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use super::ReadBuf;
use std::io;
use std::ops::DerefMut;
use std::pin::Pin;
use std::task::{Context, Poll};
/// Reads bytes from a source.
///
/// This trait is analogous to the [`std::io::Read`] trait, but integrates with
/// the asynchronous task system. In particular, the [`poll_read`] method,
/// unlike [`Read::read`], will automatically queue the current task for wakeup
/// and return if data is not yet available, rather than blocking the calling
/// thread.
///
/// Specifically, this means that the `poll_read` function will return one of
/// the following:
///
/// * `Poll::Ready(Ok(()))` means that data was immediately read and placed into
/// the output buffer. The amount of data read can be determined by the
/// increase in the length of the slice returned by `ReadBuf::filled`. If the
/// difference is 0, EOF has been reached.
///
/// * `Poll::Pending` means that no data was read into the buffer
/// provided. The I/O object is not currently readable but may become readable
/// in the future. Most importantly, **the current future's task is scheduled
/// to get unparked when the object is readable**. This means that like
/// `Future::poll` you'll receive a notification when the I/O object is
/// readable again.
///
/// * `Poll::Ready(Err(e))` for other errors are standard I/O errors coming from the
/// underlying object.
///
/// This trait importantly means that the `read` method only works in the
/// context of a future's task. The object may panic if used outside of a task.
///
/// Utilities for working with `AsyncRead` values are provided by
/// [`AsyncReadExt`].
///
/// [`poll_read`]: AsyncRead::poll_read
/// [`std::io::Read`]: std::io::Read
/// [`Read::read`]: std::io::Read::read
/// [`AsyncReadExt`]: crate::io::AsyncReadExt
pub trait AsyncRead {
/// Attempts to read from the `AsyncRead` into `buf`.
///
/// On success, returns `Poll::Ready(Ok(()))` and places data in the
/// unfilled portion of `buf`. If no data was read (`buf.filled().len()` is
/// unchanged), it implies that EOF has been reached.
///
/// If no data is available for reading, the method returns `Poll::Pending`
/// and arranges for the current task (via `cx.waker()`) to receive a
/// notification when the object becomes readable or is closed.
fn poll_read(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
buf: &mut ReadBuf<'_>,
) -> Poll<io::Result<()>>;
}
macro_rules! deref_async_read {
() => {
fn poll_read(
mut self: Pin<&mut Self>,
cx: &mut Context<'_>,
buf: &mut ReadBuf<'_>,
) -> Poll<io::Result<()>> {
Pin::new(&mut **self).poll_read(cx, buf)
}
};
}
impl<T: ?Sized + AsyncRead + Unpin> AsyncRead for Box<T> {
deref_async_read!();
}
impl<T: ?Sized + AsyncRead + Unpin> AsyncRead for &mut T {
deref_async_read!();
}
impl<P> AsyncRead for Pin<P>
where
P: DerefMut + Unpin,
P::Target: AsyncRead,
{
fn poll_read(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
buf: &mut ReadBuf<'_>,
) -> Poll<io::Result<()>> {
self.get_mut().as_mut().poll_read(cx, buf)
}
}
impl AsyncRead for &[u8] {
fn poll_read(
mut self: Pin<&mut Self>,
_cx: &mut Context<'_>,
buf: &mut ReadBuf<'_>,
) -> Poll<io::Result<()>> {
let amt = std::cmp::min(self.len(), buf.remaining());
let (a, b) = self.split_at(amt);
buf.put_slice(a);
*self = b;
Poll::Ready(Ok(()))
}
}
impl<T: AsRef<[u8]> + Unpin> AsyncRead for io::Cursor<T> {
fn poll_read(
mut self: Pin<&mut Self>,
_cx: &mut Context<'_>,
buf: &mut ReadBuf<'_>,
) -> Poll<io::Result<()>> {
let pos = self.position();
let slice: &[u8] = (*self).get_ref().as_ref();
// The position could technically be out of bounds, so don't panic...
if pos > slice.len() as u64 {
return Poll::Ready(Ok(()));
}
let start = pos as usize;
let amt = std::cmp::min(slice.len() - start, buf.remaining());
// Add won't overflow because of pos check above.
let end = start + amt;
buf.put_slice(&slice[start..end]);
self.set_position(end as u64);
Poll::Ready(Ok(()))
}
}
|
