presentations/futures-async-await/async.md

title	author	theme	classoption
Getting comfy with async await	Sanchayan Maity	default	aspectratio=169

Who

• Who am I?
  • Embedded Systems background
  • Prefer C, Haskell and Rust
  • Organize and speak at Rust and Haskell meet-ups in Bangalore
• Work?
  • Software Engineer @ asymptotic
  • Open source consulting firm based out of Bangalore and Toronto
  • Work on low level systems software centred around multimedia
  • GStreamer, PipeWire, PulseAudio
  • Language Polyglots

Agenda

• Future trait
• async/await
• Using futures/Runtime
• Working with multiple futures (select, join, FuturesOrdered)
• Streams
• Pitfalls
• Pin/Unpin/pin_project

Future¹

use std::future::Future;
use std::pin::Pin;
use std::task::Context;

pub trait Future {
    type Output;
    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>)
        -> Poll<Self::Output>;
}

pub enum Poll<T> {
    Ready(T),
    Pending,
}

Example

async fn hello() {
    println!("Hello from async");
}

fn main() {
    hello();
    println!("Hello from main");
}

Where's the future

async fn give_number() -> u32 {
    100
}

Sugar town²

fn give_number() -> impl Future<Output = u32> {
    GiveNumberFuture
}

struct GiveNumberFuture {}

impl Future for GiveNumberFuture {
    type Output = u32;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>)
                -> Poll<Self::Output> {
        Poll::Ready(100)
    }
}

Runtimes

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Runtimes³

• futures::executor
• tokio
• smol-rs
• embassy
• glommio
• async-std

Example

use futures::executor::block_on;

async fn hello() {
    println!("hello, world!");
}

fn main() {
    block_on(hello());
    println!("Hello from main");
}

Example

async fn hello() {
    println!("Hello from async");
}

#[tokio::main]
async fn main() {
    hello().await;
    println!("Hello from main");
}

Multiple futures

• join
• join_all
• select
• select!
• select_all
• FuturesOrdered
• FuturesUnordered
• JoinSet

join

use futures::future;

#[tokio::main]
async fn main() {
    let a = async { "Future 1" };
    let b = async { "Future 2" };
    let pair = future::join(a, b);

    println!("{:?}", pair.await);
}

join_all

use futures::future::join_all;
async fn hello(msg: String) -> String {
    msg
}

#[tokio::main]
async fn main() {
    let futures = vec![
        hello("Future 1".to_string()),
        hello("Future 2".to_string()),
        hello("Future 3".to_string()),
        hello("Future 4".to_string()),
    ];

    println!("{:?}", join_all(futures).await);
}

JoinSet

use tokio::task::JoinSet;

#[tokio::main]
async fn main() {
    let mut set = JoinSet::new();
    for i in 0..10 {
        set.spawn(async move { i });
    }

    while let Some(res) = set.join_next().await {
        println!("{}", res.unwrap());
    }
}

future::select

pub fn select<A, B>(future1: A, future2: B) -> Select<A, B>
where
    A: Future + Unpin,
    B: Future + Unpin,

future::select

use futures::{future, future::Either, future::FutureExt, select};
use tokio::time::{sleep, Duration};

async fn task1(delay: u64) -> u64 {
    sleep(Duration::from_millis(delay)).await;
    delay
}

async fn task2(delay: u64) -> String {
    sleep(Duration::from_millis(delay)).await;
    "Hello".to_string()
}

future::select

#[tokio::main]
async fn main() {
    let t1 = task1(200u64).fuse();
    let t2 = task2(300u64).fuse();

    tokio::pin!(t1, t2);

    match future::select(t1, t2).await {
        Either::Left((value1, _)) => println!("{}", value1),
        Either::Right((value2, _)) => println!("{}", value2),
    };
}

futures::select!⁴

use futures::{future::FutureExt, pin_mut, select};
use tokio::time::{sleep, Duration};
async fn task(delay: u64) {
    sleep(Duration::from_millis(delay)).await;
}

#[tokio::main]
async fn main() {
    let t1 = task(300u64).fuse();
    let t2 = task(200u64).fuse();
    pin_mut!(t1, t2);
    select! {
        () = t1 => println!("task one completed first"),
        () = t2 => println!("task two completed first"),
    }
}

tokio::select!⁵

use tokio::time::{sleep, Duration};
async fn task(delay: u64) {
    sleep(Duration::from_millis(delay)).await;
}

#[tokio::main]
async fn main() {
    let t1 = task(300u64);
    let t2 = task(200u64);
    tokio::pin!(t1, t2);
    tokio::select! {
        () = t1 => println!("task one completed first"),
        () = t2 => println!("task two completed first"),
    }
}

loop tokio::select!

#[tokio::main]
async fn main() {
    let mut count = 0;
    let t1 = task(300u64);
    let t2 = task(200u64);
    tokio::pin!(t1, t2);
    loop {
        if count > 5 {
            break;
        }
        tokio::select! {
            () = &mut t1 => println!("task one completed first"),
            () = &mut t2 => println!("task two completed first"),
        }
        count += 1;
    }
}

loop futures::select!

#[tokio::main]
async fn main() {
    let mut count = 0;
    let t1 = task(300u64).fuse();
    let t2 = task(200u64).fuse();
    tokio::pin!(t1, t2);
    loop {
        if count > 5 {
            break;
        }
        futures::select! {
            () = &mut t1 => println!("task one completed first"),
            () = &mut t2 => println!("task two completed first"),
        }
        count += 1;
    }
}

Stream⁶

pub trait Stream {
    type Item;

    // Required method
    fn poll_next(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>
    ) -> Poll<Option<Self::Item>>;
}

async-stream

fn zero_to_three() -> impl Stream<Item = u32> {
    stream! {
        for i in 0..3 {
            yield i;
        }
    }
}

#[tokio::main]
async fn main() {
    let s = zero_to_three();
    pin_mut!(s); // needed for iteration

    while let Some(value) = s.next().await {
        println!("got {}", value);
    }
}

futures::select! vs tokio::select!

• futures::select!
• tokio::select!
• SO - What's the difference between futures::select and tokio::select?
• Provide select! macro

Multiple futures

• FuturesUnordered
• FuturesOrdered
• Must read
  • FuturesUnordered and the order of futures

Cancellation

• futures::future::Abortable

Pitfalls

• Blocking in async
  • Async: What's blocking
  • TLDR: Async code should never spend a long time without reaching an .await
• Cancellation safety
• Holding a Mutex across an await
• Must read
  • Async cancellation: a case study of pub-sub in mini-redis
  • Yoshua Wuyts - Async Cancellation
  • Common mistakes with Rust Async
  • Rust tokio task cancellation patterns
  • for await and the battle of buffered streams
  • Mutex without lock, Queue without push: cancel safety in lilos

Cancellation safety with select!

So the TLDR

• futures in select! other than the future that yields Poll::Ready get dropped
• futures which own some form of state aren't cancellation safe, since the owned state gets dropped when another future returns Poll::Ready

Pinning

use std::pin::Pin;
use pin_project::pin_project;

#[pin_project]
struct Struct<T, U> {
    #[pin]
    pinned: T,
    unpinned: U,
}

impl<T, U> Struct<T, U> {
    fn method(self: Pin<&mut Self>) {
        let this = self.project();
        let _: Pin<&mut T> = this.pinned; // Pinned reference to the field
        let _: &mut U = this.unpinned; // Normal reference to the field
    }
}

Pinning

• Must read
  • std::pin
  • Pin and suffering
  • Pin, Unpin, and why Rust needs them
  • Async Book - Pinning
  • pin_project

More references

• Meetup code samples
• Tokio tutorial
• Tokio select
• Tokio internals
• How Rust optimizes async/await - I
• How Rust optimizes async/await - II

Questions

• Reach out on
  • Email: me@sanchayanmaity.net
  • Mastodon: sanchayanmaity.com
  • Telegram: https://t.me/SanchayanMaity
  • Blog: sanchayanmaity.net

---

1. Associated types ↩︎

2. Syntactic sugar for Future ↩︎

3. The state of Async Rust: Runtimes ↩︎

4. futures::select! ↩︎

5. tokio::select! ↩︎

6. Guided tour of Streams ↩︎