Unable to use standard library within audio callback

[nukep]

SDL audio callbacks are called from a native thread created by SDL. This has consequences in Rust.

I recently learned that std always assumes it's being used from a Rust task. Use of std from outside of a Rust task is undefined behavior, and can cause crashes. rust-lang/rust#17485 (comment)

Simply using std::sync::Mutex from within the callback will cause a crash:

fatal runtime error:  assertion failed: !ptr.is_null()

std is fairly ubiquitous, so it'd be preferable to make this work. I see a few solutions:

1. Do nothing. Add a caution in the Rustdocs that warns about this exact issue.
2. Find a way to run a Rust task on the callback thread. I'm unsure if this is possible, though it seems it might be.
3. Recommend use of the queued audio API (Rust bindings non-existent so far) over the callback API, which can be implemented as a std::io::Writer or similar.

Option 2 is what I'll try first. I'll update with my findings.

[AngryLawyer]

You sound like you have a much better handle on this than I do, cheers.