消除

丢弃 Future 意味着无法再对其进行轮询。这称为 取消，在任何 await 点都可能发生。请务必小心谨慎，确保即使 Future 任务被取消，系统也能正常运行。例如，系统不应死锁或丢失数据。

use std::io::{self, ErrorKind};
use std::time::Duration;
use tokio::io::{AsyncReadExt, AsyncWriteExt, DuplexStream};

struct LinesReader {
    stream: DuplexStream,
}

impl LinesReader {
    fn new(stream: DuplexStream) -> Self {
        Self { stream }
    }

    async fn next(&mut self) -> io::Result<Option<String>> {
        let mut bytes = Vec::new();
        let mut buf = [0];
        while self.stream.read(&mut buf[..]).await? != 0 {
            bytes.push(buf[0]);
            if buf[0] == b'\n' {
                break;
            }
        }
        if bytes.is_empty() {
            return Ok(None);
        }
        let s = String::from_utf8(bytes)
            .map_err(|_| io::Error::new(ErrorKind::InvalidData, "not UTF-8"))?;
        Ok(Some(s))
    }
}

async fn slow_copy(source: String, mut dest: DuplexStream) -> std::io::Result<()> {
    for b in source.bytes() {
        dest.write_u8(b).await?;
        tokio::time::sleep(Duration::from_millis(10)).await
    }
    Ok(())
}

#[tokio::main]
async fn main() -> std::io::Result<()> {
    let (client, server) = tokio::io::duplex(5);
    let handle = tokio::spawn(slow_copy("hi\nthere\n".to_owned(), client));

    let mut lines = LinesReader::new(server);
    let mut interval = tokio::time::interval(Duration::from_millis(60));
    loop {
        tokio::select! {
            _ = interval.tick() => println!("tick!"),
            line = lines.next() => if let Some(l) = line? {
                print!("{}", l)
            } else {
                break
            },
        }
    }
    handle.await.unwrap()?;
    Ok(())
}

  • 编译器无法确保取消操作的安全性。您需要阅读 API 文档，并考虑 async fn 所持状态。

  • 与 panic 和 ? 不同，取消属于正常控制流的一部分（而非错误处理）。

  • 该示例丢失了字符串的某些部分。

    • 每当 tick() 分支先完成操作时，next() 及其 buf` 均会被丢弃。

    • 通过将 buf 整合到结构体中，`LinesReader 可以确保取消操作的安全性：

      #![allow(unused)]
fn main() {
struct LinesReader {
    stream: DuplexStream,
    bytes: Vec<u8>,
    buf: [u8; 1],
}

impl LinesReader {
    fn new(stream: DuplexStream) -> Self {
        Self { stream, bytes: Vec::new(), buf: [0] }
    }
    async fn next(&mut self) -> io::Result<Option<String>> {
        // prefix buf and bytes with self.
        // ...
        let raw = std::mem::take(&mut self.bytes);
        let s = String::from_utf8(raw)
        // ...
    }
}
}

  • Interval::tick is cancellation-safe because it keeps track of whether a tick has been ‘delivered’.

  • AsyncReadExt::read is cancellation-safe because it either returns or doesn’t read data.

  • AsyncBufReadExt::read_line is similar to the example and isn’t cancellation-safe. See its documentation for details and alternatives.