Document or explain thread safety of C bindings

[numinit]

Hey @roberth

I'm experimenting with the C bindings in Rust, and am running into things like this when passing off a value between threads:

error[E0277]: `NonNull<nix_c_raw::value>` cannot be sent between threads safely
   --> src/main.rs:134:5

I've seen this with a couple other types, too. It would help to know what I have to do to get these working between threads, or whether doing so is a good idea. (From what I've seen, you may just have to register the Boehm GC onto the current thread, and squash these errors by adding Send, but I'm also not sure?)

(P.S. I know that the Rust bindings aren't "public" yet, but already got a flake evaluating with them, thanks for the work!)

[RossComputerGuy]

I'm facing similar problems. I am having to wrap things with Rc because Arc does not work, even when wrapping it with a Mutex.

[roberth]

Upstream is working on thread safety, so while it's not working perfectly on the inside just yet, I'd be open to already allowing Send.

[numinit]

Just adding on: a lot of judicious use of clone works just fine 😀

After having more experience with the API surface now, I do particularly like the design where you can clone your way into a value that has had its refcount incremented. Send probably would have been easier, but I got around it in the end with clone.

(Hopefully I'll be able to post up the result quite soon, I yanked all of the Rust bindings from nixops4 for another project, and they feel very intuitive so far.)

Edit: I've got some minor changes to the API surface I'll PR, as well as a few things like this that I'll document since they're important "gotchas" if consuming the Rust bindings, and then close this issue. I think I mostly am able to answer this question now...