callback_cell 0.1.0

//! Example of using `CallbackCell` to elevate crossbeam `SegQueue` (a concurrent queue which does
//! not support blocking, only polling) into an mpsc queue which supports both blocking and
//! async-awaiting.

use callback_cell::CallbackCell;
use std::{
    sync::Arc,
    thread::{
        self,
        sleep,
    },
    time::Duration,
};
use crossbeam::{
    queue::SegQueue,
    sync::Parker,
};
use tokio::{
    sync::Notify,
    runtime::Runtime,
};


/// Sender half to mpsc queue.
pub struct Sender<T>(Arc<State<T>>);

/// Receiver half to mpsc queue.
pub struct Receiver<T>(Arc<State<T>>);

struct State<T> {
    queue: SegQueue<T>,
    callback_cell: CallbackCell,
}

/// Create an mpsc queue.
fn new_queue<T>() -> (Sender<T>, Receiver<T>) {
    let state_1 = Arc::new(State {
        queue: SegQueue::new(),
        callback_cell: CallbackCell::new(),
    });
    let state_2 = Arc::clone(&state_1);
    (Sender(state_1), Receiver(state_2))
}

impl<T> Sender<T> {
    /// Send item into queue.
    fn send(&self, item: T) {
        self.0.queue.push(item);
        self.0.callback_cell.take_call();
    }
}

impl<T> Clone for Sender<T> {
    fn clone(&self) -> Self {
        Sender(Arc::clone(&self.0))
    }
}

impl<T> Receiver<T> {
    /// Take item from queue, blocking until one is present.
    fn recv_blocking(&mut self) -> T {
        if let Some(item) = self.0.queue.pop() {
            return item;
        }
        let parker = Parker::new();
        loop {
            let unparker = parker.unparker().clone();
            self.0.callback_cell.put(move || unparker.unpark());
            if let Some(item) = self.0.queue.pop() {
                return item;
            }
            parker.park();
        }
    }

    /// Future to take item from queue, yielding until one is present.
    async fn recv_async(&mut self) -> T {
        if let Some(item) = self.0.queue.pop() {
            return item;
        }
        let notify_1 = Arc::new(Notify::new());
        loop {
            let notify_2 = Arc::clone(&notify_1);
            self.0.callback_cell.put(move || notify_2.notify_one());
            if let Some(item) = self.0.queue.pop() {
                return item;
            }
            notify_1.notified().await;
        }
    }
}

fn main() {
    let (send, mut recv) = new_queue();
    thread::spawn(move || {
        for i in 0..10 {
            send.send(i);
            sleep(Duration::from_millis(100));
        }
    });
    for _ in 0..5 {
        let i = recv.recv_blocking();
        println!("received through blocking: {}", i);
    }
    println!("entering tokio runtime");
    Runtime::new().unwrap().block_on(async move {
        for _ in 0..5 {
            let i = recv.recv_async().await;
            println!("receiving through awaiting: {}", i);
        }
    });
}