tokio/io/
async_read.rs

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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 = std::cmp::min(self.len(), buf.remaining());
101        let (a, b) = self.split_at(amt);
102        buf.put_slice(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 = 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 = pos as usize;
123        let amt = std::cmp::min(slice.len() - start, buf.remaining());
124        // Add won't overflow because of pos check above.
125        let end = start + amt;
126        buf.put_slice(&slice[start..end]);
127        self.set_position(end as u64);
128
129        Poll::Ready(Ok(()))
130    }
131}
tokio/io/async_read.rs

tokio/io/
async_read.rs
