1use super::ReadBuf;
2use std::io;
3use std::ops::DerefMut;
4use std::pin::Pin;
5use std::task::{Context, Poll};
6
7/// Reads bytes from a source.
8///
9/// This trait is analogous to the [`std::io::Read`] trait, but integrates with
10/// the asynchronous task system. In particular, the [`poll_read`] method,
11/// unlike [`Read::read`], will automatically queue the current task for wakeup
12/// and return if data is not yet available, rather than blocking the calling
13/// thread.
14///
15/// Specifically, this means that the `poll_read` function will return one of
16/// the following:
17///
18/// * `Poll::Ready(Ok(()))` means that data was immediately read and placed into
19/// the output buffer. The amount of data read can be determined by the
20/// increase in the length of the slice returned by `ReadBuf::filled`. If the
21/// difference is 0, EOF has been reached.
22///
23/// * `Poll::Pending` means that no data was read into the buffer
24/// provided. The I/O object is not currently readable but may become readable
25/// in the future. Most importantly, **the current future's task is scheduled
26/// to get unparked when the object is readable**. This means that like
27/// `Future::poll` you'll receive a notification when the I/O object is
28/// readable again.
29///
30/// * `Poll::Ready(Err(e))` for other errors are standard I/O errors coming from the
31/// underlying object.
32///
33/// This trait importantly means that the `read` method only works in the
34/// context of a future's task. The object may panic if used outside of a task.
35///
36/// Utilities for working with `AsyncRead` values are provided by
37/// [`AsyncReadExt`].
38///
39/// [`poll_read`]: AsyncRead::poll_read
40/// [`std::io::Read`]: std::io::Read
41/// [`Read::read`]: std::io::Read::read
42/// [`AsyncReadExt`]: crate::io::AsyncReadExt
43pub trait AsyncRead {
44 /// Attempts to read from the `AsyncRead` into `buf`.
45 ///
46 /// On success, returns `Poll::Ready(Ok(()))` and places data in the
47 /// unfilled portion of `buf`. If no data was read (`buf.filled().len()` is
48 /// unchanged), it implies that EOF has been reached.
49 ///
50 /// If no data is available for reading, the method returns `Poll::Pending`
51 /// and arranges for the current task (via `cx.waker()`) to receive a
52 /// notification when the object becomes readable or is closed.
53 fn poll_read(
54 self: Pin<&mut Self>,
55 cx: &mut Context<'_>,
56 buf: &mut ReadBuf<'_>,
57 ) -> Poll<io::Result<()>>;
58}
59
60macro_rules! deref_async_read {
61 () => {
62 fn poll_read(
63 mut self: Pin<&mut Self>,
64 cx: &mut Context<'_>,
65 buf: &mut ReadBuf<'_>,
66 ) -> Poll<io::Result<()>> {
67 Pin::new(&mut **self).poll_read(cx, buf)
68 }
69 };
70}
71
72impl<T: ?Sized + AsyncRead + Unpin> AsyncRead for Box<T> {
73 deref_async_read!();
74}
75
76impl<T: ?Sized + AsyncRead + Unpin> AsyncRead for &mut T {
77 deref_async_read!();
78}
79
80impl<P> AsyncRead for Pin<P>
81where
82 P: DerefMut + Unpin,
83 P::Target: AsyncRead,
84{
85 fn poll_read(
86 self: Pin<&mut Self>,
87 cx: &mut Context<'_>,
88 buf: &mut ReadBuf<'_>,
89 ) -> Poll<io::Result<()>> {
90 self.get_mut().as_mut().poll_read(cx, buf)
91 }
92}
93
94impl AsyncRead for &[u8] {
95 fn poll_read(
96 mut self: Pin<&mut Self>,
97 _cx: &mut Context<'_>,
98 buf: &mut ReadBuf<'_>,
99 ) -> Poll<io::Result<()>> {
100 let amt: usize = std::cmp::min(self.len(), v2:buf.remaining());
101 let (a: &[u8], b: &[u8]) = self.split_at(mid:amt);
102 buf.put_slice(buf:a);
103 *self = b;
104 Poll::Ready(Ok(()))
105 }
106}
107
108impl<T: AsRef<[u8]> + Unpin> AsyncRead for io::Cursor<T> {
109 fn poll_read(
110 mut self: Pin<&mut Self>,
111 _cx: &mut Context<'_>,
112 buf: &mut ReadBuf<'_>,
113 ) -> Poll<io::Result<()>> {
114 let pos: u64 = self.position();
115 let slice: &[u8] = (*self).get_ref().as_ref();
116
117 // The position could technically be out of bounds, so don't panic...
118 if pos > slice.len() as u64 {
119 return Poll::Ready(Ok(()));
120 }
121
122 let start: usize = pos as usize;
123 let amt: usize = std::cmp::min(v1:slice.len() - start, v2:buf.remaining());
124 // Add won't overflow because of pos check above.
125 let end: usize = start + amt;
126 buf.put_slice(&slice[start..end]);
127 self.set_position(pos:end as u64);
128
129 Poll::Ready(Ok(()))
130 }
131}
132