use std::io::{self, SeekFrom};
use std::ops::DerefMut;
use std::pin::Pin;
use std::task::{Context, Poll};
/// Seek bytes asynchronously.
///
/// This trait is analogous to the [`std::io::Seek`] trait, but integrates
/// with the asynchronous task system. In particular, the `start_seek`
/// method, unlike [`Seek::seek`], will not block the calling thread.
///
/// Utilities for working with `AsyncSeek` values are provided by
/// [`AsyncSeekExt`].
///
/// [`std::io::Seek`]: std::io::Seek
/// [`Seek::seek`]: std::io::Seek::seek()
/// [`AsyncSeekExt`]: crate::io::AsyncSeekExt
pub trait AsyncSeek {
/// Attempts to seek to an offset, in bytes, in a stream.
///
/// A seek beyond the end of a stream is allowed, but behavior is defined
/// by the implementation.
///
/// If this function returns successfully, then the job has been submitted.
/// To find out when it completes, call `poll_complete`.
///
/// # Errors
///
/// This function can return [`io::ErrorKind::Other`] in case there is
/// another seek in progress. To avoid this, it is advisable that any call
/// to `start_seek` is preceded by a call to `poll_complete` to ensure all
/// pending seeks have completed.
fn start_seek(self: Pin<&mut Self>, position: SeekFrom) -> io::Result<()>;
/// Waits for a seek operation to complete.
///
/// If the seek operation completed successfully,
/// this method returns the new position from the start of the stream.
/// That position can be used later with [`SeekFrom::Start`]. Repeatedly
/// calling this function without calling `start_seek` might return the
/// same result.
///
/// # Errors
///
/// Seeking to a negative offset is considered an error.
fn poll_complete(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>>;
}
macro_rules! deref_async_seek {
() => {
fn start_seek(mut self: Pin<&mut Self>, pos: SeekFrom) -> io::Result<()> {
Pin::new(&mut **self).start_seek(pos)
}
fn poll_complete(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>> {
Pin::new(&mut **self).poll_complete(cx)
}
};
}
impl<T: ?Sized + AsyncSeek + Unpin> AsyncSeek for Box<T> {
deref_async_seek!();
}
impl<T: ?Sized + AsyncSeek + Unpin> AsyncSeek for &mut T {
deref_async_seek!();
}
impl<P> AsyncSeek for Pin<P>
where
P: DerefMut + Unpin,
P::Target: AsyncSeek,
{
fn start_seek(self: Pin<&mut Self>, pos: SeekFrom) -> io::Result<()> {
self.get_mut().as_mut().start_seek(pos)
}
fn poll_complete(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>> {
self.get_mut().as_mut().poll_complete(cx)
}
}
impl<T: AsRef<[u8]> + Unpin> AsyncSeek for io::Cursor<T> {
fn start_seek(mut self: Pin<&mut Self>, pos: SeekFrom) -> io::Result<()> {
io::Seek::seek(&mut *self, pos).map(drop)
}
fn poll_complete(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<u64>> {
Poll::Ready(Ok(self.get_mut().position()))
}
}