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use crate::io::util::DEFAULT_BUF_SIZE;
use crate::io::{AsyncBufRead, AsyncRead, AsyncWrite};
use pin_project_lite::pin_project;
use std::fmt;
use std::io::{self, Write};
use std::mem::MaybeUninit;
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,
}
}
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,
}
}
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_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)
}
}
impl<W: AsyncWrite + AsyncRead> AsyncRead for BufWriter<W> {
fn poll_read(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
buf: &mut [u8],
) -> Poll<io::Result<usize>> {
self.get_pin_mut().poll_read(cx, buf)
}
// we can't skip unconditionally because of the large buffer case in read.
unsafe fn prepare_uninitialized_buffer(&self, buf: &mut [MaybeUninit<u8>]) -> bool {
self.get_ref().prepare_uninitialized_buffer(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<()>>();
}
}