Why tokio::sync::broadcast doesn't work correctly when the sender is move out of the main thread?

[JuliaAlos]

Hi, I don't understand why when moving the receiver to another thread one of the receivers doesn't receive the message sent at the same time as the others, but instead receives it when the next message is sent. The receiver that does not receive it changes in each iteration. This case is reproduced below:

use std::{thread, time::Duration};

use tokio::sync::broadcast;

#[tokio::main]
async fn main() {
    let tx = broadcast::channel::<usize>(10).0;
    let mut handles = Vec::new();

    for thread in 0..5 {
        let mut rx = tx.subscribe();
        handles.push(tokio::spawn(async move {
            loop {
                let message = rx.recv().await.unwrap();
                println!("Thread {}: {}", thread, message);
            }
        }))
    }
    handles.push(tokio::spawn(async move {
        let mut count = 0;
        loop {
            thread::sleep(Duration::from_secs(5));
            let _ = tx.send(count);
            count += 1;
        }
    }));
    for handle in handles.into_iter() {
        handle.await.unwrap();
    }
}

On the other hand, if the messages are sent from the main thread, they are received without problem by all the receivers. As the following example shows:

use std::{thread, time::Duration};

use tokio::sync::broadcast;

#[tokio::main]
async fn main() {
    let tx = broadcast::channel::<usize>(10).0;
    let mut handles = Vec::new();

    for thread in 0..5 {
        let mut rx = tx.subscribe();
        handles.push(tokio::spawn(async move {
            loop {
                let message = rx.recv().await.unwrap();
                println!("Thread {}: {}", thread, message);
            }
        }))
    }

    let mut count = 0;
    loop {
        thread::sleep(Duration::from_secs(5));
        let _ = tx.send(count);
        count += 1;
    }
}

[Darksonn]

All async code that uses std::thread::sleep is wrong. Read this for more.