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
use std::future::ready;

use compio_buf::{buf_try, BufResult, IntoInner, IoBuf, IoVectoredBuf};

use crate::{
    buffer::Buffer,
    util::{slice_to_buf, DEFAULT_BUF_SIZE},
    AsyncWrite, IoResult,
};

/// Wraps a writer and buffers its output.
///
/// It can be excessively inefficient to work directly with something that
/// implements [`AsyncWrite`].  A `BufWriter<W>` keeps an in-memory buffer of
/// data and writes it to an underlying writer in large, infrequent batches.
//
/// `BufWriter<W>` can improve the speed of programs that make *small* and
/// *repeated* write calls to the same file or network socket. It does not
/// help when writing very large amounts at once, or writing just one or a few
/// times. It also provides no advantage when writing to a destination that is
/// in memory, like a `Vec<u8>`.
///
/// Dropping `BufWriter<W>` also discards any bytes left in the buffer, so it is
/// critical to call [`flush`] before `BufWriter<W>` is dropped. Calling
/// [`flush`] ensures that the buffer is empty and thus no data is lost.
///
/// [`flush`]: AsyncWrite::flush

#[derive(Debug)]
pub struct BufWriter<W> {
    writer: W,
    buf: Buffer,
}

impl<W> BufWriter<W> {
    /// Creates a new `BufWriter` with a default buffer capacity. The default is
    /// currently 8 KB, but may change in the future.
    pub fn new(writer: W) -> Self {
        Self::with_capacity(DEFAULT_BUF_SIZE, writer)
    }

    /// Creates a new `BufWriter` with the specified buffer capacity.
    pub fn with_capacity(cap: usize, writer: W) -> Self {
        Self {
            writer,
            buf: Buffer::with_capacity(cap),
        }
    }
}

impl<W: AsyncWrite> BufWriter<W> {
    async fn flush_if_needed(&mut self) -> IoResult<()> {
        if self.buf.need_flush() {
            self.flush().await?;
        }
        Ok(())
    }
}

impl<W: AsyncWrite> AsyncWrite for BufWriter<W> {
    async fn write<T: IoBuf>(&mut self, mut buf: T) -> BufResult<usize, T> {
        // The previous flush may error because disk full. We need to make the buffer
        // all-done before writing new data to it.
        (_, buf) = buf_try!(self.flush_if_needed().await, buf);

        let written = self
            .buf
            .with_sync(|w| {
                let len = w.buf_len();
                let mut w = w.slice(len..);
                let written = slice_to_buf(buf.as_slice(), &mut w);
                BufResult(Ok(written), w.into_inner())
            })
            .expect("Closure always return Ok");

        (_, buf) = buf_try!(self.flush_if_needed().await, buf);

        BufResult(Ok(written), buf)
    }

    async fn write_vectored<T: IoVectoredBuf>(&mut self, mut buf: T) -> BufResult<usize, T> {
        (_, buf) = buf_try!(self.flush_if_needed().await, buf);

        let written = self
            .buf
            .with(|mut w| {
                let mut written = 0;
                for buf in buf.iter_buf() {
                    let len = w.buf_len();
                    let mut slice = w.slice(len..);
                    written += slice_to_buf(buf.as_slice(), &mut slice);
                    w = slice.into_inner();

                    if w.buf_len() == w.buf_capacity() {
                        break;
                    }
                }
                ready(BufResult(Ok(written), w))
            })
            .await
            .expect("Closure always return Ok");

        (_, buf) = buf_try!(self.flush_if_needed().await, buf);

        BufResult(Ok(written), buf)
    }

    async fn flush(&mut self) -> IoResult<()> {
        let Self { writer, buf } = self;

        buf.flush_to(writer).await?;

        Ok(())
    }

    async fn shutdown(&mut self) -> IoResult<()> {
        self.flush().await?;
        self.writer.shutdown().await
    }
}

impl<W> IntoInner for BufWriter<W> {
    type Inner = W;

    fn into_inner(self) -> Self::Inner {
        self.writer
    }
}