tokio::time::interval is not working

[rcitsz]

Version

root@6a2e14c7eff5:~/projects/rustprj/rs-proxy/server# cargo version
cargo 1.78.0 (54d8815d0 2024-03-26)

root@6a2e14c7eff5:~/projects/rustprj/rs-proxy/server# cargo tree | grep tokio
│   │   │   │   │   │   ├── tokio v1.37.0
│   │   │   │   │   │   │   └── tokio-macros v2.2.0 (proc-macro)
│   │   │   │   │   │   ├── tokio-util v0.7.10
│   │   │   │   │   │   │   ├── tokio v1.37.0 (*)
│   │   │   │   │   ├── tokio v1.37.0 (*)
│   │   │   │   │   ├── tokio v1.37.0 (*)
│   │   │   │   │   ├── tokio-util v0.7.10 (*)
│   │   │   │   ├── tokio v1.37.0 (*)
│   │   │   │   └── tokio-io-timeout v1.2.0
│   │   │   │       └── tokio v1.37.0 (*)
│   │   │   ├── tokio v1.37.0 (*)
│   │   │   ├── tokio-stream v0.1.15
│   │   │   │   └── tokio v1.37.0 (*)
│   ├── tokio v1.37.0 (*)
│   ├── tokio-stream v0.1.15 (*)
│   │   ├── tokio v1.37.0 (*)
│   │   └── tokio-native-tls v0.3.1
│   │       └── tokio v1.37.0 (*)
│   ├── tokio v1.37.0 (*)
│   ├── tokio-native-tls v0.3.1 (*)
├── tokio v1.37.0 (*)
├── tokio-rustls v0.24.1
│   └── tokio v1.37.0 (*)

Platform

root@6a2e14c7eff5:~/projects/rustprj/rs-proxy/server# uname -a
Linux 6a2e14c7eff5 6.6.16-linuxkit #1 SMP Fri Feb 16 11:54:02 UTC 2024 aarch64 aarch64 aarch64 GNU/Linux

Description

Test code is as follows:

#[test]
fn test() {
    tokio::runtime::Runtime::new().unwrap().block_on(async {
        tokio::spawn(async move {
            let mut timer = tokio::time::interval(tokio::time::Duration::from_secs(1));
            let mut s = true; 
            loop {
                tokio::select! {
                    _= timer.tick() => {
                       println!("-----tick");
                    }
                 _= async {
                    if s {
                        s = false; // set off
                        tokio::time::sleep(tokio::time::Duration::from_secs(3)).await;  //  If comment out this line, the timer will work properly
                    }
                 } => {}
                }
            }
        })
        .await
        .unwrap();
    });
}

I expect to output the following content every second, but it doesn't work:
-----tick
-----tick
-----tick
-----tick
-----tick
-----tick
...

[mox692]

After the completion of the first loop, subsequent loops are almost always expected to terminate the second arm first, so the tick print will not continue as you expected.

[Darksonn]

You need to use an if precondition instead of placing the if inside the async block. That way, the branch will be disabled instead of triggering immediately.

See the documentation on select! for more info on if preconditions.

[rcitsz]

@Darksonn
Thanks!
But in another form.
If the select arm is not async block but async function：

async fn waitfor(s: &mut bool) {
    if *s {
        *s = false;

       //   If comment out this line, the timer will work properly
        tokio::time::sleep(tokio::time::Duration::from_secs(3)).await;
    }
}

#[test]
fn test() {
    tokio::runtime::Runtime::new().unwrap().block_on(async {
        tokio::spawn(async move {
            let mut timer = tokio::time::interval(tokio::time::Duration::from_secs(1));
            let mut s = true; 
            loop {
                tokio::select! {
                    _= timer.tick() => {
                       println!("----tick");
                    }
                _ = waitfor(&mut s) => {}
                }
            }
        })
        .await
        .unwrap();
    });
}

[Darksonn]

You can also await std::future::pending() to have waitfor sleep forever / until timer.tick() triggers.