tokio/io/
async_read.rs

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

tokio/io/
async_read.rs
