Added a pubsub channel implementation
This commit is contained in:
Dion Dokter
@@ -1,5 +1,5 @@
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//! Async channels
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pub mod mpmc;
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pub mod pubsub;
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pub mod signal;
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590
embassy/src/channel/pubsub.rs
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590
embassy/src/channel/pubsub.rs
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@@ -0,0 +1,590 @@
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//! Implementation of [PubSubChannel], a queue where published messages get received by all subscribers.
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use core::cell::RefCell;
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use core::fmt::Debug;
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use core::future::Future;
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use core::pin::Pin;
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use core::task::{Context, Poll, Waker};
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use heapless::Deque;
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use crate::blocking_mutex::raw::RawMutex;
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use crate::blocking_mutex::Mutex;
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use crate::waitqueue::WakerRegistration;
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/// A broadcast channel implementation where multiple publishers can send messages to multiple subscribers
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///
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/// Any published message can be read by all subscribers.
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/// A publisher can choose how it sends its message.
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///
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/// - With [Publisher::publish] the publisher has to wait until there is space in the internal message queue.
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/// - With [Publisher::publish_immediate] the publisher doesn't await and instead lets the oldest message
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/// in the queue drop if necessary. This will cause any [Subscriber] that missed the message to receive
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/// an error to indicate that it has lagged.
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pub struct PubSubChannel<M: RawMutex, T: Clone, const CAP: usize, const SUBS: usize, const PUBS: usize> {
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inner: Mutex<M, RefCell<PubSubState<T, CAP, SUBS, PUBS>>>,
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}
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impl<M: RawMutex, T: Clone, const CAP: usize, const SUBS: usize, const PUBS: usize> PubSubChannel<M, T, CAP, SUBS, PUBS> {
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/// Create a new channel
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pub const fn new() -> Self {
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Self {
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inner: Mutex::const_new(M::INIT, RefCell::new(PubSubState::new())),
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}
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}
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/// Create a new subscriber. It will only receive messages that are published after its creation.
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///
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/// If there are no subscriber slots left, an error will be returned.
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pub fn subscriber(&self) -> Result<Subscriber<'_, T>, Error> {
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self.inner.lock(|inner| {
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let mut s = inner.borrow_mut();
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// Search for an empty subscriber spot
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for (i, sub_spot) in s.subscriber_wakers.iter_mut().enumerate() {
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if sub_spot.is_none() {
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// We've found a spot, so now fill it and create the subscriber
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*sub_spot = Some(WakerRegistration::new());
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return Ok(Subscriber {
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subscriber_index: i,
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next_message_id: s.next_message_id,
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channel: self,
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});
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}
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}
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// No spot was found, we're full
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Err(Error::MaximumSubscribersReached)
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})
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}
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/// Create a new publisher
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///
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/// If there are no publisher slots left, an error will be returned.
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pub fn publisher(&self) -> Result<Publisher<'_, T>, Error> {
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self.inner.lock(|inner| {
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let mut s = inner.borrow_mut();
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// Search for an empty publisher spot
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for (i, pub_spot) in s.publisher_wakers.iter_mut().enumerate() {
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if pub_spot.is_none() {
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// We've found a spot, so now fill it and create the subscriber
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*pub_spot = Some(WakerRegistration::new());
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return Ok(Publisher {
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publisher_index: i,
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channel: self,
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});
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}
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}
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// No spot was found, we're full
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Err(Error::MaximumPublishersReached)
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})
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}
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/// Create a new publisher that can only send immediate messages.
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/// This kind of publisher does not take up a publisher slot.
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pub fn immediate_publisher(&self) -> ImmediatePublisher<'_, T> {
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ImmediatePublisher { channel: self }
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}
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}
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impl<M: RawMutex, T: Clone, const CAP: usize, const SUBS: usize, const PUBS: usize> PubSubBehavior<T>
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for PubSubChannel<M, T, CAP, SUBS, PUBS>
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{
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fn try_publish(&self, message: T) -> Result<(), T> {
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self.inner.lock(|inner| {
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let mut s = inner.borrow_mut();
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let active_subscriber_count = s.subscriber_wakers.iter().flatten().count();
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if active_subscriber_count == 0 {
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// We don't need to publish anything because there is no one to receive it
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return Ok(());
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}
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if s.queue.is_full() {
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return Err(message);
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}
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// We just did a check for this
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unsafe {
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s.queue.push_back_unchecked((message, active_subscriber_count));
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}
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s.next_message_id += 1;
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// Wake all of the subscribers
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for active_subscriber in s.subscriber_wakers.iter_mut().flatten() {
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active_subscriber.wake()
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}
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Ok(())
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})
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}
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fn publish_immediate(&self, message: T) {
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self.inner.lock(|inner| {
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let mut s = inner.borrow_mut();
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// Make space in the queue if required
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if s.queue.is_full() {
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s.queue.pop_front();
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}
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// We are going to call something is Self again.
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// The lock is fine, but we need to get rid of the refcell borrow
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drop(s);
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// This will succeed because we made sure there is space
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unsafe { self.try_publish(message).unwrap_unchecked() };
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});
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}
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fn get_message(&self, message_id: u64) -> Option<WaitResult<T>> {
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self.inner.lock(|inner| {
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let mut s = inner.borrow_mut();
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let start_id = s.next_message_id - s.queue.len() as u64;
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if message_id < start_id {
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return Some(WaitResult::Lagged(start_id - message_id));
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}
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let current_message_index = (message_id - start_id) as usize;
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if current_message_index >= s.queue.len() {
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return None;
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}
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// We've checked that the index is valid
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unsafe {
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let queue_item = s.queue.iter_mut().nth(current_message_index).unwrap_unchecked();
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// We're reading this item, so decrement the counter
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queue_item.1 -= 1;
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let message = queue_item.0.clone();
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if current_message_index == 0 && queue_item.1 == 0 {
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s.queue.pop_front();
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s.publisher_wakers.iter_mut().flatten().for_each(|w| w.wake());
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}
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Some(WaitResult::Message(message))
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}
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})
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}
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unsafe fn register_subscriber_waker(&self, subscriber_index: usize, waker: &Waker) {
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self.inner.lock(|inner| {
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let mut s = inner.borrow_mut();
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s.subscriber_wakers
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.get_unchecked_mut(subscriber_index)
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.as_mut()
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.unwrap_unchecked()
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.register(waker);
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})
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}
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unsafe fn register_publisher_waker(&self, publisher_index: usize, waker: &Waker) {
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self.inner.lock(|inner| {
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let mut s = inner.borrow_mut();
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s.publisher_wakers
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.get_unchecked_mut(publisher_index)
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.as_mut()
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.unwrap_unchecked()
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.register(waker);
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})
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}
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unsafe fn unregister_subscriber(&self, subscriber_index: usize, subscriber_next_message_id: u64) {
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self.inner.lock(|inner| {
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let mut s = inner.borrow_mut();
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// Remove the subscriber from the wakers
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*s.subscriber_wakers.get_unchecked_mut(subscriber_index) = None;
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// All messages that haven't been read yet by this subscriber must have their counter decremented
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let start_id = s.next_message_id - s.queue.len() as u64;
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if subscriber_next_message_id >= start_id {
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let current_message_index = (subscriber_next_message_id - start_id) as usize;
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s.queue
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.iter_mut()
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.skip(current_message_index)
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.for_each(|(_, counter)| *counter -= 1);
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}
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})
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}
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unsafe fn unregister_publisher(&self, publisher_index: usize) {
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self.inner.lock(|inner| {
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let mut s = inner.borrow_mut();
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// Remove the publisher from the wakers
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*s.publisher_wakers.get_unchecked_mut(publisher_index) = None;
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})
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}
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}
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/// Internal state for the PubSub channel
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struct PubSubState<T: Clone, const CAP: usize, const SUBS: usize, const PUBS: usize> {
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/// The queue contains the last messages that have been published and a countdown of how many subscribers are yet to read it
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queue: Deque<(T, usize), CAP>,
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/// Every message has an id.
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/// Don't worry, we won't run out.
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/// If a million messages were published every second, then the ID's would run out in about 584942 years.
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next_message_id: u64,
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/// Collection of wakers for Subscribers that are waiting.
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/// The [Subscriber::subscriber_index] field indexes into this array.
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subscriber_wakers: [Option<WakerRegistration>; SUBS],
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/// Collection of wakers for Publishers that are waiting.
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/// The [Publisher::publisher_index] field indexes into this array.
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publisher_wakers: [Option<WakerRegistration>; PUBS],
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}
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impl<T: Clone, const CAP: usize, const SUBS: usize, const PUBS: usize> PubSubState<T, CAP, SUBS, PUBS> {
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/// Create a new internal channel state
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const fn new() -> Self {
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const WAKER_INIT: Option<WakerRegistration> = None;
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Self {
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queue: Deque::new(),
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next_message_id: 0,
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subscriber_wakers: [WAKER_INIT; SUBS],
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publisher_wakers: [WAKER_INIT; PUBS],
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}
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}
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}
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/// A subscriber to a channel
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///
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/// This instance carries a reference to the channel, but uses a trait object for it so that the channel's
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/// generics are erased on this subscriber
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pub struct Subscriber<'a, T: Clone> {
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/// Our index into the channel
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subscriber_index: usize,
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/// The message id of the next message we are yet to receive
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next_message_id: u64,
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/// The channel we are a subscriber to
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channel: &'a dyn PubSubBehavior<T>,
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}
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impl<'a, T: Clone> Subscriber<'a, T> {
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/// Wait for a published message
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pub fn wait<'s>(&'s mut self) -> SubscriberWaitFuture<'s, 'a, T> {
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SubscriberWaitFuture { subscriber: self }
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}
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/// Try to see if there's a published message we haven't received yet.
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///
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/// This function does not peek. The message is received if there is one.
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pub fn check(&mut self) -> Option<WaitResult<T>> {
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match self.channel.get_message(self.next_message_id) {
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Some(WaitResult::Lagged(amount)) => {
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self.next_message_id += amount;
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Some(WaitResult::Lagged(amount))
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}
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result => {
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self.next_message_id += 1;
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result
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}
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}
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}
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}
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impl<'a, T: Clone> Drop for Subscriber<'a, T> {
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fn drop(&mut self) {
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unsafe {
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self.channel
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.unregister_subscriber(self.subscriber_index, self.next_message_id)
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}
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}
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}
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/// A publisher to a channel
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///
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/// This instance carries a reference to the channel, but uses a trait object for it so that the channel's
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/// generics are erased on this subscriber
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pub struct Publisher<'a, T: Clone> {
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/// Our index into the channel
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publisher_index: usize,
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/// The channel we are a publisher for
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channel: &'a dyn PubSubBehavior<T>,
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}
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impl<'a, T: Clone> Publisher<'a, T> {
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/// Publish a message right now even when the queue is full.
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/// This may cause a subscriber to miss an older message.
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pub fn publish_immediate(&self, message: T) {
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self.channel.publish_immediate(message)
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}
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/// Publish a message. But if the message queue is full, wait for all subscribers to have read the last message
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pub fn publish<'s>(&'s self, message: T) -> PublisherWaitFuture<'s, 'a, T> {
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PublisherWaitFuture {
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message: Some(message),
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publisher: self,
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}
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}
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/// Publish a message if there is space in the message queue
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pub fn try_publish(&self, message: T) -> Result<(), T> {
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self.channel.try_publish(message)
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}
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}
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impl<'a, T: Clone> Drop for Publisher<'a, T> {
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fn drop(&mut self) {
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unsafe { self.channel.unregister_publisher(self.publisher_index) }
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}
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}
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/// A publisher that can only use the `publish_immediate` function, but it doesn't have to be registered with the channel.
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/// (So an infinite amount is possible)
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pub struct ImmediatePublisher<'a, T: Clone> {
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/// The channel we are a publisher for
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channel: &'a dyn PubSubBehavior<T>,
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}
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impl<'a, T: Clone> ImmediatePublisher<'a, T> {
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/// Publish the message right now even when the queue is full.
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/// This may cause a subscriber to miss an older message.
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pub fn publish_immediate(&mut self, message: T) {
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self.channel.publish_immediate(message)
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}
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/// Publish a message if there is space in the message queue
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pub fn try_publish(&self, message: T) -> Result<(), T> {
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self.channel.try_publish(message)
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}
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}
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/// Error type for the [PubSubChannel]
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#[derive(Debug, PartialEq, Clone)]
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pub enum Error {
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/// All subscriber slots are used. To add another subscriber, first another subscriber must be dropped or
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/// the capacity of the channels must be increased.
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MaximumSubscribersReached,
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/// All publisher slots are used. To add another publisher, first another publisher must be dropped or
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/// the capacity of the channels must be increased.
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MaximumPublishersReached,
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}
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trait PubSubBehavior<T> {
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/// Try to publish a message. If the queue is full it won't succeed
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fn try_publish(&self, message: T) -> Result<(), T>;
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/// Publish a message immediately. If the queue is full, just throw out the oldest one.
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fn publish_immediate(&self, message: T);
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/// Tries to read the message if available
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fn get_message(&self, message_id: u64) -> Option<WaitResult<T>>;
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/// Register the given waker for the given subscriber.
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///
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/// ## Safety
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///
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/// The subscriber index must be of a valid and active subscriber
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unsafe fn register_subscriber_waker(&self, subscriber_index: usize, waker: &Waker);
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/// Register the given waker for the given publisher.
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///
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/// ## Safety
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///
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/// The subscriber index must be of a valid and active publisher
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unsafe fn register_publisher_waker(&self, publisher_index: usize, waker: &Waker);
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/// Make the channel forget the subscriber.
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///
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/// ## Safety
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///
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/// The subscriber index must be of a valid and active subscriber which must not be used again
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/// unless a new subscriber takes on that index.
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unsafe fn unregister_subscriber(&self, subscriber_index: usize, subscriber_next_message_id: u64);
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/// Make the channel forget the publisher.
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///
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/// ## Safety
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///
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/// The publisher index must be of a valid and active publisher which must not be used again
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/// unless a new publisher takes on that index.
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unsafe fn unregister_publisher(&self, publisher_index: usize);
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}
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/// Future for the subscriber wait action
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pub struct SubscriberWaitFuture<'s, 'a, T: Clone> {
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subscriber: &'s mut Subscriber<'a, T>,
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}
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impl<'s, 'a, T: Clone> Future for SubscriberWaitFuture<'s, 'a, T> {
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type Output = WaitResult<T>;
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fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
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// Check if we can read a message
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match self.subscriber.channel.get_message(self.subscriber.next_message_id) {
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// Yes, so we are done polling
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Some(WaitResult::Message(message)) => {
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self.subscriber.next_message_id += 1;
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Poll::Ready(WaitResult::Message(message))
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}
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// No, so we need to reregister our waker and sleep again
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None => {
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unsafe {
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self.subscriber
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.channel
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.register_subscriber_waker(self.subscriber.subscriber_index, cx.waker());
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}
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Poll::Pending
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}
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// We missed a couple of messages. We must do our internal bookkeeping and return that we lagged
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Some(WaitResult::Lagged(amount)) => {
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self.subscriber.next_message_id += amount;
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Poll::Ready(WaitResult::Lagged(amount))
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}
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}
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}
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}
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/// Future for the publisher wait action
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pub struct PublisherWaitFuture<'s, 'a, T: Clone> {
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/// The message we need to publish
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message: Option<T>,
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publisher: &'s Publisher<'a, T>,
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}
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impl<'s, 'a, T: Clone> Future for PublisherWaitFuture<'s, 'a, T> {
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type Output = ();
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fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
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let this = unsafe { self.get_unchecked_mut() };
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// Try to publish the message
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match this.publisher.channel.try_publish(this.message.take().unwrap()) {
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// We did it, we are ready
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Ok(()) => Poll::Ready(()),
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// The queue is full, so we need to reregister our waker and go to sleep
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Err(message) => {
|
||||
this.message = Some(message);
|
||||
unsafe {
|
||||
this.publisher
|
||||
.channel
|
||||
.register_publisher_waker(this.publisher.publisher_index, cx.waker());
|
||||
}
|
||||
Poll::Pending
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
/// The result of the subscriber wait procedure
|
||||
#[derive(Debug, Clone, PartialEq)]
|
||||
pub enum WaitResult<T> {
|
||||
/// The subscriber did not receive all messages and lagged by the given amount of messages.
|
||||
/// (This is the amount of messages that were missed)
|
||||
Lagged(u64),
|
||||
/// A message was received
|
||||
Message(T),
|
||||
}
|
||||
|
||||
#[cfg(test)]
|
||||
mod tests {
|
||||
use crate::blocking_mutex::raw::NoopRawMutex;
|
||||
use super::*;
|
||||
|
||||
#[futures_test::test]
|
||||
async fn all_subscribers_receive() {
|
||||
let channel = PubSubChannel::<NoopRawMutex, u32, 4, 4, 4>::new();
|
||||
|
||||
let mut sub0 = channel.subscriber().unwrap();
|
||||
let mut sub1 = channel.subscriber().unwrap();
|
||||
let pub0 = channel.publisher().unwrap();
|
||||
|
||||
pub0.publish(42).await;
|
||||
|
||||
assert_eq!(sub0.wait().await, WaitResult::Message(42));
|
||||
assert_eq!(sub1.wait().await, WaitResult::Message(42));
|
||||
|
||||
assert_eq!(sub0.check(), None);
|
||||
assert_eq!(sub1.check(), None);
|
||||
}
|
||||
|
||||
#[futures_test::test]
|
||||
async fn lag_when_queue_full_on_immediate_publish() {
|
||||
let channel = PubSubChannel::<NoopRawMutex, u32, 4, 4, 4>::new();
|
||||
|
||||
let mut sub0 = channel.subscriber().unwrap();
|
||||
let pub0 = channel.publisher().unwrap();
|
||||
|
||||
pub0.publish_immediate(42);
|
||||
pub0.publish_immediate(43);
|
||||
pub0.publish_immediate(44);
|
||||
pub0.publish_immediate(45);
|
||||
pub0.publish_immediate(46);
|
||||
pub0.publish_immediate(47);
|
||||
|
||||
assert_eq!(sub0.check(), Some(WaitResult::Lagged(2)));
|
||||
assert_eq!(sub0.wait().await, WaitResult::Message(44));
|
||||
assert_eq!(sub0.wait().await, WaitResult::Message(45));
|
||||
assert_eq!(sub0.wait().await, WaitResult::Message(46));
|
||||
assert_eq!(sub0.wait().await, WaitResult::Message(47));
|
||||
assert_eq!(sub0.check(), None);
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn limited_subs_and_pubs() {
|
||||
let channel = PubSubChannel::<NoopRawMutex, u32, 4, 4, 4>::new();
|
||||
|
||||
let sub0 = channel.subscriber();
|
||||
let sub1 = channel.subscriber();
|
||||
let sub2 = channel.subscriber();
|
||||
let sub3 = channel.subscriber();
|
||||
let sub4 = channel.subscriber();
|
||||
|
||||
assert!(sub0.is_ok());
|
||||
assert!(sub1.is_ok());
|
||||
assert!(sub2.is_ok());
|
||||
assert!(sub3.is_ok());
|
||||
assert_eq!(sub4.err().unwrap(), Error::MaximumSubscribersReached);
|
||||
|
||||
drop(sub0);
|
||||
|
||||
let sub5 = channel.subscriber();
|
||||
assert!(sub5.is_ok());
|
||||
|
||||
// publishers
|
||||
|
||||
let pub0 = channel.publisher();
|
||||
let pub1 = channel.publisher();
|
||||
let pub2 = channel.publisher();
|
||||
let pub3 = channel.publisher();
|
||||
let pub4 = channel.publisher();
|
||||
|
||||
assert!(pub0.is_ok());
|
||||
assert!(pub1.is_ok());
|
||||
assert!(pub2.is_ok());
|
||||
assert!(pub3.is_ok());
|
||||
assert_eq!(pub4.err().unwrap(), Error::MaximumPublishersReached);
|
||||
|
||||
drop(pub0);
|
||||
|
||||
let pub5 = channel.publisher();
|
||||
assert!(pub5.is_ok());
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn publisher_wait_on_full_queue() {
|
||||
let channel = PubSubChannel::<NoopRawMutex, u32, 4, 4, 4>::new();
|
||||
|
||||
let pub0 = channel.publisher().unwrap();
|
||||
|
||||
// There are no subscribers, so the queue will never be full
|
||||
assert_eq!(pub0.try_publish(0), Ok(()));
|
||||
assert_eq!(pub0.try_publish(0), Ok(()));
|
||||
assert_eq!(pub0.try_publish(0), Ok(()));
|
||||
assert_eq!(pub0.try_publish(0), Ok(()));
|
||||
assert_eq!(pub0.try_publish(0), Ok(()));
|
||||
|
||||
let sub0 = channel.subscriber().unwrap();
|
||||
|
||||
assert_eq!(pub0.try_publish(0), Ok(()));
|
||||
assert_eq!(pub0.try_publish(0), Ok(()));
|
||||
assert_eq!(pub0.try_publish(0), Ok(()));
|
||||
assert_eq!(pub0.try_publish(0), Ok(()));
|
||||
assert_eq!(pub0.try_publish(0), Err(0));
|
||||
|
||||
drop(sub0);
|
||||
}
|
||||
|
||||
}
|
||||
Reference in New Issue
Block a user