futures_util/io/
read_to_end.rs

1use futures_core::future::Future;
2use futures_core::ready;
3use futures_core::task::{Context, Poll};
4use futures_io::AsyncRead;
5use std::io;
6use std::iter;
7use std::pin::Pin;
8use std::vec::Vec;
9
10/// Future for the [`read_to_end`](super::AsyncReadExt::read_to_end) method.
11#[derive(Debug)]
12#[must_use = "futures do nothing unless you `.await` or poll them"]
13pub struct ReadToEnd<'a, R: ?Sized> {
14    reader: &'a mut R,
15    buf: &'a mut Vec<u8>,
16    start_len: usize,
17}
18
19impl<R: ?Sized + Unpin> Unpin for ReadToEnd<'_, R> {}
20
21impl<'a, R: AsyncRead + ?Sized + Unpin> ReadToEnd<'a, R> {
22    pub(super) fn new(reader: &'a mut R, buf: &'a mut Vec<u8>) -> Self {
23        let start_len = buf.len();
24        Self { reader, buf, start_len }
25    }
26}
27
28struct Guard<'a> {
29    buf: &'a mut Vec<u8>,
30    len: usize,
31}
32
33impl Drop for Guard<'_> {
34    fn drop(&mut self) {
35        unsafe {
36            self.buf.set_len(self.len);
37        }
38    }
39}
40
41// This uses an adaptive system to extend the vector when it fills. We want to
42// avoid paying to allocate and zero a huge chunk of memory if the reader only
43// has 4 bytes while still making large reads if the reader does have a ton
44// of data to return. Simply tacking on an extra DEFAULT_BUF_SIZE space every
45// time is 4,500 times (!) slower than this if the reader has a very small
46// amount of data to return.
47//
48// Because we're extending the buffer with uninitialized data for trusted
49// readers, we need to make sure to truncate that if any of this panics.
50pub(super) fn read_to_end_internal<R: AsyncRead + ?Sized>(
51    mut rd: Pin<&mut R>,
52    cx: &mut Context<'_>,
53    buf: &mut Vec<u8>,
54    start_len: usize,
55) -> Poll<io::Result<usize>> {
56    let mut g = Guard { len: buf.len(), buf };
57    loop {
58        if g.len == g.buf.len() {
59            g.buf.reserve(32);
60            let spare_capacity = g.buf.capacity() - g.buf.len();
61            // FIXME: switch to `Vec::resize` once rust-lang/rust#120050 is fixed
62            g.buf.extend(iter::repeat(0).take(spare_capacity));
63        }
64
65        let buf = &mut g.buf[g.len..];
66        match ready!(rd.as_mut().poll_read(cx, buf)) {
67            Ok(0) => return Poll::Ready(Ok(g.len - start_len)),
68            Ok(n) => {
69                // We can't allow bogus values from read. If it is too large, the returned vec could have its length
70                // set past its capacity, or if it overflows the vec could be shortened which could create an invalid
71                // string if this is called via read_to_string.
72                assert!(n <= buf.len());
73                g.len += n;
74            }
75            Err(e) => return Poll::Ready(Err(e)),
76        }
77    }
78}
79
80impl<A> Future for ReadToEnd<'_, A>
81where
82    A: AsyncRead + ?Sized + Unpin,
83{
84    type Output = io::Result<usize>;
85
86    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
87        let this = &mut *self;
88        read_to_end_internal(Pin::new(&mut this.reader), cx, this.buf, this.start_len)
89    }
90}
futures_util/io/read_to_end.rs

futures_util/io/
read_to_end.rs
