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use std::sync::{Arc, Mutex};
use futures::{Stream, future, stream::StreamExt as _};
use tokio::sync::broadcast::{Sender, channel};
use tokio_stream::wrappers::{BroadcastStream, errors::BroadcastStreamRecvError};
// Clones will share the same sender.
#[derive(Clone)]
pub struct Broadcaster<M> {
// The use of std::sync::Mutex, and not tokio::sync::Mutex, follows Tokio's
// own advice: <https://tokio.rs/tokio/tutorial/shared-state>. Methods that
// lock it must be sync.
senders: Arc<Mutex<Sender<M>>>,
}
impl<M> Default for Broadcaster<M>
where
M: Clone + Send + std::fmt::Debug + 'static,
{
fn default() -> Self {
let sender = Self::make_sender();
Self {
senders: Arc::new(Mutex::new(sender)),
}
}
}
impl<M> Broadcaster<M>
where
M: Clone + Send + std::fmt::Debug + 'static,
{
// panic: if ``message.channel.id`` has not been previously registered,
// and was not part of the initial set of channels.
pub fn broadcast(&self, message: impl Into<M>) {
let tx = self.sender();
// Per the Tokio docs, the returned error is only used to indicate that
// there are no receivers. In this use case, that's fine; a lack of
// listening consumers (chat clients) when a message is sent isn't an
// error.
//
// The successful return value, which includes the number of active
// receivers, also isn't that interesting to us.
let _ = tx.send(message.into());
}
// panic: if ``channel`` has not been previously registered, and was not
// part of the initial set of channels.
pub fn subscribe(&self) -> impl Stream<Item = M> + std::fmt::Debug + use<M> {
let rx = self.sender().subscribe();
BroadcastStream::from(rx).scan((), |(), r| {
// The following could technically be `r.ok()`, and is exactly
// equivalent to it, but spelling out the match arms means we'll
// find out at compile time if new errors get added to
// `BroadcastStreamRecvError`.
#[allow(clippy::manual_ok_err)]
future::ready(match r {
Ok(event) => Some(event),
// Stop the stream here. This will disconnect SSE clients
// (see `routes.rs`), who will then resume from
// `Last-Event-ID`, allowing them to catch up by reading
// the skipped messages from the database.
//
// See also:
// <https://users.rust-lang.org/t/taking-from-stream-while-ok/48854>
Err(BroadcastStreamRecvError::Lagged(_)) => None,
})
})
}
fn sender(&self) -> Sender<M> {
self.senders.lock().unwrap().clone()
}
fn make_sender() -> Sender<M> {
// Queue depth of 16 chosen entirely arbitrarily. Don't read too much
// into it.
let (tx, _) = channel(16);
tx
}
}
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