forked from mirror/async-std
0f30ab8c0a
The BufWriter docs inaccurately stated that it flushes on drop, which it does not do. This PR changes the docs, as well as the example, to highlight that the user must explicitly flush a bufwriter. There were also two places where the BufWriter code referred to it as a BufReader: in the link to the std docs, and in the Debug output. Those have also been fixed.
350 lines
11 KiB
Rust
350 lines
11 KiB
Rust
use std::fmt;
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use std::pin::Pin;
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use pin_project_lite::pin_project;
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use crate::io::write::WriteExt;
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use crate::io::{self, Seek, SeekFrom, Write, DEFAULT_BUF_SIZE};
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use crate::task::{Context, Poll, ready};
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pin_project! {
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/// Wraps a writer and buffers its output.
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///
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/// It can be excessively inefficient to work directly with something that
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/// implements [`Write`]. For example, every call to
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/// [`write`][`TcpStream::write`] on [`TcpStream`] results in a system call. A
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/// `BufWriter` keeps an in-memory buffer of data and writes it to an underlying
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/// writer in large, infrequent batches.
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///
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/// `BufWriter` can improve the speed of programs that make *small* and
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/// *repeated* write calls to the same file or network socket. It does not
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/// help when writing very large amounts at once, or writing just one or a few
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/// times. It also provides no advantage when writing to a destination that is
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/// in memory, like a `Vec<u8>`.
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///
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/// Unlike the `BufWriter` type in `std`, this type does not write out the
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/// contents of its buffer when it is dropped. Therefore, it is absolutely
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/// critical that users explicitly flush the buffer before dropping a
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/// `BufWriter`.
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///
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/// This type is an async version of [`std::io::BufWriter`].
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///
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/// [`std::io::BufWriter`]: https://doc.rust-lang.org/std/io/struct.BufWriter.html
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///
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/// # Examples
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///
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/// Let's write the numbers one through ten to a [`TcpStream`]:
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///
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/// ```no_run
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/// # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
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/// use async_std::net::TcpStream;
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/// use async_std::prelude::*;
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///
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/// let mut stream = TcpStream::connect("127.0.0.1:34254").await?;
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///
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/// for i in 0..10 {
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/// let arr = [i+1];
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/// stream.write(&arr).await?;
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/// }
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/// #
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/// # Ok(()) }) }
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/// ```
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///
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/// Because we're not buffering, we write each one in turn, incurring the
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/// overhead of a system call per byte written. We can fix this with a
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/// `BufWriter`:
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///
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/// ```no_run
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/// # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
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/// use async_std::io::BufWriter;
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/// use async_std::net::TcpStream;
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/// use async_std::prelude::*;
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///
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/// let mut stream = BufWriter::new(TcpStream::connect("127.0.0.1:34254").await?);
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///
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/// for i in 0..10 {
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/// let arr = [i+1];
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/// stream.write(&arr).await?;
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/// };
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///
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/// stream.flush().await?;
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/// #
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/// # Ok(()) }) }
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/// ```
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///
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/// By wrapping the stream with a `BufWriter`, these ten writes are all grouped
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/// together by the buffer, and will all be written out in one system call when
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/// the `stream` is dropped.
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///
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/// [`Write`]: trait.Write.html
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/// [`TcpStream::write`]: ../net/struct.TcpStream.html#method.write
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/// [`TcpStream`]: ../net/struct.TcpStream.html
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/// [`flush`]: trait.Write.html#tymethod.flush
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pub struct BufWriter<W> {
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#[pin]
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inner: W,
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buf: Vec<u8>,
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written: usize,
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}
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}
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/// An error returned by `into_inner` which combines an error that
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/// happened while writing out the buffer, and the buffered writer object
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/// which may be used to recover from the condition.
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///
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/// # Examples
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///
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/// ```no_run
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/// # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
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/// use async_std::io::BufWriter;
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/// use async_std::net::TcpStream;
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///
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/// let buf_writer = BufWriter::new(TcpStream::connect("127.0.0.1:34251").await?);
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///
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/// // unwrap the TcpStream and flush the buffer
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/// let stream = match buf_writer.into_inner().await {
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/// Ok(s) => s,
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/// Err(e) => {
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/// // Here, e is an IntoInnerError
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/// panic!("An error occurred");
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/// }
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/// };
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/// #
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/// # Ok(()) }) }
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///```
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#[derive(Debug)]
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pub struct IntoInnerError<W>(W, crate::io::Error);
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impl<W: Write> BufWriter<W> {
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/// Creates a new `BufWriter` with a default buffer capacity. The default is currently 8 KB,
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/// but may change in the future.
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///
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/// # Examples
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///
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/// ```no_run
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/// # #![allow(unused_mut)]
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/// # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
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/// use async_std::io::BufWriter;
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/// use async_std::net::TcpStream;
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///
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/// let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").await?);
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/// #
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/// # Ok(()) }) }
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/// ```
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pub fn new(inner: W) -> BufWriter<W> {
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BufWriter::with_capacity(DEFAULT_BUF_SIZE, inner)
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}
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/// Creates a new `BufWriter` with the specified buffer capacity.
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///
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/// # Examples
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///
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/// Creating a buffer with a buffer of a hundred bytes.
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///
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/// ```no_run
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/// # #![allow(unused_mut)]
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/// # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
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/// use async_std::io::BufWriter;
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/// use async_std::net::TcpStream;
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///
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/// let stream = TcpStream::connect("127.0.0.1:34254").await?;
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/// let mut buffer = BufWriter::with_capacity(100, stream);
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/// #
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/// # Ok(()) }) }
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/// ```
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pub fn with_capacity(capacity: usize, inner: W) -> BufWriter<W> {
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BufWriter {
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inner,
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buf: Vec::with_capacity(capacity),
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written: 0,
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}
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}
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/// Gets a reference to the underlying writer.
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///
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/// # Examples
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///
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/// ```no_run
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/// # #![allow(unused_mut)]
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/// # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
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/// use async_std::io::BufWriter;
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/// use async_std::net::TcpStream;
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///
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/// let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").await?);
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///
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/// // We can use reference just like buffer
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/// let reference = buffer.get_ref();
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/// #
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/// # Ok(()) }) }
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/// ```
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pub fn get_ref(&self) -> &W {
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&self.inner
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}
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/// Gets a mutable reference to the underlying writer.
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///
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/// It is inadvisable to directly write to the underlying writer.
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///
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/// # Examples
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///
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/// ```no_run
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/// # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
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/// use async_std::io::BufWriter;
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/// use async_std::net::TcpStream;
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///
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/// let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").await?);
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///
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/// // We can use reference just like buffer
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/// let reference = buffer.get_mut();
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/// #
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/// # Ok(()) }) }
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/// ```
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pub fn get_mut(&mut self) -> &mut W {
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&mut self.inner
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}
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/// Gets a pinned mutable reference to the underlying writer.
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///
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/// It is inadvisable to directly write to the underlying writer.
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fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut W> {
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self.project().inner
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}
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/// Consumes BufWriter, returning the underlying writer
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///
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/// This method will not write leftover data, it will be lost.
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/// For method that will attempt to write before returning the writer see [`poll_into_inner`]
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///
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/// [`poll_into_inner`]: #method.poll_into_inner
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/// # Examples
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///
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/// ```no_run
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/// # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
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/// use async_std::io::BufWriter;
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/// use async_std::net::TcpStream;
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///
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/// let buf_writer = BufWriter::new(TcpStream::connect("127.0.0.1:34251").await?);
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///
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/// // unwrap the TcpStream and flush the buffer
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/// let stream = buf_writer.into_inner().await.unwrap();
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/// #
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/// # Ok(()) }) }
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/// ```
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pub async fn into_inner(mut self) -> Result<W, IntoInnerError<BufWriter<W>>>
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where
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Self: Unpin,
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{
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match self.flush().await {
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Err(e) => Err(IntoInnerError(self, e)),
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Ok(()) => Ok(self.inner),
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}
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}
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/// Returns a reference to the internally buffered data.
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///
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/// # Examples
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///
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/// ```no_run
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/// # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
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/// use async_std::io::BufWriter;
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/// use async_std::net::TcpStream;
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///
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/// let buf_writer = BufWriter::new(TcpStream::connect("127.0.0.1:34251").await?);
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///
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/// // See how many bytes are currently buffered
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/// let bytes_buffered = buf_writer.buffer().len();
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/// #
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/// # Ok(()) }) }
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/// ```
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pub fn buffer(&self) -> &[u8] {
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&self.buf
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}
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/// Poll buffer flushing until completion
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///
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/// This is used in types that wrap around BufWrite, one such example: [`LineWriter`]
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///
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/// [`LineWriter`]: struct.LineWriter.html
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fn poll_flush_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
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let mut this = self.project();
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let len = this.buf.len();
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let mut ret = Ok(());
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while *this.written < len {
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match this
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.inner
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.as_mut()
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.poll_write(cx, &this.buf[*this.written..])
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{
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Poll::Ready(Ok(0)) => {
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ret = Err(io::Error::new(
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io::ErrorKind::WriteZero,
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"Failed to write buffered data",
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));
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break;
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}
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Poll::Ready(Ok(n)) => *this.written += n,
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Poll::Ready(Err(ref e)) if e.kind() == io::ErrorKind::Interrupted => {}
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Poll::Ready(Err(e)) => {
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ret = Err(e);
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break;
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}
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Poll::Pending => return Poll::Pending,
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}
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}
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if *this.written > 0 {
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this.buf.drain(..*this.written);
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}
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*this.written = 0;
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Poll::Ready(ret)
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}
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}
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impl<W: Write> Write for BufWriter<W> {
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fn poll_write(
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mut self: Pin<&mut Self>,
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cx: &mut Context<'_>,
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buf: &[u8],
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) -> Poll<io::Result<usize>> {
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if self.buf.len() + buf.len() > self.buf.capacity() {
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ready!(self.as_mut().poll_flush_buf(cx))?;
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}
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if buf.len() >= self.buf.capacity() {
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self.get_pin_mut().poll_write(cx, buf)
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} else {
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Pin::new(&mut *self.project().buf).poll_write(cx, buf)
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}
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}
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fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
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ready!(self.as_mut().poll_flush_buf(cx))?;
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self.get_pin_mut().poll_flush(cx)
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}
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fn poll_close(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
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ready!(self.as_mut().poll_flush_buf(cx))?;
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self.get_pin_mut().poll_close(cx)
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}
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}
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impl<W: Write + fmt::Debug> fmt::Debug for BufWriter<W> {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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f.debug_struct("BufWriter")
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.field("writer", &self.inner)
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.field("buf", &self.buf)
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.finish()
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}
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}
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impl<W: Write + Seek> Seek for BufWriter<W> {
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/// Seek to the offset, in bytes, in the underlying writer.
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///
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/// Seeking always writes out the internal buffer before seeking.
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fn poll_seek(
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mut self: Pin<&mut Self>,
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cx: &mut Context<'_>,
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pos: SeekFrom,
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) -> Poll<io::Result<u64>> {
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ready!(self.as_mut().poll_flush_buf(cx))?;
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self.get_pin_mut().poll_seek(cx, pos)
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}
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}
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