怎么读这一卷

git clone https://github.com/tokio-rs/tokio
cd tokio
git checkout tokio-1.54.1

use std::time::Duration;
use tokio::io::{AsyncReadExt, AsyncWriteExt};
use tokio::net::{TcpListener, TcpStream};

// 任务主线：建运行时，Runtime::block_on 驱动 main 的 future
#[tokio::main]
async fn main() -> std::io::Result<()> {
  let listener = TcpListener::bind("127.0.0.1:0").await?;
  let addr = listener.local_addr()?;

  // 任务主线：tokio::spawn 把任务放进队列，worker 取出后 poll
  let reader = tokio::spawn(async move {
    let (mut socket, _) = listener.accept().await?;
    let mut buf = [0u8; 5];
    // IO 主线：数据还没到，read 返回 Pending，ScheduledIo 记下 waker，任务挂起
    socket.read_exact(&mut buf).await?;
    Ok::<_, std::io::Error>(buf)
  });

  let mut client = TcpStream::connect(addr).await?;
  // 等 100ms 再写，保证 reader 先等在 read 上
  tokio::time::sleep(Duration::from_millis(100)).await;
  // IO 主线：数据到达，mio 报告可读，wake 把 reader 放回队列，再次 poll 读到数据
  client.write_all(b"hello").await?;

  // 任务主线：reader 返回 Ready，JoinHandle 拿到结果
  let buf = reader.await??;
  println!("{}", String::from_utf8_lossy(&buf));
  Ok(())
}