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<h1 class="menu-title">Futures Explained in 200 Lines of Rust</h1>
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<h1><a class="header" href="#introduction" id="introduction">Introduction</a></h1>
|
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<h1><a class="header" href="#some-background-information" id="some-background-information">Some background information</a></h1>
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<p>Before we start implementing our <code>Futures</code> , we'll go through some background
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information that will help demystify some of the concepts we encounter.</p>
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<h2><a class="header" href="#concurrency-in-general" id="concurrency-in-general">Concurrency in general</a></h2>
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<p>If you find the concepts of concurrency and async programming confusing in
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general, I know where you're coming from and I have written some resources to
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try to give a high level overview that will make it easier to learn Rusts
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<code>Futures</code> afterwards:</p>
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<ul>
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<li><a href="https://cfsamson.github.io/book-exploring-async-basics/1_concurrent_vs_parallel.html">Async Basics - The difference between concurrency and parallelism</a></li>
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|
<li><a href="https://cfsamson.github.io/book-exploring-async-basics/2_async_history.html">Async Basics - Async history</a></li>
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<li><a href="https://cfsamson.github.io/book-exploring-async-basics/5_strategies_for_handling_io.html">Async Basics - Strategies for handling I/O</a></li>
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<li><a href="https://cfsamson.github.io/book-exploring-async-basics/6_epoll_kqueue_iocp.html">Async Basics - Epoll, Kqueue and IOCP</a></li>
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</ul>
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<h2><a class="header" href="#trait-objects-and-dynamic-dispatch" id="trait-objects-and-dynamic-dispatch">Trait objects and dynamic dispatch</a></h2>
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<p>The single most confusing topic we encounter when implementing our own <code>Futures</code>
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is how we implement a <code>Waker</code> . Creating a <code>Waker</code> involves creating a <code>vtable</code>
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which allows using dynamic dispatch to call methods on a <em>type erased</em> trait
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object we construct our selves.</p>
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<p>If you want to know more about dynamic dispatch in Rust I can recommend this article:</p>
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<p>https://alschwalm.com/blog/static/2017/03/07/exploring-dynamic-dispatch-in-rust/</p>
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<p>Let's explain this a bit more in detail.</p>
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<h2><a class="header" href="#fat-pointers-in-rust" id="fat-pointers-in-rust">Fat pointers in Rust</a></h2>
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<p>Let's take a look at the size of some different pointer types in Rust. If we
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run the following code:</p>
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<pre><pre class="playpen"><code class="language-rust"><span class="boring">use std::mem::size_of;
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</span>trait SomeTrait { }
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fn main() {
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println!("Size of Box<i32>: {}", size_of::<Box<i32>>());
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println!("Size of &i32: {}", size_of::<&i32>());
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println!("Size of &Box<i32>: {}", size_of::<&Box<i32>>());
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println!("Size of Box<Trait>: {}", size_of::<Box<SomeTrait>>());
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println!("Size of &dyn Trait: {}", size_of::<&dyn SomeTrait>());
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println!("Size of &[i32]: {}", size_of::<&[i32]>());
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println!("Size of &[&dyn Trait]: {}", size_of::<&[&dyn SomeTrait]>());
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println!("Size of [i32; 10]: {}", size_of::<[i32; 10]>());
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println!("Size of [&dyn Trait; 10]: {}", size_of::<[&dyn SomeTrait; 10]>());
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}
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</code></pre></pre>
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<p>As you see from the output after running this, the sizes of the references varies.
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Most are 8 bytes (which is a pointer size on 64 bit systems), but some are 16
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bytes.</p>
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<p>The 16 byte sized pointers are called "fat pointers" since they carry more extra
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information.</p>
|
|
<p><strong>In the case of <code>&[i32]</code> :</strong> </p>
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|
<ul>
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<li>The first 8 bytes is the actual pointer to the first element in the array</li>
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|
</ul>
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<p>(or part of an array the slice refers to)</p>
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|
<ul>
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<li>The second 8 bytes is the length of the slice.</li>
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|
</ul>
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|
<p>The one we'll concern ourselves about is the references to traits, or
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<em>trait objects</em> as they're called in Rust.</p>
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|
<p><code>&dyn SomeTrait</code> is an example of a <em>trait object</em> </p>
|
|
<p>The layout for a pointer to a <em>trait object</em> looks like this: </p>
|
|
<ul>
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<li>The first 8 bytes points to the <code>data</code> for the trait object</li>
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|
<li>The second 8 bytes points to the <code>vtable</code> for the trait object</li>
|
|
</ul>
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|
<p>The reason for this is to allow us to refer to an object we know nothing about
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|
except that it implements the methods defined by our trait. To allow this we use
|
|
dynamic dispatch.</p>
|
|
<p>Let's explain this in code instead of words by implementing our own trait
|
|
object from these parts:</p>
|
|
<pre><pre class="playpen"><code class="language-rust">// A reference to a trait object is a fat pointer: (data_ptr, vtable_ptr)
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trait Test {
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|
fn add(&self) -> i32;
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fn sub(&self) -> i32;
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fn mul(&self) -> i32;
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}
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// This will represent our home brewn fat pointer to a trait object
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#[repr(C)]
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struct FatPointer<'a> {
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/// A reference is a pointer to an instantiated `Data` instance
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data: &'a mut Data,
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/// Since we need to pass in literal values like length and alignment it's
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/// easiest for us to convert pointers to usize-integers instead of the other way around.
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vtable: *const usize,
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}
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// This is the data in our trait object. It's just two numbers we want to operate on.
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struct Data {
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a: i32,
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b: i32,
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}
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// ====== function definitions ======
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|
fn add(s: &Data) -> i32 {
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s.a + s.b
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}
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fn sub(s: &Data) -> i32 {
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s.a - s.b
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}
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fn mul(s: &Data) -> i32 {
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s.a * s.b
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}
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fn main() {
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|
let mut data = Data {a: 3, b: 2};
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|
// vtable is like special purpose array of pointer-length types with a fixed
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|
// format where the three first values has a special meaning like the
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// length of the array is encoded in the array itself as the second value.
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|
let vtable = vec![
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|
0, // pointer to `Drop` (which we're not implementing here)
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|
6, // lenght of vtable
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|
8, // alignment
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// we need to make sure we add these in the same order as defined in the Trait.
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|
// Try changing the order of add and sub and see what happens.
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add as usize, // function pointer
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sub as usize, // function pointer
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mul as usize, // function pointer
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];
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let fat_pointer = FatPointer { data: &mut data, vtable: vtable.as_ptr()};
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let test = unsafe { std::mem::transmute::<FatPointer, &dyn Test>(fat_pointer) };
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|
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// And voalá, it's now a trait object we can call methods on
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println!("Add: 3 + 2 = {}", test.add());
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println!("Sub: 3 - 2 = {}", test.sub());
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println!("Mul: 3 * 2 = {}", test.mul());
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}
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</code></pre></pre>
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|
<p>If you run this code by pressing the "play" button at the top you'll se it
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|
outputs just what we expect.</p>
|
|
<p>This code example is editable so you can change it
|
|
and run it to see what happens.</p>
|
|
<p>The reason we go through this will be clear later on when we implement our own
|
|
<code>Waker</code> we'll actually set up a <code>vtable</code> like we do here to and knowing what
|
|
it is will make this much less mysterious.</p>
|
|
<h2><a class="header" href="#reactorexecutor-pattern" id="reactorexecutor-pattern">Reactor/Executor pattern</a></h2>
|
|
<p>If you don't know what this is, you should take a few minutes and read about
|
|
it. You will encounter the term <code>Reactor</code> and <code>Executor</code> a lot when working
|
|
with async code in Rust.</p>
|
|
<p>I have written a quick introduction explaining this pattern before which you
|
|
can take a look at here:</p>
|
|
<p><a href="https://cfsamsonbooks.gitbook.io/epoll-kqueue-iocp-explained/appendix-1/reactor-executor-pattern"><img src="./assets/reactorexecutor.png" alt="homepage" /></a></p>
|
|
<div style="text-align:center">
|
|
<a href="https://cfsamsonbooks.gitbook.io/epoll-kqueue-iocp-explained/appendix-1/reactor-executor-pattern">Epoll, Kqueue and IOCP Explained - The Reactor-Executor Pattern</a>
|
|
</div>
|
|
<p>I'll re-iterate the most important parts here.</p>
|
|
<p>This pattern consists of at least 2 parts:</p>
|
|
<ol>
|
|
<li>A reactor
|
|
<ul>
|
|
<li>handles some kind of event queue</li>
|
|
<li>has the responsibility of respoonding to events</li>
|
|
</ul>
|
|
</li>
|
|
<li>An executor
|
|
<ul>
|
|
<li>Often has a scheduler</li>
|
|
<li>Holds a set of suspended tasks, and has the responsibility of resuming
|
|
them when an event has occurred</li>
|
|
</ul>
|
|
</li>
|
|
<li>The concept of a task
|
|
<ul>
|
|
<li>A set of operations that can be stopped half way and resumed later on</li>
|
|
</ul>
|
|
</li>
|
|
</ol>
|
|
<p>This is a pattern not only used in Rust, but it's very popular in Rust due to
|
|
how well it separates concerns between handling and scheduling tasks, and queing
|
|
and responding to I/O events.</p>
|
|
<p>The only thing Rust as a language defines is the <em>task</em>. In Rust we call an
|
|
incorruptible task a <code>Future</code>. Futures has a well defined interface, which means
|
|
they can be used across the entire ecosystem.</p>
|
|
<p>In addition, Rust provides a way for the Reactor and Executor to communicate
|
|
through the <code>Waker</code>. We'll get to know these in the following chapters.</p>
|
|
<p>Providing these pieces let's Rust take care a lot of the ergonomic "friction"
|
|
programmers meet when faced with async code, and still not dictate any
|
|
preferred runtime to actually do the scheduling and I/O queues.</p>
|
|
<p>It's important to know that Rust doesn't provide a runtime, so you have to choose
|
|
one. <a href="https://github.com/async-rs/async-std">async std</a> and <a href="https://github.com/tokio-rs/tokio">tokio</a> are two popular ones.</p>
|
|
<p>With that out of the way, let's move on to our main example.</p>
|
|
<h1><a class="header" href="#naive-example" id="naive-example">Naive example</a></h1>
|
|
<pre><pre class="playpen"><code class="language-rust">use std::sync::atomic::{AtomicUsize, Ordering};
|
|
use std::sync::mpsc::{channel, Sender};
|
|
use std::sync::{Arc, Mutex};
|
|
use std::thread::{self, JoinHandle};
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|
use std::time::{Duration, Instant};
|
|
|
|
fn main() {
|
|
let readylist = Arc::new(Mutex::new(vec![]));
|
|
let mut reactor = Reactor::new();
|
|
|
|
let mywaker = MyWaker::new(1, thread::current(), readylist.clone());
|
|
reactor.register(2, mywaker);
|
|
|
|
let mywaker = MyWaker::new(2, thread::current(), readylist.clone());
|
|
reactor.register(2, mywaker);
|
|
|
|
executor_run(reactor, readylist);
|
|
}
|
|
// ====== EXECUTOR ======
|
|
fn executor_run(mut reactor: Reactor, rl: Arc<Mutex<Vec<usize>>>) {
|
|
let start = Instant::now();
|
|
loop {
|
|
let mut rl_locked = rl.lock().unwrap();
|
|
while let Some(event) = rl_locked.pop() {
|
|
let dur = (Instant::now() - start).as_secs_f32();
|
|
println!("Event {} just happened at time: {:.2}.", event, dur);
|
|
reactor.outstanding.fetch_sub(1, Ordering::Relaxed);
|
|
}
|
|
drop(rl_locked);
|
|
|
|
if reactor.outstanding.load(Ordering::Relaxed) == 0 {
|
|
reactor.close();
|
|
break;
|
|
}
|
|
|
|
thread::park();
|
|
}
|
|
}
|
|
|
|
// ====== "FUTURE" IMPL ======
|
|
#[derive(Debug)]
|
|
struct MyWaker {
|
|
id: usize,
|
|
thread: thread::Thread,
|
|
readylist: Arc<Mutex<Vec<usize>>>,
|
|
}
|
|
|
|
impl MyWaker {
|
|
fn new(id: usize, thread: thread::Thread, readylist: Arc<Mutex<Vec<usize>>>) -> Self {
|
|
MyWaker {
|
|
id,
|
|
thread,
|
|
readylist,
|
|
}
|
|
}
|
|
|
|
fn wake(&self) {
|
|
self.readylist.lock().map(|mut rl| rl.push(self.id)).unwrap();
|
|
self.thread.unpark();
|
|
}
|
|
}
|
|
|
|
|
|
#[derive(Debug, Clone)]
|
|
pub struct Task {
|
|
id: usize,
|
|
pending: bool,
|
|
}
|
|
|
|
// ===== REACTOR =====
|
|
struct Reactor {
|
|
dispatcher: Sender<Event>,
|
|
handle: Option<JoinHandle<()>>,
|
|
outstanding: AtomicUsize,
|
|
}
|
|
#[derive(Debug)]
|
|
enum Event {
|
|
Close,
|
|
Simple(MyWaker, u64),
|
|
}
|
|
|
|
impl Reactor {
|
|
fn new() -> Self {
|
|
let (tx, rx) = channel::<Event>();
|
|
let mut handles = vec![];
|
|
let handle = thread::spawn(move || {
|
|
// This simulates some I/O resource
|
|
for event in rx {
|
|
match event {
|
|
Event::Close => break,
|
|
Event::Simple(mywaker, duration) => {
|
|
let event_handle = thread::spawn(move || {
|
|
thread::sleep(Duration::from_secs(duration));
|
|
mywaker.wake();
|
|
});
|
|
handles.push(event_handle);
|
|
}
|
|
}
|
|
}
|
|
|
|
for handle in handles {
|
|
handle.join().unwrap();
|
|
}
|
|
});
|
|
|
|
Reactor {
|
|
dispatcher: tx,
|
|
handle: Some(handle),
|
|
outstanding: AtomicUsize::new(0),
|
|
}
|
|
}
|
|
|
|
fn register(&mut self, duration: u64, mywaker: MyWaker) {
|
|
self.dispatcher
|
|
.send(Event::Simple(mywaker, duration))
|
|
.unwrap();
|
|
self.outstanding.fetch_add(1, Ordering::Relaxed);
|
|
}
|
|
|
|
fn close(&mut self) {
|
|
self.dispatcher.send(Event::Close).unwrap();
|
|
}
|
|
}
|
|
|
|
impl Drop for Reactor {
|
|
fn drop(&mut self) {
|
|
self.handle.take().map(|h| h.join().unwrap()).unwrap();
|
|
}
|
|
}
|
|
</code></pre></pre>
|
|
<pre><pre class="playpen"><code class="language-rust">use std::sync::atomic::{AtomicUsize, Ordering};
|
|
use std::sync::mpsc::{channel, Sender};
|
|
use std::sync::{Arc, Mutex};
|
|
use std::thread::{self, JoinHandle};
|
|
use std::time::{Duration, Instant};
|
|
|
|
fn main() {
|
|
let readylist = Arc::new(Mutex::new(vec![]));
|
|
let mut reactor = Reactor::new();
|
|
|
|
let mywaker = MyWaker::new(1, thread::current(), readylist.clone());
|
|
reactor.register(2, mywaker);
|
|
|
|
let mywaker = MyWaker::new(2, thread::current(), readylist.clone());
|
|
reactor.register(2, mywaker);
|
|
|
|
executor_run(reactor, readylist);
|
|
}
|
|
<span class="boring">// ====== EXECUTOR ======
|
|
</span><span class="boring">fn executor_run(mut reactor: Reactor, rl: Arc<Mutex<Vec<usize>>>) {
|
|
</span><span class="boring"> let start = Instant::now();
|
|
</span><span class="boring"> loop {
|
|
</span><span class="boring"> let mut rl_locked = rl.lock().unwrap();
|
|
</span><span class="boring"> while let Some(event) = rl_locked.pop() {
|
|
</span><span class="boring"> let dur = (Instant::now() - start).as_secs_f32();
|
|
</span><span class="boring"> println!("Event {} just happened at time: {:.2}.", event, dur);
|
|
</span><span class="boring"> reactor.outstanding.fetch_sub(1, Ordering::Relaxed);
|
|
</span><span class="boring"> }
|
|
</span><span class="boring"> drop(rl_locked);
|
|
</span><span class="boring">
|
|
</span><span class="boring"> if reactor.outstanding.load(Ordering::Relaxed) == 0 {
|
|
</span><span class="boring"> reactor.close();
|
|
</span><span class="boring"> break;
|
|
</span><span class="boring"> }
|
|
</span><span class="boring">
|
|
</span><span class="boring"> thread::park();
|
|
</span><span class="boring"> }
|
|
</span><span class="boring">}
|
|
</span><span class="boring">
|
|
</span><span class="boring">// ====== "FUTURE" IMPL ======
|
|
</span><span class="boring">#[derive(Debug)]
|
|
</span><span class="boring">struct MyWaker {
|
|
</span><span class="boring"> id: usize,
|
|
</span><span class="boring"> thread: thread::Thread,
|
|
</span><span class="boring"> readylist: Arc<Mutex<Vec<usize>>>,
|
|
</span><span class="boring">}
|
|
</span><span class="boring">
|
|
</span><span class="boring">impl MyWaker {
|
|
</span><span class="boring"> fn new(id: usize, thread: thread::Thread, readylist: Arc<Mutex<Vec<usize>>>) -> Self {
|
|
</span><span class="boring"> MyWaker {
|
|
</span><span class="boring"> id,
|
|
</span><span class="boring"> thread,
|
|
</span><span class="boring"> readylist,
|
|
</span><span class="boring"> }
|
|
</span><span class="boring"> }
|
|
</span><span class="boring">
|
|
</span><span class="boring"> fn wake(&self) {
|
|
</span><span class="boring"> self.readylist.lock().map(|mut rl| rl.push(self.id)).unwrap();
|
|
</span><span class="boring"> self.thread.unpark();
|
|
</span><span class="boring"> }
|
|
</span><span class="boring">}
|
|
</span><span class="boring">
|
|
</span><span class="boring">
|
|
</span><span class="boring">#[derive(Debug, Clone)]
|
|
</span><span class="boring">pub struct Task {
|
|
</span><span class="boring"> id: usize,
|
|
</span><span class="boring"> pending: bool,
|
|
</span><span class="boring">}
|
|
</span><span class="boring">
|
|
</span><span class="boring">// ===== REACTOR =====
|
|
</span><span class="boring">struct Reactor {
|
|
</span><span class="boring"> dispatcher: Sender<Event>,
|
|
</span><span class="boring"> handle: Option<JoinHandle<()>>,
|
|
</span><span class="boring"> outstanding: AtomicUsize,
|
|
</span><span class="boring">}
|
|
</span><span class="boring">#[derive(Debug)]
|
|
</span><span class="boring">enum Event {
|
|
</span><span class="boring"> Close,
|
|
</span><span class="boring"> Simple(MyWaker, u64),
|
|
</span><span class="boring">}
|
|
</span><span class="boring">
|
|
</span><span class="boring">impl Reactor {
|
|
</span><span class="boring"> fn new() -> Self {
|
|
</span><span class="boring"> let (tx, rx) = channel::<Event>();
|
|
</span><span class="boring"> let mut handles = vec![];
|
|
</span><span class="boring"> let handle = thread::spawn(move || {
|
|
</span><span class="boring"> // This simulates some I/O resource
|
|
</span><span class="boring"> for event in rx {
|
|
</span><span class="boring"> match event {
|
|
</span><span class="boring"> Event::Close => break,
|
|
</span><span class="boring"> Event::Simple(mywaker, duration) => {
|
|
</span><span class="boring"> let event_handle = thread::spawn(move || {
|
|
</span><span class="boring"> thread::sleep(Duration::from_secs(duration));
|
|
</span><span class="boring"> mywaker.wake();
|
|
</span><span class="boring"> });
|
|
</span><span class="boring"> handles.push(event_handle);
|
|
</span><span class="boring"> }
|
|
</span><span class="boring"> }
|
|
</span><span class="boring"> }
|
|
</span><span class="boring">
|
|
</span><span class="boring"> for handle in handles {
|
|
</span><span class="boring"> handle.join().unwrap();
|
|
</span><span class="boring"> }
|
|
</span><span class="boring"> });
|
|
</span><span class="boring">
|
|
</span><span class="boring"> Reactor {
|
|
</span><span class="boring"> dispatcher: tx,
|
|
</span><span class="boring"> handle: Some(handle),
|
|
</span><span class="boring"> outstanding: AtomicUsize::new(0),
|
|
</span><span class="boring"> }
|
|
</span><span class="boring"> }
|
|
</span><span class="boring">
|
|
</span><span class="boring"> fn register(&mut self, duration: u64, mywaker: MyWaker) {
|
|
</span><span class="boring"> self.dispatcher
|
|
</span><span class="boring"> .send(Event::Simple(mywaker, duration))
|
|
</span><span class="boring"> .unwrap();
|
|
</span><span class="boring"> self.outstanding.fetch_add(1, Ordering::Relaxed);
|
|
</span><span class="boring"> }
|
|
</span><span class="boring">
|
|
</span><span class="boring"> fn close(&mut self) {
|
|
</span><span class="boring"> self.dispatcher.send(Event::Close).unwrap();
|
|
</span><span class="boring"> }
|
|
</span><span class="boring">}
|
|
</span><span class="boring">
|
|
</span><span class="boring">impl Drop for Reactor {
|
|
</span><span class="boring"> fn drop(&mut self) {
|
|
</span><span class="boring"> self.handle.take().map(|h| h.join().unwrap()).unwrap();
|
|
</span><span class="boring"> }
|
|
</span><span class="boring">}
|
|
</span></code></pre></pre>
|
|
<h1><a class="header" href="#proper-waker" id="proper-waker">Proper Waker</a></h1>
|
|
<h1><a class="header" href="#proper-future" id="proper-future">Proper Future</a></h1>
|
|
<h1><a class="header" href="#supporting-asyncawait" id="supporting-asyncawait">Supporting async/await</a></h1>
|
|
<h1><a class="header" href="#bonus-concurrent-futures" id="bonus-concurrent-futures">Bonus: concurrent futures</a></h1>
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