custom RdKafka::PartitionerCb doesn't work in certain circumstances

[wmorgan6796]

Librdkafka Version: 2.11.1
GCC Version: 15.2
OS: Ubuntu 20.04

Bug:

When I set a RdKafka::PartitionerCb* on the default topic configuration that correct gets set on the C++ class, but never gets set on the underlying rkt conf struct. This means when the Default Topic Configuration is added to the Global Conf via

Conf::ConfResult set(const std::string &name,
                       const Conf *topic_conf,
                       std::string &errstr)

the internal call to rd_kafka_topic_conf_dup loses the pointer to the RdKafka::PartitionerCb* that was set in the default Topic Conf prior.

This means when we're calling a produce call that doesn't instantiate a RdKafka::Topic from the given Topic Conf, we will see the partitioner_cb_ ignored and we will find that it will instead use the default consistent_random partitioner (since it sees the underlying partitioner in the default rkt as NULL

Potential Fix:

When duplicating the default topic conf in the C++ function, you need to set the trampoline partitioner_cb_trampoline to the partitioner so that the C++ child class correctly gets called. The issue I can't seem to solve is how to get a pointer to the partitioner_cb_ into the *rkt_opaque

I think at a minimum there needs to be a note that unless you're using a produce call that takes a RdKafka::Topic*, you need to utilize the C API to set a custom partitioner.