360 lines
18 KiB
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360 lines
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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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<p><a href="https://cfsamson.github.io/book-exploring-async-basics/1_concurrent_vs_parallel.html">Async Basics - The difference between concurrency and parallelism</a>
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<a href="https://cfsamson.github.io/book-exploring-async-basics/2_async_history.html">Async Basics - Async history</a>
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<a href="https://cfsamson.github.io/book-exploring-async-basics/5_strategies_for_handling_io.html">Async Basics - Strategies for handling I/O</a>
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<a href="https://cfsamson.github.io/book-exploring-async-basics/6_epoll_kqueue_iocp.html">Async Basics - Epoll, Kqueue and IOCP</a></p>
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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>
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<p><strong>In the case of <code>&[i32]</code>:</strong> </p>
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<ul>
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<li>
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<p>The first 8 bytes is the actual pointer to the first element in the array
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(or part of an array the slice refers to)</p>
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</li>
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<li>
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<p>The second 8 bytes is the length of the slice.</p>
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</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>
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<p>The layout for a pointer to a <em>trait object</em> looks like this: </p>
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<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>
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</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
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dynamic dispatch.</p>
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<p>Let's explain this in code instead of words by implementing our own trait
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object from these parts:</p>
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<pre><pre class="playpen"><code class="language-rust editable">// 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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// 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>
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<p>This code example is editable so you can change it
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and run it to see what happens.</p>
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<p>The reason we go through this will be clear later on when we implement our own
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<code>Waker</code> we'll actually set up a <code>vtable</code> like we do here to and knowing what
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it is will make this much less mysterious.</p>
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<p>With that out of the way, let's move on to our main example.</p>
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<h1><a class="header" href="#naive-example" id="naive-example">Naive example</a></h1>
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<h1><a class="header" href="#proper-waker" id="proper-waker">Proper Waker</a></h1>
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<h1><a class="header" href="#proper-future" id="proper-future">Proper Future</a></h1>
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<h1><a class="header" href="#supporting-asyncawait" id="supporting-asyncawait">Supporting async/await</a></h1>
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<h1><a class="header" href="#bonus-concurrent-futures" id="bonus-concurrent-futures">Bonus: concurrent futures</a></h1>
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