1use crate::io::{AsyncRead, ReadBuf};
2
3use pin_project_lite::pin_project;
4use std::future::Future;
5use std::io;
6use std::marker::PhantomPinned;
7use std::marker::Unpin;
8use std::pin::Pin;
9use std::task::{ready, Context, Poll};
10
11/// A future which can be used to easily read exactly enough bytes to fill
12/// a buffer.
13///
14/// Created by the [`AsyncReadExt::read_exact`][read_exact].
15/// [`read_exact`]: [`crate::io::AsyncReadExt::read_exact`]
16pub(crate) fn read_exact<'a, A>(reader: &'a mut A, buf: &'a mut [u8]) -> ReadExact<'a, A>
17where
18 A: AsyncRead + Unpin + ?Sized,
19{
20 ReadExact {
21 reader,
22 buf: ReadBuf::new(buf),
23 _pin: PhantomPinned,
24 }
25}
26
27pin_project! {
28 /// Creates a future which will read exactly enough bytes to fill `buf`,
29 /// returning an error if EOF is hit sooner.
30 ///
31 /// On success the number of bytes is returned
32 #[derive(Debug)]
33 #[must_use = "futures do nothing unless you `.await` or poll them"]
34 pub struct ReadExact<'a, A: ?Sized> {
35 reader: &'a mut A,
36 buf: ReadBuf<'a>,
37 // Make this future `!Unpin` for compatibility with async trait methods.
38 #[pin]
39 _pin: PhantomPinned,
40 }
41}
42
43fn eof() -> io::Error {
44 io::Error::new(kind:io::ErrorKind::UnexpectedEof, error:"early eof")
45}
46
47impl<A> Future for ReadExact<'_, A>
48where
49 A: AsyncRead + Unpin + ?Sized,
50{
51 type Output = io::Result<usize>;
52
53 fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<usize>> {
54 let me: Projection<'_, '_, A> = self.project();
55
56 loop {
57 // if our buffer is empty, then we need to read some data to continue.
58 let rem: usize = me.buf.remaining();
59 if rem != 0 {
60 ready!(Pin::new(&mut *me.reader).poll_read(cx, me.buf))?;
61 if me.buf.remaining() == rem {
62 return Err(eof()).into();
63 }
64 } else {
65 return Poll::Ready(Ok(me.buf.capacity()));
66 }
67 }
68 }
69}
70