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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<()>>();
}
}