Channel sender downgrades not behaving as expected

[shutton]

Using the .downgrade() method to prevent a sender from keeping an MPSC receiver channel doesn't appear to be working as I'd expect. Take the following example:

async fn completes() {
    let (sender, mut reader) = tokio::sync::mpsc::unbounded_channel::<i32>();

    {
        let sender = sender.clone();
        tokio::spawn(async move {
            sender.send(1).unwrap();
        });
    }

    {
        let sender = sender.clone();
        tokio::spawn(async move {
            sender.send(2).unwrap();
        });
    }

    drop(sender);
    while let Some(msg) = reader.recv().await {
        dbg!(msg);
    }
    eprintln!("done");
}

async fn hangs() {
    let (sender, mut reader) = tokio::sync::mpsc::unbounded_channel::<i32>();

    {
        let sender = sender.clone();
        tokio::spawn(async move {
            sender.send(3).unwrap();
        });
    }

    {
        let sender = sender.clone();
        tokio::spawn(async move {
            sender.send(4).unwrap();
        });
    }

    let _ = sender.downgrade();
    while let Some(msg) = reader.recv().await {
        dbg!(msg);
    }
    eprintln!("never done");
}

#[tokio::main]
async fn main() {
    completes().await;
    hangs().await;
}

Output

[src/main.rs:20] msg = 2
[src/main.rs:20] msg = 1
done
[src/main.rs:44] msg = 3
[src/main.rs:44] msg = 4

Dropping the sender works as expected (reader.recv() eventually returns None). However, downgrading the sender and holding onto the downgraded instance doesn't permit the channel to close, as would be expected from the documentation.

This is with tokio v1.32.0

[Darksonn]

The downgrade method takes &self, so the original sender is still there keeping the channel alive.

(By the way, let _ = drops it immediately.)

[shutton]

Ah, I didn't catch that subtlety. This fixes it:

    let weak_sender = sender.downgrade();
    drop(sender);

Thanks for the insight.