Run native blocking code

I'm trying out shuttle for https://github.com/anacrolix/possum. In some tests that are multithreaded, I need to call in to rusqlite code which then calls into C code which blocks the calling thread. After lots of trial and error I think I discovered that shuttle is not using native threads under the hood but its own kind of green threading. This doesn't play well with calling into code that hasn't been instrumented with shuttle primitives.

How can I call into that code? I was going to create a wrapper that spawned a native thread to run blocking code in rusqlite, then wait on the result from a shuttle mpsc channel but then I realised I can't send the result into a shuttle channel from a thread that wasn't created by shuttle.

The issue names no repository files or tests. Start by tracing Shuttle's native-thread and mpsc-channel behavior alongside the rusqlite call path; done means there is a documented, supported way to call blocking native code and return its result from a Shuttle test.

Written by the indexing model from the issue text.