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use futures_core::ready;
use futures_core::task::{Context, Poll};
use futures_io::{AsyncBufRead, AsyncRead};
use pin_project_lite::pin_project;
use std::pin::Pin;
use std::{cmp, io};
pin_project! {
/// Reader for the [`take`](super::AsyncReadExt::take) method.
#[derive(Debug)]
#[must_use = "readers do nothing unless you `.await` or poll them"]
pub struct Take<R> {
#[pin]
inner: R,
limit: u64,
}
}
impl<R: AsyncRead> Take<R> {
pub(super) fn new(inner: R, limit: u64) -> Self {
Self { inner, limit }
}
/// Returns the remaining number of bytes that can be
/// read before this instance will return EOF.
///
/// # Note
///
/// This instance may reach `EOF` after reading fewer bytes than indicated by
/// this method if the underlying [`AsyncRead`] instance reaches EOF.
///
/// # Examples
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::io::{AsyncReadExt, Cursor};
///
/// let reader = Cursor::new(&b"12345678"[..]);
/// let mut buffer = [0; 2];
///
/// let mut take = reader.take(4);
/// let n = take.read(&mut buffer).await?;
///
/// assert_eq!(take.limit(), 2);
/// # Ok::<(), Box<dyn std::error::Error>>(()) }).unwrap();
/// ```
pub fn limit(&self) -> u64 {
self.limit
}
/// Sets the number of bytes that can be read before this instance will
/// return EOF. This is the same as constructing a new `Take` instance, so
/// the amount of bytes read and the previous limit value don't matter when
/// calling this method.
///
/// # Examples
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::io::{AsyncReadExt, Cursor};
///
/// let reader = Cursor::new(&b"12345678"[..]);
/// let mut buffer = [0; 4];
///
/// let mut take = reader.take(4);
/// let n = take.read(&mut buffer).await?;
///
/// assert_eq!(n, 4);
/// assert_eq!(take.limit(), 0);
///
/// take.set_limit(10);
/// let n = take.read(&mut buffer).await?;
/// assert_eq!(n, 4);
///
/// # Ok::<(), Box<dyn std::error::Error>>(()) }).unwrap();
/// ```
pub fn set_limit(&mut self, limit: u64) {
self.limit = limit
}
delegate_access_inner!(inner, R, ());
}
impl<R: AsyncRead> AsyncRead for Take<R> {
fn poll_read(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
buf: &mut [u8],
) -> Poll<Result<usize, io::Error>> {
let this = self.project();
if *this.limit == 0 {
return Poll::Ready(Ok(0));
}
let max = cmp::min(buf.len() as u64, *this.limit) as usize;
let n = ready!(this.inner.poll_read(cx, &mut buf[..max]))?;
*this.limit -= n as u64;
Poll::Ready(Ok(n))
}
}
impl<R: AsyncBufRead> AsyncBufRead for Take<R> {
fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
let this = self.project();
// Don't call into inner reader at all at EOF because it may still block
if *this.limit == 0 {
return Poll::Ready(Ok(&[]));
}
let buf = ready!(this.inner.poll_fill_buf(cx)?);
let cap = cmp::min(buf.len() as u64, *this.limit) as usize;
Poll::Ready(Ok(&buf[..cap]))
}
fn consume(self: Pin<&mut Self>, amt: usize) {
let this = self.project();
// Don't let callers reset the limit by passing an overlarge value
let amt = cmp::min(amt as u64, *this.limit) as usize;
*this.limit -= amt as u64;
this.inner.consume(amt);
}
}