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76 lines
3 KiB
Markdown
76 lines
3 KiB
Markdown
## Writing an Accept Loop
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Let's implement the scaffold of the server: a loop that binds a TCP socket to an address and starts accepting connections.
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First of all, let's add required import boilerplate:
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```rust
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#![feature(async_await)]
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use std::net::ToSocketAddrs; // 1
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use async_std::{
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prelude::*, // 2
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task, // 3
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net::TcpListener, // 4
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};
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type Result<T> = std::result::Result<T, Box<dyn std::error::Error + Send + Sync>>; // 5
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```
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1. `async_std` uses `std` types where appropriate.
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We'll need `ToSocketAddrs` to specify address to listen on.
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2. `prelude` re-exports some traits required to work with futures and streams.
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3. The `task` module roughly corresponds to the `std::thread` module, but tasks are much lighter weight.
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A single thread can run many tasks.
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4. For the socket type, we use `TcpListener` from `async_std`, which is just like `std::net::TcpListener`, but is non-blocking and uses `async` API.
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5. We will skip implementing comprehensive error handling in this example.
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To propagate the errors, we will use a boxed error trait object.
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Do you know that there's `From<&'_ str> for Box<dyn Error>` implementation in stdlib, which allows you to use strings with `?` operator?
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Now we can write the server's accept loop:
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```rust
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async fn server(addr: impl ToSocketAddrs) -> Result<()> { // 1
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let listener = TcpListener::bind(addr).await?; // 2
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let mut incoming = listener.incoming();
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while let Some(stream) = incoming.next().await { // 3
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// TODO
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}
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Ok(())
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}
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```
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1. We mark the `server` function as `async`, which allows us to use `.await` syntax inside.
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2. `TcpListener::bind` call returns a future, which we `.await` to extract the `Result`, and then `?` to get a `TcpListener`.
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Note how `.await` and `?` work nicely together.
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This is exactly how `std::net::TcpListener` works, but with `.await` added.
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Mirroring API of `std` is an explicit design goal of `async_std`.
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3. Here, we would like to iterate incoming sockets, just how one would do in `std`:
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```rust
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let listener: std::net::TcpListener = unimplemented!();
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for stream in listener.incoming() {
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}
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```
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Unfortunately this doesn't quite work with `async` yet, because there's no support for `async` for-loops in the language yet.
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For this reason we have to implement the loop manually, by using `while let Some(item) = iter.next().await` pattern.
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Finally, let's add main:
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```rust
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fn main() -> Result<()> {
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let fut = server("127.0.0.1:8080");
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task::block_on(fut)
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}
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```
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The crucial thing to realise that is in Rust, unlike other languages, calling an async function does **not** run any code.
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Async functions only construct futures, which are inert state machines.
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To start stepping through the future state-machine in an async function, you should use `.await`.
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In a non-async function, a way to execute a future is to handle it to the executor.
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In this case, we use `task::block_on` to execute a future on the current thread and block until it's done.
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