Tokio select! Macro Issue with Barrier

[suxb201]

Version
tokio v1.37.0
tokio-macros v2.2.0 (proc-macro)

Platform
Darwin Kernel Version 23.0.0

Description
The Barrier implementation might be incrementing the state.generation field every time the select! is called.

I tried this code:

use tokio;
use std::sync::Arc;
use tokio::select;
use tokio::time::sleep;
use tokio::sync::Barrier;
use tokio::task::JoinSet;

#[tokio::main]
async fn main() {
    let barrier = Arc::new(Barrier::new(2));
    let mut tasks = JoinSet::new();
    tasks.spawn({
        let barrier = barrier.clone();
        async move {
            loop {
                sleep(std::time::Duration::from_secs(1)).await;
                barrier.wait().await;
            }
        }
    });
    tasks.spawn(async move {
        let mut interval = tokio::time::interval(std::time::Duration::from_secs(1));
        loop {
            select! {
                _ = interval.tick() => {
                    println ! ("interval tick");
                }
                _ = barrier.wait() => {
                    println ! ("barrier wait");
                }
            }
        }
    });
    for _ in 0..2 {
        tasks.join_next().await;
    }
}

I expected to see this happen:

interval tick
barrier wait
interval tick
barrier wait
interval tick
barrier wait
...

Instead, this happened:

interval tick
barrier wait
barrier wait
interval tick
barrier wait
barrier wait
interval tick
barrier wait
barrier wait
...

[Alice Ryhl]

Thank you for reporting this. Barriers are not cancel safe, so you can't use them in a tokio::select! like that. Unfortunately, fixing this would take a breaking change.

I have opened #6494 to document this issue.