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。

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Speaker Notes

  • 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.