1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132

use futures_core::ready;
use futures_core::task::{Context, Poll};
#[cfg(feature = "read-initializer")]
use futures_io::Initializer;
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))
    }

    #[cfg(feature = "read-initializer")]
    unsafe fn initializer(&self) -> Initializer {
        self.inner.initializer()
    }
}

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);
    }
}