1use crate::io::AsyncRead;
2
3use std::io;
4use std::ops::DerefMut;
5use std::pin::Pin;
6use std::task::{Context, Poll};
7
8/// Reads bytes asynchronously.
9///
10/// This trait is analogous to [`std::io::BufRead`], but integrates with
11/// the asynchronous task system. In particular, the [`poll_fill_buf`] method,
12/// unlike [`BufRead::fill_buf`], will automatically queue the current task for wakeup
13/// and return if data is not yet available, rather than blocking the calling
14/// thread.
15///
16/// Utilities for working with `AsyncBufRead` values are provided by
17/// [`AsyncBufReadExt`].
18///
19/// [`std::io::BufRead`]: std::io::BufRead
20/// [`poll_fill_buf`]: AsyncBufRead::poll_fill_buf
21/// [`BufRead::fill_buf`]: std::io::BufRead::fill_buf
22/// [`AsyncBufReadExt`]: crate::io::AsyncBufReadExt
23pub trait AsyncBufRead: AsyncRead {
24 /// Attempts to return the contents of the internal buffer, filling it with more data
25 /// from the inner reader if it is empty.
26 ///
27 /// On success, returns `Poll::Ready(Ok(buf))`.
28 ///
29 /// If no data is available for reading, the method returns
30 /// `Poll::Pending` and arranges for the current task (via
31 /// `cx.waker().wake_by_ref()`) to receive a notification when the object becomes
32 /// readable or is closed.
33 ///
34 /// This function is a lower-level call. It needs to be paired with the
35 /// [`consume`] method to function properly. When calling this
36 /// method, none of the contents will be "read" in the sense that later
37 /// calling [`poll_read`] may return the same contents. As such, [`consume`] must
38 /// be called with the number of bytes that are consumed from this buffer to
39 /// ensure that the bytes are never returned twice.
40 ///
41 /// An empty buffer returned indicates that the stream has reached EOF.
42 ///
43 /// [`poll_read`]: AsyncRead::poll_read
44 /// [`consume`]: AsyncBufRead::consume
45 fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>>;
46
47 /// Tells this buffer that `amt` bytes have been consumed from the buffer,
48 /// so they should no longer be returned in calls to [`poll_read`].
49 ///
50 /// This function is a lower-level call. It needs to be paired with the
51 /// [`poll_fill_buf`] method to function properly. This function does
52 /// not perform any I/O, it simply informs this object that some amount of
53 /// its buffer, returned from [`poll_fill_buf`], has been consumed and should
54 /// no longer be returned. As such, this function may do odd things if
55 /// [`poll_fill_buf`] isn't called before calling it.
56 ///
57 /// The `amt` must be `<=` the number of bytes in the buffer returned by
58 /// [`poll_fill_buf`].
59 ///
60 /// [`poll_read`]: AsyncRead::poll_read
61 /// [`poll_fill_buf`]: AsyncBufRead::poll_fill_buf
62 fn consume(self: Pin<&mut Self>, amt: usize);
63}
64
65macro_rules! deref_async_buf_read {
66 () => {
67 fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
68 Pin::new(&mut **self.get_mut()).poll_fill_buf(cx)
69 }
70
71 fn consume(mut self: Pin<&mut Self>, amt: usize) {
72 Pin::new(&mut **self).consume(amt)
73 }
74 };
75}
76
77impl<T: ?Sized + AsyncBufRead + Unpin> AsyncBufRead for Box<T> {
78 deref_async_buf_read!();
79}
80
81impl<T: ?Sized + AsyncBufRead + Unpin> AsyncBufRead for &mut T {
82 deref_async_buf_read!();
83}
84
85impl<P> AsyncBufRead for Pin<P>
86where
87 P: DerefMut + Unpin,
88 P::Target: AsyncBufRead,
89{
90 fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
91 self.get_mut().as_mut().poll_fill_buf(cx)
92 }
93
94 fn consume(self: Pin<&mut Self>, amt: usize) {
95 self.get_mut().as_mut().consume(amt)
96 }
97}
98
99impl AsyncBufRead for &[u8] {
100 fn poll_fill_buf(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
101 Poll::Ready(Ok(*self))
102 }
103
104 fn consume(mut self: Pin<&mut Self>, amt: usize) {
105 *self = &self[amt..];
106 }
107}
108
109impl<T: AsRef<[u8]> + Unpin> AsyncBufRead for io::Cursor<T> {
110 fn poll_fill_buf(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
111 Poll::Ready(io::BufRead::fill_buf(self.get_mut()))
112 }
113
114 fn consume(self: Pin<&mut Self>, amt: usize) {
115 io::BufRead::consume(self.get_mut(), amt)
116 }
117}
118