/
lib.rs
191 lines (171 loc) · 5.87 KB
/
lib.rs
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use futures::{future, ready, Stream};
use std::sync::{Arc, Weak};
use std::task::{Context, Poll};
use std::{fmt, future::Future, mem, pin::Pin};
use tokio::sync::{mpsc, OwnedSemaphorePermit as Permit, Semaphore};
use self::error::{SendError, TrySendError};
pub use tokio::sync::mpsc::error;
/// Returns a new pollable, bounded MPSC channel.
///
/// Unlike `tokio::sync`'s `MPSC` channel, this channel exposes a `poll_ready`
/// function, at the cost of an allocation when driving it to readiness.
pub fn channel<T>(buffer: usize) -> (Sender<T>, Receiver<T>) {
assert!(buffer > 0, "mpsc bounded channel requires buffer > 0");
let semaphore = Arc::new(Semaphore::new(buffer));
let (tx, rx) = mpsc::unbounded_channel();
let rx = Receiver {
rx,
semaphore: Arc::downgrade(&semaphore),
buffer,
};
let tx = Sender {
tx,
semaphore,
state: State::Empty,
};
(tx, rx)
}
/// A bounded, pollable MPSC sender.
///
/// This is similar to Tokio's bounded MPSC channel's `Sender` type, except that
/// it exposes a `poll_ready` function, at the cost of an allocation when
/// driving it to readiness.
pub struct Sender<T> {
tx: mpsc::UnboundedSender<(T, Permit)>,
semaphore: Arc<Semaphore>,
state: State,
}
/// A bounded MPSC receiver.
///
/// This is similar to Tokio's bounded MPSC channel's `Receiver` type.
pub struct Receiver<T> {
rx: mpsc::UnboundedReceiver<(T, Permit)>,
semaphore: Weak<Semaphore>,
buffer: usize,
}
enum State {
Waiting(Pin<Box<dyn Future<Output = Permit> + Send + Sync>>),
Acquired(Permit),
Empty,
}
impl<T> Sender<T> {
pub fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), SendError<()>>> {
loop {
self.state = match self.state {
State::Empty => State::Waiting(Box::pin(self.semaphore.clone().acquire_owned())),
State::Waiting(ref mut f) => State::Acquired(ready!(Pin::new(f).poll(cx))),
State::Acquired(_) if self.tx.is_closed() => {
return Poll::Ready(Err(SendError(())))
}
State::Acquired(_) => return Poll::Ready(Ok(())),
}
}
}
pub async fn ready(&mut self) -> Result<(), SendError<()>> {
future::poll_fn(|cx| self.poll_ready(cx)).await
}
pub fn try_send(&mut self, value: T) -> Result<(), TrySendError<T>> {
if self.tx.is_closed() {
return Err(TrySendError::Closed(value));
}
self.state = match mem::replace(&mut self.state, State::Empty) {
// Have we previously acquired a permit?
State::Acquired(permit) => {
self.send2(value, permit);
return Ok(());
}
// Okay, can we acquire a permit now?
State::Empty => {
if let Ok(permit) = self.semaphore.clone().try_acquire_owned() {
self.send2(value, permit);
return Ok(());
}
State::Empty
}
state => state,
};
Err(TrySendError::Full(value))
}
pub async fn send(&mut self, value: T) -> Result<(), SendError<T>> {
if let Err(_) = self.ready().await {
return Err(SendError(value));
}
match mem::replace(&mut self.state, State::Empty) {
State::Acquired(permit) => {
self.send2(value, permit);
Ok(())
}
state => panic!("unexpected state after poll_ready: {:?}", state),
}
}
fn send2(&mut self, value: T, permit: Permit) {
self.tx.send((value, permit)).ok().expect("was not closed");
}
}
impl<T> Clone for Sender<T> {
fn clone(&self) -> Self {
Self {
tx: self.tx.clone(),
semaphore: self.semaphore.clone(),
state: State::Empty,
}
}
}
impl<T> fmt::Debug for Sender<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Sender")
.field("message_type", &std::any::type_name::<T>())
.field("state", &self.state)
.field("semaphore", &self.semaphore)
.finish()
}
}
// === impl Receiver ===
impl<T> Receiver<T> {
pub async fn recv(&mut self) -> Option<T> {
self.rx.recv().await.map(|(t, _)| t)
}
pub fn poll_recv(&mut self, cx: &mut Context<'_>) -> Poll<Option<T>> {
let res = ready!(Pin::new(&mut self.rx).poll_next(cx));
Poll::Ready(res.map(|(t, _)| t))
}
}
impl<T> Stream for Receiver<T> {
type Item = T;
fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
let res = ready!(Pin::new(&mut self.as_mut().rx).poll_next(cx));
Poll::Ready(res.map(|(t, _)| t))
}
}
impl<T> Drop for Receiver<T> {
fn drop(&mut self) {
if let Some(semaphore) = self.semaphore.upgrade() {
// Close the buffer by releasing any senders waiting on channel capacity.
// If more than `usize::MAX >> 3` permits are added to the semaphore, it
// will panic.
const MAX: usize = std::usize::MAX >> 4;
semaphore.add_permits(MAX - self.buffer - semaphore.available_permits());
}
}
}
impl<T> fmt::Debug for Receiver<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Receiver")
.field("message_type", &std::any::type_name::<T>())
.field("semaphore", &self.semaphore)
.finish()
}
}
// === impl State ===
impl fmt::Debug for State {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Display::fmt(
match self {
State::Acquired(_) => "State::Acquired(..)",
State::Waiting(_) => "State::Waiting(..)",
State::Empty => "State::Empty",
},
f,
)
}
}