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Carl Fredrik Samson
2020-01-25 20:20:43 +01:00
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<!DOCTYPE HTML>
<html lang="en" class="sidebar-visible no-js">
<html lang="en" class="sidebar-visible no-js light">
<head>
<!-- Book generated using mdBook -->
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<nav id="sidebar" class="sidebar" aria-label="Table of contents">
<div class="sidebar-scrollbox">
<ol class="chapter"><li><a href="0_0_Introduction.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><a href="0_1_BackgroundInformation.html"><strong aria-hidden="true">2.</strong> Some background information</a></li></ol>
<div id="sidebar-scrollbox" class="sidebar-scrollbox">
<ol class="chapter"><li class="expanded "><a href="0_0_introduction.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li class="expanded "><a href="0_1_background_information.html"><strong aria-hidden="true">2.</strong> Some background information</a></li><li class="expanded "><a href="0_2_naive_implementation.html"><strong aria-hidden="true">3.</strong> Naive example</a></li><li class="expanded "><a href="0_3_proper_waker.html"><strong aria-hidden="true">4.</strong> Proper Waker</a></li><li class="expanded "><a href="0_4_proper_future.html"><strong aria-hidden="true">5.</strong> Proper Future</a></li><li class="expanded "><a href="0_5_async_wait.html"><strong aria-hidden="true">6.</strong> Supporting async/await</a></li><li class="expanded "><a href="0_6_concurrent_futures.html"><strong aria-hidden="true">7.</strong> Bonus: concurrent futures</a></li></ol>
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<main>
<h1><a class="header" href="#introduction" id="introduction">Introduction</a></h1>
<h1><a class="header" href="#some-background-information" id="some-background-information">Some background information</a></h1>
<p>Before we start implementing our <code>Futures</code>, we'll go through some background
information that will help demystify some of the concepts we encounter.</p>
<h2><a class="header" href="#concurrency-in-general" id="concurrency-in-general">Concurrency in general</a></h2>
<p>If you find the concepts of concurrency and async programming confusing in
general, I know where you're coming from and I have written some resources to
try to give a high level overview that will make it easier to learn Rusts
<code>Futures</code> afterwards:</p>
<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>
<a href="https://cfsamson.github.io/book-exploring-async-basics/2_async_history.html">Async Basics - Async history</a>
<a href="https://cfsamson.github.io/book-exploring-async-basics/5_strategies_for_handling_io.html">Async Basics - Strategies for handling I/O</a>
<a href="https://cfsamson.github.io/book-exploring-async-basics/6_epoll_kqueue_iocp.html">Async Basics - Epoll, Kqueue and IOCP</a></p>
<h2><a class="header" href="#trait-objects-and-dynamic-dispatch" id="trait-objects-and-dynamic-dispatch">Trait objects and dynamic dispatch</a></h2>
<p>The single most confusing topic we encounter when implementing our own <code>Futures</code>
is how we implement a <code>Waker</code>. Creating a <code>Waker</code> involves creating a <code>vtable</code>
which allows using dynamic dispatch to call methods on a <em>type erased</em> trait
object we construct our selves.</p>
<p>If you want to know more about dynamic dispatch in Rust I can recommend this article:</p>
<p>https://alschwalm.com/blog/static/2017/03/07/exploring-dynamic-dispatch-in-rust/</p>
<p>Let's explain this a bit more in detail.</p>
<h2><a class="header" href="#fat-pointers-in-rust" id="fat-pointers-in-rust">Fat pointers in Rust</a></h2>
<p>Let's take a look at the size of some different pointer types in Rust. If we
run the following code:</p>
<pre><pre class="playpen"><code class="language-rust"><span class="boring">use std::mem::size_of;
</span>trait SomeTrait { }
fn main() {
println!(&quot;Size of Box&lt;i32&gt;: {}&quot;, size_of::&lt;Box&lt;i32&gt;&gt;());
println!(&quot;Size of &amp;i32: {}&quot;, size_of::&lt;&amp;i32&gt;());
println!(&quot;Size of &amp;Box&lt;i32&gt;: {}&quot;, size_of::&lt;&amp;Box&lt;i32&gt;&gt;());
println!(&quot;Size of Box&lt;Trait&gt;: {}&quot;, size_of::&lt;Box&lt;SomeTrait&gt;&gt;());
println!(&quot;Size of &amp;dyn Trait: {}&quot;, size_of::&lt;&amp;dyn SomeTrait&gt;());
println!(&quot;Size of &amp;[i32]: {}&quot;, size_of::&lt;&amp;[i32]&gt;());
println!(&quot;Size of &amp;[&amp;dyn Trait]: {}&quot;, size_of::&lt;&amp;[&amp;dyn SomeTrait]&gt;());
println!(&quot;Size of [i32; 10]: {}&quot;, size_of::&lt;[i32; 10]&gt;());
println!(&quot;Size of [&amp;dyn Trait; 10]: {}&quot;, size_of::&lt;[&amp;dyn SomeTrait; 10]&gt;());
}
</code></pre></pre>
<p>As you see from the output after running this, the sizes of the references varies.
Most are 8 bytes (which is a pointer size on 64 bit systems), but some are 16
bytes.</p>
<p>The 16 byte sized pointers are called &quot;fat pointers&quot; since they carry more extra
information. </p>
<p><strong>In the case of <code>&amp;[i32]</code>:</strong> </p>
<ul>
<li>
<p>The first 8 bytes is the actual pointer to the first element in the array
(or part of an array the slice refers to)</p>
</li>
<li>
<p>The second 8 bytes is the length of the slice.</p>
</li>
</ul>
<p>The one we'll concern ourselves about is the references to traits, or
<em>trait objects</em> as they're called in Rust.</p>
<p><code>&amp;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>
<li>The first 8 bytes points to the <code>data</code> for the trait object</li>
<li>The second 8 bytes points to the <code>vtable</code> for the trait object</li>
</ul>
<p>The reason for this is to allow us to refer to an object we know nothing about
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 editable">// A reference to a trait object is a fat pointer: (data_ptr, vtable_ptr)
trait Test {
fn add(&amp;self) -&gt; i32;
fn sub(&amp;self) -&gt; i32;
fn mul(&amp;self) -&gt; i32;
}
// This will represent our home brewn fat pointer to a trait object
#[repr(C)]
struct FatPointer&lt;'a&gt; {
/// A reference is a pointer to an instantiated `Data` instance
data: &amp;'a mut Data,
/// Since we need to pass in literal values like length and alignment it's
/// easiest for us to convert pointers to usize-integers instead of the other way around.
vtable: *const usize,
}
// This is the data in our trait object. It's just two numbers we want to operate on.
struct Data {
a: i32,
b: i32,
}
// ====== function definitions ======
fn add(s: &amp;Data) -&gt; i32 {
s.a + s.b
}
fn sub(s: &amp;Data) -&gt; i32 {
s.a - s.b
}
fn mul(s: &amp;Data) -&gt; i32 {
s.a * s.b
}
fn main() {
let mut data = Data {a: 3, b: 2};
// vtable is like special purpose array of pointer-length types with a fixed
// format where the three first values has a special meaning like the
// length of the array is encoded in the array itself as the second value.
let vtable = vec![
0, // pointer to `Drop` (which we're not implementing here)
6, // lenght of vtable
8, // alignment
// we need to make sure we add these in the same order as defined in the Trait.
// Try changing the order of add and sub and see what happens.
add as usize, // function pointer
sub as usize, // function pointer
mul as usize, // function pointer
];
let fat_pointer = FatPointer { data: &amp;mut data, vtable: vtable.as_ptr()};
let test = unsafe { std::mem::transmute::&lt;FatPointer, &amp;dyn Test&gt;(fat_pointer) };
// And voalá, it's now a trait object we can call methods on
println!(&quot;Add: 3 + 2 = {}&quot;, test.add());
println!(&quot;Sub: 3 - 2 = {}&quot;, test.sub());
println!(&quot;Mul: 3 * 2 = {}&quot;, test.mul());
}
</code></pre></pre>
<p>If you run this code by pressing the &quot;play&quot; button at the top you'll se it
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>
<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>
<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>
</main>
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