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use crate::io::AsyncRead;
use std::future::Future;
use std::io;
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 [`AsyncRead::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,
pos: 0,
}
}
/// 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: &'a mut [u8],
pos: usize,
}
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(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<usize>> {
loop {
// if our buffer is empty, then we need to read some data to continue.
if self.pos < self.buf.len() {
let me = &mut *self;
let n = ready!(Pin::new(&mut *me.reader).poll_read(cx, &mut me.buf[me.pos..]))?;
me.pos += n;
if n == 0 {
return Err(eof()).into();
}
}
if self.pos >= self.buf.len() {
return Poll::Ready(Ok(self.pos));
}
}
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn assert_unpin() {
use std::marker::PhantomPinned;
crate::is_unpin::<ReadExact<'_, PhantomPinned>>();
}
}
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