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use std::pin::Pin;
use tokio::sync::broadcast::error::RecvError;
use tokio::sync::broadcast::Receiver;
use futures_core::Stream;
use tokio_util::sync::ReusableBoxFuture;
use std::fmt;
use std::task::{Context, Poll};
/// A wrapper around [`tokio::sync::broadcast::Receiver`] that implements [`Stream`].
///
/// [`tokio::sync::broadcast::Receiver`]: struct@tokio::sync::broadcast::Receiver
/// [`Stream`]: trait@crate::Stream
#[cfg_attr(docsrs, doc(cfg(feature = "sync")))]
pub struct BroadcastStream<T> {
inner: ReusableBoxFuture<'static, (Result<T, RecvError>, Receiver<T>)>,
}
/// An error returned from the inner stream of a [`BroadcastStream`].
#[derive(Debug, PartialEq, Eq, Clone)]
pub enum BroadcastStreamRecvError {
/// The receiver lagged too far behind. Attempting to receive again will
/// return the oldest message still retained by the channel.
///
/// Includes the number of skipped messages.
Lagged(u64),
}
impl fmt::Display for BroadcastStreamRecvError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self {
BroadcastStreamRecvError::Lagged(amt) => write!(f, "channel lagged by {}", amt),
}
}
}
impl std::error::Error for BroadcastStreamRecvError {}
async fn make_future<T: Clone>(mut rx: Receiver<T>) -> (Result<T, RecvError>, Receiver<T>) {
let result = rx.recv().await;
(result, rx)
}
impl<T: 'static + Clone + Send> BroadcastStream<T> {
/// Create a new `BroadcastStream`.
pub fn new(rx: Receiver<T>) -> Self {
Self {
inner: ReusableBoxFuture::new(make_future(rx)),
}
}
}
impl<T: 'static + Clone + Send> Stream for BroadcastStream<T> {
type Item = Result<T, BroadcastStreamRecvError>;
fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
let (result, rx) = ready!(self.inner.poll(cx));
self.inner.set(make_future(rx));
match result {
Ok(item) => Poll::Ready(Some(Ok(item))),
Err(RecvError::Closed) => Poll::Ready(None),
Err(RecvError::Lagged(n)) => {
Poll::Ready(Some(Err(BroadcastStreamRecvError::Lagged(n))))
}
}
}
}
impl<T> fmt::Debug for BroadcastStream<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("BroadcastStream").finish()
}
}
impl<T: 'static + Clone + Send> From<Receiver<T>> for BroadcastStream<T> {
fn from(recv: Receiver<T>) -> Self {
Self::new(recv)
}
}
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