Pin

When you await a future, all local variables (that would ordinarily be stored on a stack frame) are instead stored in the Future for the current async block. If your future has pointers to data on the stack, those pointers might get invalidated. This is unsafe.

Therefore, you must guarantee that the addresses your future points to don’t change. That is why we need to “pin” futures. Using the same future repeatedly in a select! often leads to issues with pinned values.

use tokio::sync::{mpsc, oneshot};
use tokio::task::spawn;
use tokio::time::{sleep, Duration};

// A work item. In this case, just sleep for the given time and respond
// with a message on the `respond_on` channel.
#[derive(Debug)]
struct Work {
    input: u32,
    respond_on: oneshot::Sender<u32>,
}

// A worker which listens for work on a queue and performs it.
async fn worker(mut work_queue: mpsc::Receiver<Work>) {
    let mut iterations = 0;
    loop {
        tokio::select! {
            Some(work) = work_queue.recv() => {
                sleep(Duration::from_millis(10)).await; // Pretend to work.
                work.respond_on
                    .send(work.input * 1000)
                    .expect("failed to send response");
                iterations += 1;
            }
            // TODO: report number of iterations every 100ms
        }
    }
}

// A requester which requests work and waits for it to complete.
async fn do_work(work_queue: &mpsc::Sender<Work>, input: u32) -> u32 {
    let (tx, rx) = oneshot::channel();
    work_queue
        .send(Work {
            input,
            respond_on: tx,
        })
        .await
        .expect("failed to send on work queue");
    rx.await.expect("failed waiting for response")
}

#[tokio::main]
async fn main() {
    let (tx, rx) = mpsc::channel(10);
    spawn(worker(rx));
    for i in 0..100 {
        let resp = do_work(&tx, i).await;
        println!("work result for iteration {i}: {resp}");
    }
}

  • You may recognize this as an example of the actor pattern. Actors typically call select! in a loop.

  • This serves as a summation of a few of the previous lessons, so take your time with it.

    • Naively add a _ = sleep(Duration::from_millis(100)) => { println!(..) } to the select!. This will never execute. Why?

    • Instead, add a timeout_fut containing that future outside of the loop:

      #![allow(unused)]
fn main() {
let mut timeout_fut = sleep(Duration::from_millis(100));
loop {
    select! {
        ..,
        _ = timeout_fut => { println!(..); },
    }
}
}

    • This still doesn’t work. Follow the compiler errors, adding &mut to the timeout_fut in the select! to work around the move, then using Box::pin:

      #![allow(unused)]
fn main() {
let mut timeout_fut = Box::pin(sleep(Duration::from_millis(100)));
loop {
    select! {
        ..,
        _ = &mut timeout_fut => { println!(..); },
    }
}
}

    • This compiles, but once the timeout expires it is Poll::Ready on every iteration (a fused future would help with this). Update to reset timeout_fut every time it expires.

  • Box allocates on the heap. In some cases, std::pin::pin! (only recently stabilized, with older code often using tokio::pin!) is also an option, but that is difficult to use for a future that is reassigned.

  • Another alternative is to not use pin at all but spawn another task that will send to a oneshot channel every 100ms.