On kafka_topic_map_ and destructors

[javierguerragiraldez]

Hi.

As far as I can tell, the kafka_topic_map_ table is used only to relate the topics created with each producer, so that they can be destroyed before the producer. but:

1. it uses the rd_kafka_t cdata object as the table key; but that's specifically advised against in the FFI docs:

Lua tables may be indexed by cdata objects, but this doesn't provide any useful semantics — cdata objects are unsuitable as table keys!

I guess in this specific case it's workable, since the only library function that returns a rd_kafka_t pointer is the creator, so there shouldn't be two different cdata objects with the same content. Still, it's a fragile situation.

2. kafka_topic_map_ is a regular, strong, Lua table; so the rd_kafka_t cdata objects registered there won't be GC'ed, even if the producer Lua wrapper is collected. Effectively, it's made useless by its own existence.