Why does this compile? (Possible unsoundness?)

[SUPERCILEX]

Version

tokio v1.17.0
rustc 1.61.0-nightly (76d770ac2 2022-04-02)

Description

async fn test() {
    let s = UdpSocket::bind("0.0.0.0:12235").await.unwrap();
    let mut rng = thread_rng();

    loop {
        let mut bytes = [0; 10];
        let (len, addr) = s.recv_from(&mut bytes).await.unwrap();
        s.send_to(&bytes[..rng.gen::<usize>() % len], addr)
            .await
            .unwrap();
    }
}

I'm probably missing something, but I feel like this shouldn't compile. ThreadRng is not Send so shouldn't it fail to cross the await point? I thought you could end up on any thread after hitting an await which would break the ThreadRng.

[Darksonn]

It's only an issue once you actually spawn it on a runtime on which it can be moved across threads. So, the error will happen at the tokio::spawn call.

If you only use it from your #[tokio::main], then it will compile because the main function cannot be moved across threads.

[SUPERCILEX]

Ohhh, interesting. I found this code. As I understand it, await calls cause the main thread to park/block/sleep/whatever until the reactor presumably unparks it with the waker once things are ready. Is my understanding correct?

Appendix: repro

fn root1() {
    let runtime = tokio::runtime::Builder::new_multi_thread().build().unwrap();
    runtime.block_on(test()); // WORKS
}

fn root2() {
    let runtime = tokio::runtime::Builder::new_multi_thread().build().unwrap();
    runtime.block_on(async {
        tokio::spawn(test()); // FAILS
    });
}

async fn test() {
    let s = UdpSocket::bind("0.0.0.0:12235").await.unwrap();
    let mut rng = thread_rng();

    loop {
        let mut bytes = [0; 10];
        let (len, addr) = s.recv_from(&mut bytes).await.unwrap();
        s.send_to(&bytes[..rng.gen::<usize>() % len], addr)
            .await
            .unwrap();
    }
}