Why isn't there a watch::Sender::new in tokio::sync

help
6

I actually ran into a problem when I worked with Senders that have no Receiver. Consider the following example:

use tokio::sync::watch;

#[allow(unused_variables)]
fn main() {
    let (sender, receiver) = watch::channel("initial");
    //drop(receiver); // try to uncomment
    sender.send("updated").ok();
    let receiver2 = sender.subscribe();
    let s = *receiver2.borrow();
    assert_eq!(s, "updated"); // fails if `receiver` is dropped before `send`
}

(Playground)

Apparently the Sender doesn't retain the last value if it was sent when there was no Receiver. Is this intended?

The current behavior is kind-of documented here:

pub fn send(&self, value: T) -> Result<(), SendError>

Sends a new value via the channel, notifying all receivers.

This method fails if the channel has been closed, which happens when every receiver has been dropped.

It's not 100% clear what "fails" means in this context (i.e. returning Err(_) or failing to update the value). Either way, I think it would be helpful if the sent value was retained for a future subscriber.

Due to the current behavior, I have to keep a "dummy" Receiver in my code:

pub struct Receiver<T> {
    watch: watch::Sender<Option<broadcast::Sender<Message<T>>>>,
    _dummy: watch::Receiver<Option<broadcast::Sender<Message<T>>>>,
}

impl<T> Receiver<T>
where
    T: Clone,
{
    pub fn new() -> Self {
        let (sender, receiver) = watch::channel(None);
        Self {
            watch: sender,
            _dummy: receiver,
        }
    }
}

Update: Using Sender::send_replace is a simpler solution than keeping a dummy receiver around.