Presentation on Rust future/async/await

futures-async-await/Makefile

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+all:
+	make slide slideshow
+
+slide:
+	pandoc -t beamer async.md -f markdown-implicit_figures -V colorlinks=true -V linkcolor=blue -V urlcolor=red -o async.pdf
+
+slideshow:
+	pandoc -t beamer async.md -f markdown-implicit_figures -V colorlinks=true -V linkcolor=blue -V urlcolor=red -i -o async-slideshow.pdf
+
+view:
+	zathura --mode=presentation async.pdf &

futures-async-await/async.md

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+---
+title:
+- Getting comfy with async await
+author:
+- Sanchayan Maity
+theme:
+- default
+classoption:
+- 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](https://asymptotic.io/)
+  * 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`
+
+# Future[^1]
+
+```rust
+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,
+}
+```
+
+[^1]: [Associated types](https://doc.rust-lang.org/reference/items/associated-items.html)
+
+# Example
+
+```rust
+async fn hello() {
+    println!("Hello from async");
+}
+
+fn main() {
+    hello();
+    println!("Hello from main");
+}
+```
+
+# Where's the future
+
+```rust
+async fn give_number() -> u32 {
+    100
+}
+```
+
+# Sugar town[^2]
+
+```rust
+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)
+    }
+}
+```
+
+[^2]: [Syntactic sugar for Future](https://ruspiro.github.io/ruspiro-async-book/02-03-async.html)
+
+
+# Runtimes
+
+![*_Run Futures_*](future-meme.jpg){width=80%}
+
+
+# Runtimes[^3]
+
+- `futures::executor`
+- `tokio`
+- `smol-rs`
+- `embassy`
+- `glommio`
+- `async-std`
+
+[^3]: [The state of Async Rust: Runtimes](https://corrode.dev/blog/async/)
+
+
+# Example
+
+```rust
+use futures::executor::block_on;
+
+async fn hello() {
+    println!("hello, world!");
+}
+
+fn main() {
+    block_on(hello());
+    println!("Hello from main");
+}
+```
+
+# Example
+
+```rust
+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`
+
+```rust
+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`
+
+```rust
+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`
+
+```rust
+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`
+
+```rust
+pub fn select<A, B>(future1: A, future2: B) -> Select<A, B>
+where
+    A: Future + Unpin,
+    B: Future + Unpin,
+```
+
+
+# `future::select`
+
+
+```rust
+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`
+
+```rust
+#[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]
+
+```rust
+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"),
+    }
+}
+```
+
+[^4]: [futures::select!](https://docs.rs/futures/latest/futures/macro.select.html)
+
+
+# `tokio::select!`[^5]
+
+```rust
+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"),
+    }
+}
+```
+
+[^5]: [tokio::select!](https://docs.rs/tokio/latest/tokio/macro.select.html)
+
+# `loop tokio::select!`
+
+```rust
+#[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!`
+
+```rust
+#[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;
+    }
+}
+```
+
+# `Stream`[^6]
+
+```rust
+pub trait Stream {
+    type Item;
+
+    // Required method
+    fn poll_next(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>
+    ) -> Poll<Option<Self::Item>>;
+}
+```
+
+[^6]: [Guided tour of Streams](https://www.qovery.com/blog/a-guided-tour-of-streams-in-rust/)
+
+
+# `async-stream`
+
+```rust
+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!](https://docs.rs/futures/latest/futures/macro.select.html)
+- [tokio::select!](https://docs.rs/tokio/latest/tokio/macro.select.html)
+- [SO - What's the difference between futures::select and tokio::select?](https://stackoverflow.com/questions/60811657/what-is-the-difference-between-futuresselect-and-tokioselect)
+- [Provide `select!` macro](https://stackoverflow.com/questions/60811657/what-is-the-difference-between-futuresselect-and-tokioselect)
+
+# Multiple futures
+
+- [FuturesUnordered](https://docs.rs/futures/latest/futures/stream/struct.FuturesUnordered.html)
+- [FuturesOrdered](https://docs.rs/futures/latest/futures/stream/struct.FuturesOrdered.html)
+- Must read
+    - [FuturesUnordered and the order of futures](https://without.boats/blog/futures-unordered/)
+
+# Cancellation
+
+- [futures::future::Abortable](https://docs.rs/futures/latest/futures/future/struct.Abortable.html)
+
+# Pitfalls
+
+- Blocking in `async`
+    - [Async: What's blocking](https://ryhl.io/blog/async-what-is-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](https://smallcultfollowing.com/babysteps/blog/2022/06/13/async-cancellation-a-case-study-of-pub-sub-in-mini-redis/)
+    - [Yoshua Wuyts - Async Cancellation](https://blog.yoshuawuyts.com/async-cancellation-1/)
+    - [Common mistakes with Rust Async](https://www.qovery.com/blog/common-mistakes-with-rust-async/)
+    - [Rust tokio task cancellation patterns](https://cybernetist.com/2024/04/19/rust-tokio-task-cancellation-patterns/)
+    - [`for await` and the battle of buffered streams](https://tmandry.gitlab.io/blog/posts/for-await-buffered-streams/)
+    - [Mutex without lock, Queue without push: cancel safety in lilos](https://cliffle.com/blog/lilos-cancel-safety/)
+
+
+# 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
+
+```rust
+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](https://doc.rust-lang.org/std/pin/index.html#projections-and-structural-pinning)
+    - [Pin and suffering](https://fasterthanli.me/articles/pin-and-suffering)
+    - [Pin, Unpin, and why Rust needs them](https://blog.cloudflare.com/pin-and-unpin-in-rust/)
+    - [Async Book - Pinning](https://rust-lang.github.io/async-book/04_pinning/01_chapter.html)
+    - [pin_project](https://docs.rs/pin-project/latest/pin_project/)
+
+# More references
+
+- [Meetup code samples](https://git.sanchayanmaity.net/sanchayanmaity/async-await-rust-meetup-examples)
+- [Tokio tutorial](https://tokio.rs/tokio/tutorial)
+- [Tokio select](https://tokio.rs/tokio/tutorial/select)
+- [Tokio internals](https://cafbit.com/post/tokio_internals/)
+- [How Rust optimizes async/await - I](https://tmandry.gitlab.io/blog/posts/optimizing-await-1/)
+- [How Rust optimizes async/await - II](https://tmandry.gitlab.io/blog/posts/optimizing-await-2/)
+
+# Questions
+
+- Reach out on
+  * Email: me@sanchayanmaity.net
+  * Mastodon: [sanchayanmaity.com](https://sanchayanmaity.com/@sanchayan)
+  * Telegram: https://t.me/SanchayanMaity
+  * Blog: [sanchayanmaity.net](https://sanchayanmaity.net/)