Using threads in FDW for read-ahead

I have created a FDW for a storage backend and it is working well, and now
I would like to overlap processing with I/O by performing read-ahead. I
started by using a thread to do background I/O and this worked, but
problems started to arise when I tried to do predicate filtering in the
thread.

In particular, it seems as though `check_stack_depth` is built to assume a
single threaded environment (`stack_base_ptr` is global).

I'm wondering if there is a solution to this problem, or if there are
examples of overlapping tuple I/O and predicate filtering using
non-multithreading techniques?

Thanks,
Noah