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//! Initially a fork of the [`async_watch`](https://github.com/cynecx/async-watch) crate.
//!
//! A single-producer, multi-consumer channel that only retains the *last* sent
//! value.
//!
//! Extracted from [Tokio's](https://github.com/tokio-rs/tokio/) `tokio::sync::watch`
//! implementation, which was written by [Carl Lerche](https://github.com/carllerche).
//!
//! This channel is useful for watching for changes to a value from multiple
//! points in the code base, for example, changes to configuration values.
//!
//! # Usage
//!
//! [`channel`] returns a [`Sender`] / [`Receiver`] pair. These are
//! the producer and sender halves of the channel. The channel is
//! created with an initial value. The **latest** value stored in the channel is accessed with
//! [`Receiver::borrow()`]. Awaiting [`Receiver::changed()`] waits for a new
//! value to sent by the [`Sender`] half. Awaiting [`Receiver::recv()`] combines
//! [`Receiver::changed()`] and [`Receiver::borrow()`] where the borrowed value
//! is cloned and returned.
//!
//!
//! # Examples
//!
//! ```
//! # let executor = async_executor::LocalExecutor::new();
//! # executor.run(async {
//! let (tx, mut rx) = just_watch::channel("hello");
//! let mut rx2 = rx.clone();
//!
//! // First variant
//! executor.spawn(async move {
//! while let Ok(value) = rx.recv().await {
//! println!("received = {:?}", value);
//! }
//! });
//!
//! // Second variant
//! executor.spawn(async move {
//! while rx2.changed().await.is_ok() {
//! println!("received = {:?}", *rx2.borrow());
//! }
//! });
//!
//! tx.send("world").unwrap();
//! # });
//! ```
//!
//! # Closing
//!
//! [`Sender::closed`] allows the producer to detect when all [`Receiver`]
//! handles have been dropped. This indicates that there is no further interest
//! in the values being produced and work can be stopped.
//!
//! # Thread safety
//!
//! Both [`Sender`] and [`Receiver`] are thread safe. They can be moved to other
//! threads and can be used in a concurrent environment. Clones of [`Receiver`]
//! handles may be moved to separate threads and also used concurrently.
//!
//! [`Sender`]: crate::Sender
//! [`Receiver`]: crate::Receiver
//! [`Receiver::recv`]: crate::Receiver::recv
//! [`channel`]: crate::channel
//! [`Sender::closed`]: crate::Sender::closed
#![warn(missing_docs)]
pub mod error;
use event_listener::Event;
use std::ops;
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering::{Relaxed, SeqCst};
use std::sync::{Arc, RwLock, RwLockReadGuard};
/// The initial version starts at zero.
const VERSION_0: usize = 0b00;
/// The version counter shifted by one position to the left to leave space for the closed bit.
const VERSION_1: usize = 0b10;
/// The least significant bit signifies a closed channel.
const CLOSED: usize = 0b01;
/// Receives values from the associated [`Sender`](struct@Sender).
///
/// Instances are created by the [`channel`](fn@channel) function.
#[derive(Debug)]
pub struct Receiver<T> {
/// Pointer to the shared state
shared: Arc<Shared<T>>,
/// Last observed version.
version: usize,
}
/// Sends values to the associated [`Receiver`](struct@Receiver).
///
/// Instances are created by the [`channel`](fn@channel) function.
#[derive(Debug)]
pub struct Sender<T> {
shared: Arc<Shared<T>>,
}
/// Returns a reference to the inner value.
///
/// Outstanding borrows hold a read lock on the inner value. This means that
/// long lived borrows could cause the produce half to block. It is recommended
/// to keep the borrow as short lived as possible.
#[derive(Debug)]
pub struct Ref<'a, T> {
inner: RwLockReadGuard<'a, T>,
}
#[derive(Debug)]
struct Shared<T> {
/// The most recent value
value: RwLock<T>,
/// The current version
///
/// The lowest bit represents a "closed" state. The rest of the bits
/// represent the current version.
version: AtomicUsize,
/// Tracks the number of `Receiver` instances.
ref_count_rx: AtomicUsize,
/// Event when the value has changed or the `Sender` has been dropped.
event_value_changed: Event,
/// Event when all `Receiver`s have been dropped.
event_all_recv_dropped: Event,
}
/// Creates a new watch channel, returning the "send" and "receive" handles.
///
/// All values sent by [`Sender`] will become visible to the [`Receiver`] handles.
/// Only the last value sent is made available to the [`Receiver`] half. All
/// intermediate values are dropped.
///
/// # Examples
///
/// ```
/// # let executor = async_executor::LocalExecutor::new();
/// # executor.run(async {
/// let (tx, mut rx) = just_watch::channel("hello");
///
/// executor.spawn(async move {
/// while let Ok(value) = rx.recv().await {
/// println!("received = {:?}", value);
/// }
/// });
///
/// tx.send("world").unwrap();
/// # });
/// ```
///
/// [`Sender`]: struct@Sender
/// [`Receiver`]: struct@Receiver
pub fn channel<T>(init: T) -> (Sender<T>, Receiver<T>) {
let shared = Arc::new(Shared {
value: RwLock::new(init),
version: AtomicUsize::new(VERSION_0),
ref_count_rx: AtomicUsize::new(1),
event_value_changed: Event::new(),
event_all_recv_dropped: Event::new(),
});
let tx = Sender {
shared: shared.clone(),
};
let rx = Receiver {
shared,
version: VERSION_0,
};
(tx, rx)
}
impl<T> Receiver<T> {
/// Returns a reference to the most recently sent value.
///
/// Outstanding borrows hold a read lock. This means that long lived borrows
/// could cause the send half to block. It is recommended to keep the borrow
/// as short lived as possible.
///
/// # Examples
///
/// ```
/// let (_, rx) = just_watch::channel("hello");
/// assert_eq!(*rx.borrow(), "hello");
/// ```
pub fn borrow(&self) -> Ref<'_, T> {
let inner = self.shared.value.read().unwrap();
Ref { inner }
}
/// Wait for a change notification.
///
/// Returns when a new value has been sent by the [`Sender`] since the last
/// time `changed()` was called. When the `Sender` half is dropped, `Err` is
/// returned.
///
/// [`Sender`]: struct@Sender
///
/// # Examples
///
/// ```
/// # let executor = async_executor::LocalExecutor::new();
/// # executor.run(async {
/// let (tx, mut rx) = just_watch::channel("hello");
///
/// let task = executor.spawn(async move {
/// tx.send("goodbye").unwrap();
/// });
///
/// assert!(rx.changed().await.is_ok());
/// assert_eq!(*rx.borrow(), "goodbye");
///
/// // The `tx` handle has been dropped
/// assert!(rx.changed().await.is_err());
///
/// task.await;
/// });
/// ```
pub async fn changed(&mut self) -> Result<(), error::RecvError> {
// Fast path: Check the state first.
if let Some(ret) = self.maybe_changed() {
return ret;
}
// In order to avoid a race condition, we first request a notification,
// **then** check the current value's version. If a new version exists,
// the notification request is dropped.
let listener = self.shared.event_value_changed.listen();
if let Some(ret) = self.maybe_changed() {
return ret;
}
listener.await;
self.maybe_changed()
.expect("[bug] failed to observe change after notificaton.")
}
fn maybe_changed(&mut self) -> Option<Result<(), error::RecvError>> {
// Load the version from the state
let state = self.shared.version.load(SeqCst);
let new_version = state & !CLOSED;
if self.version != new_version {
// Observe the new version and return
self.version = new_version;
return Some(Ok(()));
}
if CLOSED == state & CLOSED {
// All receivers have dropped.
return Some(Err(error::RecvError {}));
}
// No changes.
None
}
}
impl<T: Clone> Receiver<T> {
/// A convenience helper which combines calling [`Receiver::changed()`] and
/// [`Receiver::borrow()`] where the borrowed value is cloned and returned.
///
/// Note: If this is the first time the function is called on a `Receiver`
/// instance, then the function **will wait** until a new value is sent into the channel.
///
/// `None` is returned if the `Sender` half is dropped.
///
/// # Examples
///
/// ```
/// # let executor = async_executor::LocalExecutor::new();
/// # executor.run(async {
/// let (tx, mut rx) = just_watch::channel("hello");
///
/// let task = executor.spawn(async move {
/// tx.send("goodbye").unwrap();
/// });
///
/// assert_eq!(*rx.borrow(), "hello");
///
/// // Waits for the new task to spawn and send the value.
/// let v = rx.recv().await.unwrap();
/// assert_eq!(v, "goodbye");
///
/// let v = rx.recv().await;
/// assert!(v.is_err());
///
/// task.await;
/// # });
/// ```
pub async fn recv(&mut self) -> Result<T, error::RecvError> {
self.changed().await?;
Ok(self.borrow().clone())
}
}
impl<T> Clone for Receiver<T> {
fn clone(&self) -> Self {
self.shared.ref_count_rx.fetch_add(1, Relaxed);
Receiver {
shared: self.shared.clone(),
version: self.version,
}
}
}
impl<T> Drop for Receiver<T> {
fn drop(&mut self) {
if self.shared.ref_count_rx.fetch_sub(1, Relaxed) == 1 {
// Notify the single sender.
self.shared.event_all_recv_dropped.notify(usize::MAX);
}
}
}
impl<T> Sender<T> {
/// Creates a new `Receiver` connected to a `Sender`.
pub fn subscribe(&self) -> Receiver<T> {
self.shared.ref_count_rx.fetch_add(1, Relaxed);
let version = self.shared.version.load(SeqCst);
Receiver {
shared: self.shared.clone(),
version,
}
}
/// Sends a new value via the channel, notifying all receivers.
pub fn send(&self, value: T) -> Result<(), error::SendError<T>> {
if self.shared.ref_count_rx.load(Relaxed) == 0 {
// All watchers (`Receiver`s) have been dropped.
return Err(error::SendError { inner: value });
}
// Replace the value.
*self.shared.value.write().unwrap() = value;
// Update the version. 2 (`VERSION_1`) is used so that the CLOSED bit is not set.
self.shared.version.fetch_add(VERSION_1, SeqCst);
// Notify all watchers.
self.shared.event_value_changed.notify(usize::MAX);
Ok(())
}
/// Applies a function to the existing value.
///
/// A function gets a mutable reference that can be used to change a stored value.
pub fn send_modify<F>(&self, func: F)
where
F: FnOnce(&mut T),
{
// Replace the value.
func(&mut *self.shared.value.write().unwrap());
// Update the version. 2 (`VERSION_1`) is used so that the CLOSED bit is not set.
self.shared.version.fetch_add(VERSION_1, SeqCst);
// Notify all watchers.
self.shared.event_value_changed.notify(usize::MAX);
}
/// Completes when all receivers have dropped.
///
/// This allows the producer to get notified when interest in the produced
/// values is canceled and immediately stop doing work.
pub async fn closed(&self) {
// Fast path.
if self.shared.ref_count_rx.load(Relaxed) == 0 {
return;
}
// Listen for events now and check the reference count afterwards to avoid race condition.
let listener = self.shared.event_all_recv_dropped.listen();
if self.shared.ref_count_rx.load(Relaxed) == 0 {
return;
}
listener.await;
}
}
impl<T> Drop for Sender<T> {
fn drop(&mut self) {
self.shared.version.fetch_or(CLOSED, SeqCst);
self.shared.event_value_changed.notify(usize::MAX);
}
}
// ===== impl Ref =====
impl<T> ops::Deref for Ref<'_, T> {
type Target = T;
fn deref(&self) -> &T {
self.inner.deref()
}
}