tokio/io/
async_seek.rs

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1use std::io::{self, SeekFrom};
2use std::ops::DerefMut;
3use std::pin::Pin;
4use std::task::{Context, Poll};
5
6/// Seek bytes asynchronously.
7///
8/// This trait is analogous to the [`std::io::Seek`] trait, but integrates
9/// with the asynchronous task system. In particular, the `start_seek`
10/// method, unlike [`Seek::seek`], will not block the calling thread.
11///
12/// Utilities for working with `AsyncSeek` values are provided by
13/// [`AsyncSeekExt`].
14///
15/// [`std::io::Seek`]: std::io::Seek
16/// [`Seek::seek`]: std::io::Seek::seek()
17/// [`AsyncSeekExt`]: crate::io::AsyncSeekExt
18pub trait AsyncSeek {
19    /// Attempts to seek to an offset, in bytes, in a stream.
20    ///
21    /// A seek beyond the end of a stream is allowed, but behavior is defined
22    /// by the implementation.
23    ///
24    /// If this function returns successfully, then the job has been submitted.
25    /// To find out when it completes, call `poll_complete`.
26    ///
27    /// # Errors
28    ///
29    /// This function can return [`io::ErrorKind::Other`] in case there is
30    /// another seek in progress. To avoid this, it is advisable that any call
31    /// to `start_seek` is preceded by a call to `poll_complete` to ensure all
32    /// pending seeks have completed.
33    fn start_seek(self: Pin<&mut Self>, position: SeekFrom) -> io::Result<()>;
34
35    /// Waits for a seek operation to complete.
36    ///
37    /// If the seek operation completed successfully,
38    /// this method returns the new position from the start of the stream.
39    /// That position can be used later with [`SeekFrom::Start`]. Repeatedly
40    /// calling this function without calling `start_seek` might return the
41    /// same result.
42    ///
43    /// # Errors
44    ///
45    /// Seeking to a negative offset is considered an error.
46    fn poll_complete(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>>;
47}
48
49macro_rules! deref_async_seek {
50    () => {
51        fn start_seek(mut self: Pin<&mut Self>, pos: SeekFrom) -> io::Result<()> {
52            Pin::new(&mut **self).start_seek(pos)
53        }
54
55        fn poll_complete(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>> {
56            Pin::new(&mut **self).poll_complete(cx)
57        }
58    };
59}
60
61impl<T: ?Sized + AsyncSeek + Unpin> AsyncSeek for Box<T> {
62    deref_async_seek!();
63}
64
65impl<T: ?Sized + AsyncSeek + Unpin> AsyncSeek for &mut T {
66    deref_async_seek!();
67}
68
69impl<P> AsyncSeek for Pin<P>
70where
71    P: DerefMut + Unpin,
72    P::Target: AsyncSeek,
73{
74    fn start_seek(self: Pin<&mut Self>, pos: SeekFrom) -> io::Result<()> {
75        self.get_mut().as_mut().start_seek(pos)
76    }
77
78    fn poll_complete(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>> {
79        self.get_mut().as_mut().poll_complete(cx)
80    }
81}
82
83impl<T: AsRef<[u8]> + Unpin> AsyncSeek for io::Cursor<T> {
84    fn start_seek(mut self: Pin<&mut Self>, pos: SeekFrom) -> io::Result<()> {
85        io::Seek::seek(&mut *self, pos).map(drop)
86    }
87    fn poll_complete(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<u64>> {
88        Poll::Ready(Ok(self.get_mut().position()))
89    }
90}
tokio/io/async_seek.rs

tokio/io/
async_seek.rs
