add receiver_count to sync::watch::Sender

[soerenmeier]

Add a method to sync::watch::Sender to be able to check how many receivers currently exist.

Motivation

I wanted to be able to get a new Receiver from a Sender and stop sending if there aren't any receivers. To get a new Receiver however i need to store one (a Sender cannot create a Receiver). Which makes it impossible with the current methods to detect if the channel has no active receivers left.

Example

struct Sender<T> {
    tx: watch::Sender<T>,
    rx: watch::Receiver<T>
}

impl<T> Sender<T> {
    pub fn subscribe(&self) -> watch::Receiver<T> {
       self.rx.clone()
   }
    /// Returns Err(()) if no receivers exist
    pub fn send(&mut self, msg: T) -> Result<(), ()> {
        if self.tx.receiver_count() <= 1 {
            Err(())
        } else {
            self.sender.send(msg).unwrap();
            Ok(())
        }
    }
}

Solution

Adding receiver_count seams to be the smallest change that solves my problem.
However if your open to it and feel that watch::Sender could benefit from subscribe i can add that to this PR.

[soerenmeier]

As it seems, I'm probably not the only one wanting watch::Sender::subscribe see #3757.
Would it be possible to expose Sender::subscribe since it's already implemented as @arthurprs pointed out.

tokio/tokio/src/sync/watch.rs

Lines 413 to 418 in d4075a4

	pub(crate) fn subscribe(&self) -> Receiver<T> {
	let shared = self.shared.clone();
	let version = shared.version.load(SeqCst);
	Receiver::from_shared(version, shared)
	}

Or are there any concerns exposing this?

[Darksonn]

I don't have time to review until friday.

[Darksonn]

This seems fine to me.