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use futures_sink::Sink;
use std::pin::Pin;
use std::task::{Context, Poll};
use std::{fmt, mem};
use tokio::sync::mpsc::OwnedPermit;
use tokio::sync::mpsc::Sender;
use super::ReusableBoxFuture;
/// Error returned by the `PollSender` when the channel is closed.
#[derive(Debug)]
pub struct PollSendError<T>(Option<T>);
impl<T> PollSendError<T> {
/// Consumes the stored value, if any.
///
/// If this error was encountered when calling `start_send`/`send_item`, this will be the item
/// that the caller attempted to send. Otherwise, it will be `None`.
pub fn into_inner(self) -> Option<T> {
self.0
}
}
impl<T> fmt::Display for PollSendError<T> {
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(fmt, "channel closed")
}
}
impl<T: fmt::Debug> std::error::Error for PollSendError<T> {}
#[derive(Debug)]
enum State<T> {
Idle(Sender<T>),
Acquiring,
ReadyToSend(OwnedPermit<T>),
Closed,
}
/// A wrapper around [`mpsc::Sender`] that can be polled.
///
/// [`mpsc::Sender`]: tokio::sync::mpsc::Sender
#[derive(Debug)]
pub struct PollSender<T> {
sender: Option<Sender<T>>,
state: State<T>,
acquire: ReusableBoxFuture<'static, Result<OwnedPermit<T>, PollSendError<T>>>,
}
// Creates a future for acquiring a permit from the underlying channel. This is used to ensure
// there's capacity for a send to complete.
//
// By reusing the same async fn for both `Some` and `None`, we make sure every future passed to
// ReusableBoxFuture has the same underlying type, and hence the same size and alignment.
async fn make_acquire_future<T>(
data: Option<Sender<T>>,
) -> Result<OwnedPermit<T>, PollSendError<T>> {
match data {
Some(sender) => sender
.reserve_owned()
.await
.map_err(|_| PollSendError(None)),
None => unreachable!("this future should not be pollable in this state"),
}
}
impl<T: Send + 'static> PollSender<T> {
/// Creates a new `PollSender`.
pub fn new(sender: Sender<T>) -> Self {
Self {
sender: Some(sender.clone()),
state: State::Idle(sender),
acquire: ReusableBoxFuture::new(make_acquire_future(None)),
}
}
fn take_state(&mut self) -> State<T> {
mem::replace(&mut self.state, State::Closed)
}
/// Attempts to prepare the sender to receive a value.
///
/// This method must be called and return `Poll::Ready(Ok(()))` prior to each call to
/// `send_item`.
///
/// This method returns `Poll::Ready` once the underlying channel is ready to receive a value,
/// by reserving a slot in the channel for the item to be sent. If this method returns
/// `Poll::Pending`, the current task is registered to be notified (via
/// `cx.waker().wake_by_ref()`) when `poll_reserve` should be called again.
///
/// # Errors
///
/// If the channel is closed, an error will be returned. This is a permanent state.
pub fn poll_reserve(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), PollSendError<T>>> {
loop {
let (result, next_state) = match self.take_state() {
State::Idle(sender) => {
// Start trying to acquire a permit to reserve a slot for our send, and
// immediately loop back around to poll it the first time.
self.acquire.set(make_acquire_future(Some(sender)));
(None, State::Acquiring)
}
State::Acquiring => match self.acquire.poll(cx) {
// Channel has capacity.
Poll::Ready(Ok(permit)) => {
(Some(Poll::Ready(Ok(()))), State::ReadyToSend(permit))
}
// Channel is closed.
Poll::Ready(Err(e)) => (Some(Poll::Ready(Err(e))), State::Closed),
// Channel doesn't have capacity yet, so we need to wait.
Poll::Pending => (Some(Poll::Pending), State::Acquiring),
},
// We're closed, either by choice or because the underlying sender was closed.
s @ State::Closed => (Some(Poll::Ready(Err(PollSendError(None)))), s),
// We're already ready to send an item.
s @ State::ReadyToSend(_) => (Some(Poll::Ready(Ok(()))), s),
};
self.state = next_state;
if let Some(result) = result {
return result;
}
}
}
/// Sends an item to the channel.
///
/// Before calling `send_item`, `poll_reserve` must be called with a successful return
/// value of `Poll::Ready(Ok(()))`.
///
/// # Errors
///
/// If the channel is closed, an error will be returned. This is a permanent state.
///
/// # Panics
///
/// If `poll_reserve` was not successfully called prior to calling `send_item`, then this method
/// will panic.
pub fn send_item(&mut self, value: T) -> Result<(), PollSendError<T>> {
let (result, next_state) = match self.take_state() {
State::Idle(_) | State::Acquiring => {
panic!("`send_item` called without first calling `poll_reserve`")
}
// We have a permit to send our item, so go ahead, which gets us our sender back.
State::ReadyToSend(permit) => (Ok(()), State::Idle(permit.send(value))),
// We're closed, either by choice or because the underlying sender was closed.
State::Closed => (Err(PollSendError(Some(value))), State::Closed),
};
// Handle deferred closing if `close` was called between `poll_reserve` and `send_item`.
self.state = if self.sender.is_some() {
next_state
} else {
State::Closed
};
result
}
/// Checks whether this sender is been closed.
///
/// The underlying channel that this sender was wrapping may still be open.
pub fn is_closed(&self) -> bool {
matches!(self.state, State::Closed) || self.sender.is_none()
}
/// Gets a reference to the `Sender` of the underlying channel.
///
/// If `PollSender` has been closed, `None` is returned. The underlying channel that this sender
/// was wrapping may still be open.
pub fn get_ref(&self) -> Option<&Sender<T>> {
self.sender.as_ref()
}
/// Closes this sender.
///
/// No more messages will be able to be sent from this sender, but the underlying channel will
/// remain open until all senders have dropped, or until the [`Receiver`] closes the channel.
///
/// If a slot was previously reserved by calling `poll_reserve`, then a final call can be made
/// to `send_item` in order to consume the reserved slot. After that, no further sends will be
/// possible. If you do not intend to send another item, you can release the reserved slot back
/// to the underlying sender by calling [`abort_send`].
///
/// [`abort_send`]: crate::sync::PollSender::abort_send
/// [`Receiver`]: tokio::sync::mpsc::Receiver
pub fn close(&mut self) {
// Mark ourselves officially closed by dropping our main sender.
self.sender = None;
// If we're already idle, closed, or we haven't yet reserved a slot, we can quickly
// transition to the closed state. Otherwise, leave the existing permit in place for the
// caller if they want to complete the send.
match self.state {
State::Idle(_) => self.state = State::Closed,
State::Acquiring => {
self.acquire.set(make_acquire_future(None));
self.state = State::Closed;
}
_ => {}
}
}
/// Aborts the current in-progress send, if any.
///
/// Returns `true` if a send was aborted. If the sender was closed prior to calling
/// `abort_send`, then the sender will remain in the closed state, otherwise the sender will be
/// ready to attempt another send.
pub fn abort_send(&mut self) -> bool {
// We may have been closed in the meantime, after a call to `poll_reserve` already
// succeeded. We'll check if `self.sender` is `None` to see if we should transition to the
// closed state when we actually abort a send, rather than resetting ourselves back to idle.
let (result, next_state) = match self.take_state() {
// We're currently trying to reserve a slot to send into.
State::Acquiring => {
// Replacing the future drops the in-flight one.
self.acquire.set(make_acquire_future(None));
// If we haven't closed yet, we have to clone our stored sender since we have no way
// to get it back from the acquire future we just dropped.
let state = match self.sender.clone() {
Some(sender) => State::Idle(sender),
None => State::Closed,
};
(true, state)
}
// We got the permit. If we haven't closed yet, get the sender back.
State::ReadyToSend(permit) => {
let state = if self.sender.is_some() {
State::Idle(permit.release())
} else {
State::Closed
};
(true, state)
}
s => (false, s),
};
self.state = next_state;
result
}
}
impl<T> Clone for PollSender<T> {
/// Clones this `PollSender`.
///
/// The resulting `PollSender` will have an initial state identical to calling `PollSender::new`.
fn clone(&self) -> PollSender<T> {
let (sender, state) = match self.sender.clone() {
Some(sender) => (Some(sender.clone()), State::Idle(sender)),
None => (None, State::Closed),
};
Self {
sender,
state,
// We don't use `make_acquire_future` here because our relaxed bounds on `T` are not
// compatible with the transitive bounds required by `Sender<T>`.
acquire: ReusableBoxFuture::new(async { unreachable!() }),
}
}
}
impl<T: Send + 'static> Sink<T> for PollSender<T> {
type Error = PollSendError<T>;
fn poll_ready(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
Pin::into_inner(self).poll_reserve(cx)
}
fn poll_flush(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
Poll::Ready(Ok(()))
}
fn start_send(self: Pin<&mut Self>, item: T) -> Result<(), Self::Error> {
Pin::into_inner(self).send_item(item)
}
fn poll_close(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
Pin::into_inner(self).close();
Poll::Ready(Ok(()))
}
}