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a8090be3eb
* Fix book to use futures_channel and futures_util, re-enable testing * Make dev dependencies for the book explicit
46 lines
2 KiB
Markdown
46 lines
2 KiB
Markdown
## Sending Messages
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Now it's time to implement the other half -- sending messages.
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A most obvious way to implement sending is to give each `client` access to the write half of `TcpStream` of each other clients.
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That way, a client can directly `.write_all` a message to recipients.
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However, this would be wrong: if Alice sends `bob: foo`, and Charley sends `bob: bar`, Bob might actually receive `fobaor`.
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Sending a message over a socket might require several syscalls, so two concurrent `.write_all`'s might interfere with each other!
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As a rule of thumb, only a single task should write to each `TcpStream`.
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So let's create a `client_writer` task which receives messages over a channel and writes them to the socket.
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This task would be the point of serialization of messages.
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if Alice and Charley send two messages to Bob at the same time, Bob will see the messages in the same order as they arrive in the channel.
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```rust,edition2018
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# extern crate async_std;
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# extern crate futures_channel;
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# extern crate futures_util;
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# use async_std::{
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# io::Write,
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# net::TcpStream,
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# prelude::Stream,
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# };
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use futures_channel::mpsc; // 1
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use futures_util::sink::SinkExt;
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use std::sync::Arc;
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# type Result<T> = std::result::Result<T, Box<dyn std::error::Error + Send + Sync>>;
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type Sender<T> = mpsc::UnboundedSender<T>; // 2
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type Receiver<T> = mpsc::UnboundedReceiver<T>;
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async fn client_writer(
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mut messages: Receiver<String>,
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stream: Arc<TcpStream>, // 3
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) -> Result<()> {
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let mut stream = &*stream;
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while let Some(msg) = messages.next().await {
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stream.write_all(msg.as_bytes()).await?;
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}
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Ok(())
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}
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```
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1. We will use channels from the `futures` crate.
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2. For simplicity, we will use `unbounded` channels, and won't be discussing backpressure in this tutorial.
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3. As `client` and `client_writer` share the same `TcpStream`, we need to put it into an `Arc`.
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Note that because `client` only reads from the stream and `client_writer` only writes to the stream, we don't get a race here.
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