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794c12fe7e
* fix typo in tutorial * add async_std::io::BufReader to tutorial code * writers in clean_shutdown.md return unit type
94 lines
3.3 KiB
Markdown
94 lines
3.3 KiB
Markdown
## Receiving messages
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Let's implement the receiving part of the protocol.
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We need to:
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1. split incoming `TcpStream` on `\n` and decode bytes as utf-8
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2. interpret the first line as a login
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3. parse the rest of the lines as a `login: message`
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```rust
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use async_std::io::BufReader;
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use async_std::net::TcpStream;
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use async_std::io::BufReader;
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async fn server(addr: impl ToSocketAddrs) -> Result<()> {
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let listener = TcpListener::bind(addr).await?;
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let mut incoming = listener.incoming();
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while let Some(stream) = incoming.next().await {
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let stream = stream?;
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println!("Accepting from: {}", stream.peer_addr()?);
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let _handle = task::spawn(client(stream)); // 1
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}
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Ok(())
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}
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async fn client(stream: TcpStream) -> Result<()> {
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let reader = BufReader::new(&stream); // 2
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let mut lines = reader.lines();
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let name = match lines.next().await { // 3
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None => Err("peer disconnected immediately")?,
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Some(line) => line?,
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};
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println!("name = {}", name);
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while let Some(line) = lines.next().await { // 4
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let line = line?;
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let (dest, msg) = match line.find(':') { // 5
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None => continue,
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Some(idx) => (&line[..idx], line[idx + 1 ..].trim()),
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};
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let dest: Vec<String> = dest.split(',').map(|name| name.trim().to_string()).collect();
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let msg: String = msg.trim().to_string();
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}
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Ok(())
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}
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```
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1. We use `task::spawn` function to spawn an independent task for working with each client.
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That is, after accepting the client the `server` loop immediately starts waiting for the next one.
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This is the core benefit of event-driven architecture: we serve many clients concurrently, without spending many hardware threads.
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2. Luckily, the "split byte stream into lines" functionality is already implemented.
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`.lines()` call returns a stream of `String`'s.
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3. We get the first line -- login
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4. And, once again, we implement a manual async for loop.
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5. Finally, we parse each line into a list of destination logins and the message itself.
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## Managing Errors
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One serious problem in the above solution is that, while we correctly propagate errors in the `client`, we just drop the error on the floor afterwards!
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That is, `task::spawn` does not return an error immediately (it can't, it needs to run the future to completion first), only after it is joined.
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We can "fix" it by waiting for the task to be joined, like this:
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```rust
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let handle = task::spawn(client(stream));
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handle.await?
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```
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The `.await` waits until the client finishes, and `?` propagates the result.
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There are two problems with this solution however!
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*First*, because we immediately await the client, we can only handle one client at time, and that completely defeats the purpose of async!
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*Second*, if a client encounters an IO error, the whole server immediately exits.
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That is, a flaky internet connection of one peer brings down the whole chat room!
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A correct way to handle client errors in this case is log them, and continue serving other clients.
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So let's use a helper function for this:
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```rust
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fn spawn_and_log_error<F>(fut: F) -> task::JoinHandle<()>
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where
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F: Future<Output = Result<()>> + Send + 'static,
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{
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task::spawn(async move {
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if let Err(e) = fut.await {
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eprintln!("{}", e)
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}
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})
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}
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```
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