peekable 0.6.1

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

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

pub(crate) fn peek_exact<'a, A, B>(
  peekable: &'a mut AsyncPeekable<A, B>,
  buf: &'a mut [u8],
) -> PeekExact<'a, A, B>
where
  A: AsyncRead + Unpin,
{
  PeekExact {
    peekable,
    buf,
    filled: 0,
    _pin: PhantomPinned,
  }
}

pin_project! {
  /// A future which can be used to easily read exactly enough bytes to fill
  /// a buffer.
  #[derive(Debug)]
  #[must_use = "futures do nothing unless you `.await` or poll them"]
  pub struct PeekExact<'a, A, B = DefaultBuffer> {
    peekable: &'a mut AsyncPeekable<A, B>,
    buf: &'a mut [u8],
    filled: usize,
    #[pin]
    _pin: PhantomPinned,
  }
}

fn eof() -> io::Error {
  io::Error::new(io::ErrorKind::UnexpectedEof, "early eof")
}

impl<A, B> Future for PeekExact<'_, A, B>
where
  A: 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();
    let total = me.buf.len();

    // On the first poll, copy what we can from the peek buffer.
    if *me.filled == 0 {
      let peek_buf = me.peekable.buffer.as_slice();
      let from_peek = peek_buf.len().min(total);
      me.buf[..from_peek].copy_from_slice(&peek_buf[..from_peek]);
      *me.filled = from_peek;

      if *me.filled == total {
        return Poll::Ready(Ok(total));
      }
    }

    // Read from the inner reader directly into the peek buffer's tail
    // so the reader only advances for bytes recorded for replay.
    while *me.filled < total {
      let old_len = me.peekable.buffer.len();
      let want = total - *me.filled;
      me.peekable.buffer.resize(old_len + want)?;

      let mut tail = tokio::io::ReadBuf::new(&mut me.peekable.buffer.as_mut_slice()[old_len..]);
      match Pin::new(&mut me.peekable.reader).poll_read(cx, &mut tail) {
        Poll::Ready(Ok(())) => {}
        Poll::Ready(Err(e)) if e.kind() == io::ErrorKind::Interrupted => {
          me.peekable.buffer.truncate(old_len);
          continue;
        }
        Poll::Ready(Err(e)) => {
          me.peekable.buffer.truncate(old_len);
          return Poll::Ready(Err(e));
        }
        Poll::Pending => {
          me.peekable.buffer.truncate(old_len);
          return Poll::Pending;
        }
      }
      let n = tail.filled().len();
      me.peekable.buffer.truncate(old_len + n);

      if n == 0 {
        return Poll::Ready(Err(eof()));
      }

      me.buf[*me.filled..*me.filled + n]
        .copy_from_slice(&me.peekable.buffer.as_slice()[old_len..old_len + n]);
      *me.filled += n;
    }

    Poll::Ready(Ok(total))
  }
}