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use crate::io::{AsyncRead, ReadBuf};
use pin_project_lite::pin_project;
use std::future::Future;
use std::io;
use std::marker::PhantomPinned;
use std::marker::Unpin;
use std::pin::Pin;
use std::task::{Context, Poll};
/// A future which can be used to easily read exactly enough bytes to fill
/// a buffer.
///
/// Created by the [`AsyncReadExt::read_exact`][read_exact].
/// [`read_exact`]: [`crate::io::AsyncReadExt::read_exact`]
pub(crate) fn read_exact<'a, A>(reader: &'a mut A, buf: &'a mut [u8]) -> ReadExact<'a, A>
where
A: AsyncRead + Unpin + ?Sized,
{
ReadExact {
reader,
buf: ReadBuf::new(buf),
_pin: PhantomPinned,
}
}
pin_project! {
/// Creates a future which will read exactly enough bytes to fill `buf`,
/// returning an error if EOF is hit sooner.
///
/// On success the number of bytes is returned
#[derive(Debug)]
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct ReadExact<'a, A: ?Sized> {
reader: &'a mut A,
buf: ReadBuf<'a>,
// Make this future `!Unpin` for compatibility with async trait methods.
#[pin]
_pin: PhantomPinned,
}
}
fn eof() -> io::Error {
io::Error::new(io::ErrorKind::UnexpectedEof, "early eof")
}
impl<A> Future for ReadExact<'_, A>
where
A: AsyncRead + Unpin + ?Sized,
{
type Output = io::Result<usize>;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<usize>> {
let me = self.project();
loop {
// if our buffer is empty, then we need to read some data to continue.
let rem = me.buf.remaining();
if rem != 0 {
ready!(Pin::new(&mut *me.reader).poll_read(cx, me.buf))?;
if me.buf.remaining() == rem {
return Err(eof()).into();
}
} else {
return Poll::Ready(Ok(me.buf.capacity()));
}
}
}
}