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//! A zero-copy queue for sending values between asynchronous tasks.
//!
//! It can be used concurrently by multiple producers (senders) and multiple
//! consumers (receivers), i.e. it is an "MPMC channel".
//!
//! Receivers are competing for messages. So a message that is received by
//! one receiver is not received by any other.
//!
//! This queue takes a Mutex type so that various
//! targets can be attained. For example, a ThreadModeMutex can be used
//! for single-core Cortex-M targets where messages are only passed
//! between tasks running in thread mode. Similarly, a CriticalSectionMutex
//! can also be used for single-core targets where messages are to be
//! passed from exception mode e.g. out of an interrupt handler.
//!
//! This module provides a bounded channel that has a limit on the number of
//! messages that it can store, and if this limit is reached, trying to send
//! another message will result in an error being returned.
use core::cell::RefCell;
use core::future::poll_fn;
use core::marker::PhantomData;
use core::task::{Context, Poll};
use crate::blocking_mutex::raw::RawMutex;
use crate::blocking_mutex::Mutex;
use crate::waitqueue::WakerRegistration;
/// A bounded zero-copy channel for communicating between asynchronous tasks
/// with backpressure.
///
/// The channel will buffer up to the provided number of messages. Once the
/// buffer is full, attempts to `send` new messages will wait until a message is
/// received from the channel.
///
/// All data sent will become available in the same order as it was sent.
///
/// The channel requires a buffer of recyclable elements. Writing to the channel is done through
/// an `&mut T`.
pub struct Channel<'a, M: RawMutex, T> {
buf: *mut T,
phantom: PhantomData<&'a mut T>,
state: Mutex<M, RefCell<State>>,
}
impl<'a, M: RawMutex, T> Channel<'a, M, T> {
/// Initialize a new [`Channel`].
///
/// The provided buffer will be used and reused by the channel's logic, and thus dictates the
/// channel's capacity.
pub fn new(buf: &'a mut [T]) -> Self {
let len = buf.len();
assert!(len != 0);
Self {
buf: buf.as_mut_ptr(),
phantom: PhantomData,
state: Mutex::new(RefCell::new(State {
len,
front: 0,
back: 0,
full: false,
send_waker: WakerRegistration::new(),
receive_waker: WakerRegistration::new(),
})),
}
}
/// Creates a [`Sender`] and [`Receiver`] from an existing channel.
///
/// Further Senders and Receivers can be created through [`Sender::borrow`] and
/// [`Receiver::borrow`] respectively.
pub fn split(&mut self) -> (Sender<'_, M, T>, Receiver<'_, M, T>) {
(Sender { channel: self }, Receiver { channel: self })
}
}
/// Send-only access to a [`Channel`].
pub struct Sender<'a, M: RawMutex, T> {
channel: &'a Channel<'a, M, T>,
}
impl<'a, M: RawMutex, T> Sender<'a, M, T> {
/// Creates one further [`Sender`] over the same channel.
pub fn borrow(&mut self) -> Sender<'_, M, T> {
Sender { channel: self.channel }
}
/// Attempts to send a value over the channel.
pub fn try_send(&mut self) -> Option<&mut T> {
self.channel.state.lock(|s| {
let s = &mut *s.borrow_mut();
match s.push_index() {
Some(i) => Some(unsafe { &mut *self.channel.buf.add(i) }),
None => None,
}
})
}
/// Attempts to send a value over the channel.
pub fn poll_send(&mut self, cx: &mut Context) -> Poll<&mut T> {
self.channel.state.lock(|s| {
let s = &mut *s.borrow_mut();
match s.push_index() {
Some(i) => Poll::Ready(unsafe { &mut *self.channel.buf.add(i) }),
None => {
s.receive_waker.register(cx.waker());
Poll::Pending
}
}
})
}
/// Asynchronously send a value over the channel.
pub async fn send(&mut self) -> &mut T {
let i = poll_fn(|cx| {
self.channel.state.lock(|s| {
let s = &mut *s.borrow_mut();
match s.push_index() {
Some(i) => Poll::Ready(i),
None => {
s.receive_waker.register(cx.waker());
Poll::Pending
}
}
})
})
.await;
unsafe { &mut *self.channel.buf.add(i) }
}
/// Notify the channel that the sending of the value has been finalized.
pub fn send_done(&mut self) {
self.channel.state.lock(|s| s.borrow_mut().push_done())
}
}
/// Receive-only access to a [`Channel`].
pub struct Receiver<'a, M: RawMutex, T> {
channel: &'a Channel<'a, M, T>,
}
impl<'a, M: RawMutex, T> Receiver<'a, M, T> {
/// Creates one further [`Sender`] over the same channel.
pub fn borrow(&mut self) -> Receiver<'_, M, T> {
Receiver { channel: self.channel }
}
/// Attempts to receive a value over the channel.
pub fn try_receive(&mut self) -> Option<&mut T> {
self.channel.state.lock(|s| {
let s = &mut *s.borrow_mut();
match s.pop_index() {
Some(i) => Some(unsafe { &mut *self.channel.buf.add(i) }),
None => None,
}
})
}
/// Attempts to asynchronously receive a value over the channel.
pub fn poll_receive(&mut self, cx: &mut Context) -> Poll<&mut T> {
self.channel.state.lock(|s| {
let s = &mut *s.borrow_mut();
match s.pop_index() {
Some(i) => Poll::Ready(unsafe { &mut *self.channel.buf.add(i) }),
None => {
s.send_waker.register(cx.waker());
Poll::Pending
}
}
})
}
/// Asynchronously receive a value over the channel.
pub async fn receive(&mut self) -> &mut T {
let i = poll_fn(|cx| {
self.channel.state.lock(|s| {
let s = &mut *s.borrow_mut();
match s.pop_index() {
Some(i) => Poll::Ready(i),
None => {
s.send_waker.register(cx.waker());
Poll::Pending
}
}
})
})
.await;
unsafe { &mut *self.channel.buf.add(i) }
}
/// Notify the channel that the receiving of the value has been finalized.
pub fn receive_done(&mut self) {
self.channel.state.lock(|s| s.borrow_mut().pop_done())
}
}
struct State {
len: usize,
/// Front index. Always 0..=(N-1)
front: usize,
/// Back index. Always 0..=(N-1).
back: usize,
/// Used to distinguish "empty" and "full" cases when `front == back`.
/// May only be `true` if `front == back`, always `false` otherwise.
full: bool,
send_waker: WakerRegistration,
receive_waker: WakerRegistration,
}
impl State {
fn increment(&self, i: usize) -> usize {
if i + 1 == self.len {
0
} else {
i + 1
}
}
fn is_full(&self) -> bool {
self.full
}
fn is_empty(&self) -> bool {
self.front == self.back && !self.full
}
fn push_index(&mut self) -> Option<usize> {
match self.is_full() {
true => None,
false => Some(self.back),
}
}
fn push_done(&mut self) {
assert!(!self.is_full());
self.back = self.increment(self.back);
if self.back == self.front {
self.full = true;
}
self.send_waker.wake();
}
fn pop_index(&mut self) -> Option<usize> {
match self.is_empty() {
true => None,
false => Some(self.front),
}
}
fn pop_done(&mut self) {
assert!(!self.is_empty());
self.front = self.increment(self.front);
self.full = false;
self.receive_waker.wake();
}
}