peekable 0.5.0

Buffered peek for `Read` and async readers (tokio, futures): inspect incoming bytes without consuming them. Useful for protocol detection, length-prefixed framing, and header inspection.
Documentation 
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73

use super::{AsyncPeekable, AsyncRead, Buffer, DefaultBuffer};

use pin_project_lite::pin_project;
use std::{
  future::Future,
  io,
  marker::PhantomPinned,
  pin::Pin,
  task::{Context, Poll},
};

pin_project! {
  /// Future returned by [`peek_buf`](crate::io::AsyncReadExt::peek_buf).
  #[derive(Debug)]
  #[must_use = "futures do nothing unless you `.await` or poll them"]
  pub struct FillPeekBuf<'a, R, B = DefaultBuffer> {
    peeker: &'a mut AsyncPeekable<R, B>,
    original: usize,
    cap: usize,
    #[pin]
    _pin: PhantomPinned,
  }
}

impl<'a, R, B: Buffer> FillPeekBuf<'a, R, B> {
  pub(super) fn new(peeker: &'a mut AsyncPeekable<R, B>) -> Self {
    let cap = peeker.buffer.capacity();
    let cur = peeker.buffer.len();
    Self {
      peeker,
      cap,
      original: cur,
      _pin: PhantomPinned,
    }
  }
}

impl<R, B> Future for FillPeekBuf<'_, R, B>
where
  R: AsyncRead + Unpin,
  B: Buffer,
{
  type Output = io::Result<usize>;

  fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<usize>> {
    let me = self.project();

    if me.cap == me.original {
      return Poll::Ready(Ok(0));
    }

    me.peeker.buffer.resize(*me.cap)?;

    match Pin::new(&mut me.peeker.reader)
      .poll_read(cx, &mut me.peeker.buffer.as_mut_slice()[*me.original..])
    {
      Poll::Ready(Ok(n)) => {
        me.peeker.buffer.truncate(*me.original + n);
        Poll::Ready(Ok(n))
      }
      Poll::Ready(Err(e)) => {
        // Roll back the resize so the peek buffer doesn't carry ghost
        // zero-bytes that a subsequent operation would treat as data.
        me.peeker.buffer.truncate(*me.original);
        Poll::Ready(Err(e))
      }
      Poll::Pending => {
        me.peeker.buffer.truncate(*me.original);
        Poll::Pending
      }
    }
  }
}