For GenSlab the situation is less clear, as there probably are ways to make it work, but I'd vote to keep it simple for now, and simply do elog(ERROR) in the realloc() methods - both for Slab and GenSlab. The current use case (reorderbuffer) does not need that, and it seems like a can of worms to me.
Good plan. Realloc can be added later if there is an actual use case.
