GC corruption when used with either Async or Lwt

[mbacarella]

I've used Fftw3 on two separate projects now, one with Async and one with Lwt where I end up with heap corruption and had to move the FFT code into a separate sub-process to protect my runtime.

The Fftw3 C library uses threads internally so I'm guessing it has to do with tripping over Async/Lwt thread manipulations though they should theoretically be able to co-exist.

I'm aware of this https://www.fftw.org/fftw3_doc/Thread-safety.html but I'm pretty sure I'm not doing the things it's warning about. I'm creating and executing my Fftw3 calls all from the same thread.

Sorry I don't have a good repro here, I only need FFT to help me with an algorithm design right now and I resorted to dropping this simpler, possibly slower pure-OCaml implementation to make progress: https://github.com/ColinShaw/ocaml-simple-fft

If I use Fftw3 in shipping Lwt/Async code I'll circle back and try to hammer this out. Wanted to raise the possibility since I struggled with this alone for awhile first.