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
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
use crate::io::util::DEFAULT_BUF_SIZE;
use crate::io::{AsyncBufRead, AsyncRead, AsyncSeek, AsyncWrite, ReadBuf};

use pin_project_lite::pin_project;
use std::fmt;
use std::io::{self, IoSlice, SeekFrom, Write};
use std::pin::Pin;
use std::task::{Context, Poll};

pin_project! {
    /// Wraps a writer and buffers its output.
    ///
    /// It can be excessively inefficient to work directly with something that
    /// implements [`AsyncWrite`]. A `BufWriter` keeps an in-memory buffer of data and
    /// writes it to an underlying writer in large, infrequent batches.
    ///
    /// `BufWriter` 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>`.
    ///
    /// When the `BufWriter` is dropped, the contents of its buffer will be
    /// discarded. Creating multiple instances of a `BufWriter` on the same
    /// stream can cause data loss. If you need to write out the contents of its
    /// buffer, you must manually call flush before the writer is dropped.
    ///
    /// [`AsyncWrite`]: AsyncWrite
    /// [`flush`]: super::AsyncWriteExt::flush
    ///
    #[cfg_attr(docsrs, doc(cfg(feature = "io-util")))]
    pub struct BufWriter<W> {
        #[pin]
        pub(super) inner: W,
        pub(super) buf: Vec<u8>,
        pub(super) written: usize,
        pub(super) seek_state: SeekState,
    }
}

impl<W: AsyncWrite> 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(inner: W) -> Self {
        Self::with_capacity(DEFAULT_BUF_SIZE, inner)
    }

    /// Creates a new `BufWriter` with the specified buffer capacity.
    pub fn with_capacity(cap: usize, inner: W) -> Self {
        Self {
            inner,
            buf: Vec::with_capacity(cap),
            written: 0,
            seek_state: SeekState::Init,
        }
    }

    fn flush_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        let mut me = self.project();

        let len = me.buf.len();
        let mut ret = Ok(());
        while *me.written < len {
            match ready!(me.inner.as_mut().poll_write(cx, &me.buf[*me.written..])) {
                Ok(0) => {
                    ret = Err(io::Error::new(
                        io::ErrorKind::WriteZero,
                        "failed to write the buffered data",
                    ));
                    break;
                }
                Ok(n) => *me.written += n,
                Err(e) => {
                    ret = Err(e);
                    break;
                }
            }
        }
        if *me.written > 0 {
            me.buf.drain(..*me.written);
        }
        *me.written = 0;
        Poll::Ready(ret)
    }

    /// Gets a reference to the underlying writer.
    pub fn get_ref(&self) -> &W {
        &self.inner
    }

    /// Gets a mutable reference to the underlying writer.
    ///
    /// It is inadvisable to directly write to the underlying writer.
    pub fn get_mut(&mut self) -> &mut W {
        &mut self.inner
    }

    /// Gets a pinned mutable reference to the underlying writer.
    ///
    /// It is inadvisable to directly write to the underlying writer.
    pub fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut W> {
        self.project().inner
    }

    /// Consumes this `BufWriter`, returning the underlying writer.
    ///
    /// Note that any leftover data in the internal buffer is lost.
    pub fn into_inner(self) -> W {
        self.inner
    }

    /// Returns a reference to the internally buffered data.
    pub fn buffer(&self) -> &[u8] {
        &self.buf
    }
}

impl<W: AsyncWrite> AsyncWrite for BufWriter<W> {
    fn poll_write(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &[u8],
    ) -> Poll<io::Result<usize>> {
        if self.buf.len() + buf.len() > self.buf.capacity() {
            ready!(self.as_mut().flush_buf(cx))?;
        }

        let me = self.project();
        if buf.len() >= me.buf.capacity() {
            me.inner.poll_write(cx, buf)
        } else {
            Poll::Ready(me.buf.write(buf))
        }
    }

    fn poll_write_vectored(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        mut bufs: &[IoSlice<'_>],
    ) -> Poll<io::Result<usize>> {
        if self.inner.is_write_vectored() {
            let total_len = bufs
                .iter()
                .fold(0usize, |acc, b| acc.saturating_add(b.len()));
            if total_len > self.buf.capacity() - self.buf.len() {
                ready!(self.as_mut().flush_buf(cx))?;
            }
            let me = self.as_mut().project();
            if total_len >= me.buf.capacity() {
                // It's more efficient to pass the slices directly to the
                // underlying writer than to buffer them.
                // The case when the total_len calculation saturates at
                // usize::MAX is also handled here.
                me.inner.poll_write_vectored(cx, bufs)
            } else {
                bufs.iter().for_each(|b| me.buf.extend_from_slice(b));
                Poll::Ready(Ok(total_len))
            }
        } else {
            // Remove empty buffers at the beginning of bufs.
            while bufs.first().map(|buf| buf.len()) == Some(0) {
                bufs = &bufs[1..];
            }
            if bufs.is_empty() {
                return Poll::Ready(Ok(0));
            }
            // Flush if the first buffer doesn't fit.
            let first_len = bufs[0].len();
            if first_len > self.buf.capacity() - self.buf.len() {
                ready!(self.as_mut().flush_buf(cx))?;
                debug_assert!(self.buf.is_empty());
            }
            let me = self.as_mut().project();
            if first_len >= me.buf.capacity() {
                // The slice is at least as large as the buffering capacity,
                // so it's better to write it directly, bypassing the buffer.
                debug_assert!(me.buf.is_empty());
                return me.inner.poll_write(cx, &bufs[0]);
            } else {
                me.buf.extend_from_slice(&bufs[0]);
                bufs = &bufs[1..];
            }
            let mut total_written = first_len;
            debug_assert!(total_written != 0);
            // Append the buffers that fit in the internal buffer.
            for buf in bufs {
                if buf.len() > me.buf.capacity() - me.buf.len() {
                    break;
                } else {
                    me.buf.extend_from_slice(buf);
                    total_written += buf.len();
                }
            }
            Poll::Ready(Ok(total_written))
        }
    }

    fn is_write_vectored(&self) -> bool {
        true
    }

    fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        ready!(self.as_mut().flush_buf(cx))?;
        self.get_pin_mut().poll_flush(cx)
    }

    fn poll_shutdown(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        ready!(self.as_mut().flush_buf(cx))?;
        self.get_pin_mut().poll_shutdown(cx)
    }
}

#[derive(Debug, Clone, Copy)]
pub(super) enum SeekState {
    /// start_seek has not been called.
    Init,
    /// start_seek has been called, but poll_complete has not yet been called.
    Start(SeekFrom),
    /// Waiting for completion of poll_complete.
    Pending,
}

/// Seek to the offset, in bytes, in the underlying writer.
///
/// Seeking always writes out the internal buffer before seeking.
impl<W: AsyncWrite + AsyncSeek> AsyncSeek for BufWriter<W> {
    fn start_seek(self: Pin<&mut Self>, pos: SeekFrom) -> io::Result<()> {
        // We need to flush the internal buffer before seeking.
        // It receives a `Context` and returns a `Poll`, so it cannot be called
        // inside `start_seek`.
        *self.project().seek_state = SeekState::Start(pos);
        Ok(())
    }

    fn poll_complete(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>> {
        let pos = match self.seek_state {
            SeekState::Init => {
                return self.project().inner.poll_complete(cx);
            }
            SeekState::Start(pos) => Some(pos),
            SeekState::Pending => None,
        };

        // Flush the internal buffer before seeking.
        ready!(self.as_mut().flush_buf(cx))?;

        let mut me = self.project();
        if let Some(pos) = pos {
            // Ensure previous seeks have finished before starting a new one
            ready!(me.inner.as_mut().poll_complete(cx))?;
            if let Err(e) = me.inner.as_mut().start_seek(pos) {
                *me.seek_state = SeekState::Init;
                return Poll::Ready(Err(e));
            }
        }
        match me.inner.poll_complete(cx) {
            Poll::Ready(res) => {
                *me.seek_state = SeekState::Init;
                Poll::Ready(res)
            }
            Poll::Pending => {
                *me.seek_state = SeekState::Pending;
                Poll::Pending
            }
        }
    }
}

impl<W: AsyncWrite + AsyncRead> AsyncRead for BufWriter<W> {
    fn poll_read(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &mut ReadBuf<'_>,
    ) -> Poll<io::Result<()>> {
        self.get_pin_mut().poll_read(cx, buf)
    }
}

impl<W: AsyncWrite + AsyncBufRead> AsyncBufRead for BufWriter<W> {
    fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
        self.get_pin_mut().poll_fill_buf(cx)
    }

    fn consume(self: Pin<&mut Self>, amt: usize) {
        self.get_pin_mut().consume(amt)
    }
}

impl<W: fmt::Debug> fmt::Debug for BufWriter<W> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("BufWriter")
            .field("writer", &self.inner)
            .field(
                "buffer",
                &format_args!("{}/{}", self.buf.len(), self.buf.capacity()),
            )
            .field("written", &self.written)
            .finish()
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn assert_unpin() {
        crate::is_unpin::<BufWriter<()>>();
    }
}