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</pre><pre class="rust"><code><span class="kw">use </span><span class="kw">crate</span>::io::util::DEFAULT_BUF_SIZE;
<span class="kw">use </span><span class="kw">crate</span>::io::{AsyncBufRead, AsyncRead, AsyncSeek, AsyncWrite, ReadBuf};
<span class="kw">use </span>pin_project_lite::pin_project;
<span class="kw">use </span>std::fmt;
<span class="kw">use </span>std::io::{<span class="self">self</span>, IoSlice, SeekFrom, Write};
<span class="kw">use </span>std::pin::Pin;
<span class="kw">use </span>std::task::{Context, Poll};
<span class="macro">pin_project! </span>{
<span class="doccomment">/// 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&lt;u8&gt;`.
///
/// 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
///
</span><span class="attribute">#[cfg_attr(docsrs, doc(cfg(feature = <span class="string">&quot;io-util&quot;</span>)))]
</span><span class="kw">pub struct </span>BufWriter&lt;W&gt; {
<span class="attribute">#[pin]
</span><span class="kw">pub</span>(<span class="kw">super</span>) inner: W,
<span class="kw">pub</span>(<span class="kw">super</span>) buf: Vec&lt;u8&gt;,
<span class="kw">pub</span>(<span class="kw">super</span>) written: usize,
<span class="kw">pub</span>(<span class="kw">super</span>) seek_state: SeekState,
}
}
<span class="kw">impl</span>&lt;W: AsyncWrite&gt; BufWriter&lt;W&gt; {
<span class="doccomment">/// Creates a new `BufWriter` with a default buffer capacity. The default is currently 8 KB,
/// but may change in the future.
</span><span class="kw">pub fn </span>new(inner: W) -&gt; <span class="self">Self </span>{
<span class="self">Self</span>::with_capacity(DEFAULT_BUF_SIZE, inner)
}
<span class="doccomment">/// Creates a new `BufWriter` with the specified buffer capacity.
</span><span class="kw">pub fn </span>with_capacity(cap: usize, inner: W) -&gt; <span class="self">Self </span>{
<span class="self">Self </span>{
inner,
buf: Vec::with_capacity(cap),
written: <span class="number">0</span>,
seek_state: SeekState::Init,
}
}
<span class="kw">fn </span>flush_buf(<span class="self">self</span>: Pin&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>&gt;, cx: <span class="kw-2">&amp;mut </span>Context&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; Poll&lt;io::Result&lt;()&gt;&gt; {
<span class="kw">let </span><span class="kw-2">mut </span>me = <span class="self">self</span>.project();
<span class="kw">let </span>len = me.buf.len();
<span class="kw">let </span><span class="kw-2">mut </span>ret = <span class="prelude-val">Ok</span>(());
<span class="kw">while </span><span class="kw-2">*</span>me.written &lt; len {
<span class="kw">match </span><span class="macro">ready!</span>(me.inner.as_mut().poll_write(cx, <span class="kw-2">&amp;</span>me.buf[<span class="kw-2">*</span>me.written..])) {
<span class="prelude-val">Ok</span>(<span class="number">0</span>) =&gt; {
ret = <span class="prelude-val">Err</span>(io::Error::new(
io::ErrorKind::WriteZero,
<span class="string">&quot;failed to write the buffered data&quot;</span>,
));
<span class="kw">break</span>;
}
<span class="prelude-val">Ok</span>(n) =&gt; <span class="kw-2">*</span>me.written += n,
<span class="prelude-val">Err</span>(e) =&gt; {
ret = <span class="prelude-val">Err</span>(e);
<span class="kw">break</span>;
}
}
}
<span class="kw">if </span><span class="kw-2">*</span>me.written &gt; <span class="number">0 </span>{
me.buf.drain(..<span class="kw-2">*</span>me.written);
}
<span class="kw-2">*</span>me.written = <span class="number">0</span>;
Poll::Ready(ret)
}
<span class="doccomment">/// Gets a reference to the underlying writer.
</span><span class="kw">pub fn </span>get_ref(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="kw-2">&amp;</span>W {
<span class="kw-2">&amp;</span><span class="self">self</span>.inner
}
<span class="doccomment">/// Gets a mutable reference to the underlying writer.
///
/// It is inadvisable to directly write to the underlying writer.
</span><span class="kw">pub fn </span>get_mut(<span class="kw-2">&amp;mut </span><span class="self">self</span>) -&gt; <span class="kw-2">&amp;mut </span>W {
<span class="kw-2">&amp;mut </span><span class="self">self</span>.inner
}
<span class="doccomment">/// Gets a pinned mutable reference to the underlying writer.
///
/// It is inadvisable to directly write to the underlying writer.
</span><span class="kw">pub fn </span>get_pin_mut(<span class="self">self</span>: Pin&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>&gt;) -&gt; Pin&lt;<span class="kw-2">&amp;mut </span>W&gt; {
<span class="self">self</span>.project().inner
}
<span class="doccomment">/// Consumes this `BufWriter`, returning the underlying writer.
///
/// Note that any leftover data in the internal buffer is lost.
</span><span class="kw">pub fn </span>into_inner(<span class="self">self</span>) -&gt; W {
<span class="self">self</span>.inner
}
<span class="doccomment">/// Returns a reference to the internally buffered data.
</span><span class="kw">pub fn </span>buffer(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="kw-2">&amp;</span>[u8] {
<span class="kw-2">&amp;</span><span class="self">self</span>.buf
}
}
<span class="kw">impl</span>&lt;W: AsyncWrite&gt; AsyncWrite <span class="kw">for </span>BufWriter&lt;W&gt; {
<span class="kw">fn </span>poll_write(
<span class="kw-2">mut </span><span class="self">self</span>: Pin&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>&gt;,
cx: <span class="kw-2">&amp;mut </span>Context&lt;<span class="lifetime">&#39;_</span>&gt;,
buf: <span class="kw-2">&amp;</span>[u8],
) -&gt; Poll&lt;io::Result&lt;usize&gt;&gt; {
<span class="kw">if </span><span class="self">self</span>.buf.len() + buf.len() &gt; <span class="self">self</span>.buf.capacity() {
<span class="macro">ready!</span>(<span class="self">self</span>.as_mut().flush_buf(cx))<span class="question-mark">?</span>;
}
<span class="kw">let </span>me = <span class="self">self</span>.project();
<span class="kw">if </span>buf.len() &gt;= me.buf.capacity() {
me.inner.poll_write(cx, buf)
} <span class="kw">else </span>{
Poll::Ready(me.buf.write(buf))
}
}
<span class="kw">fn </span>poll_write_vectored(
<span class="kw-2">mut </span><span class="self">self</span>: Pin&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>&gt;,
cx: <span class="kw-2">&amp;mut </span>Context&lt;<span class="lifetime">&#39;_</span>&gt;,
<span class="kw-2">mut </span>bufs: <span class="kw-2">&amp;</span>[IoSlice&lt;<span class="lifetime">&#39;_</span>&gt;],
) -&gt; Poll&lt;io::Result&lt;usize&gt;&gt; {
<span class="kw">if </span><span class="self">self</span>.inner.is_write_vectored() {
<span class="kw">let </span>total_len = bufs
.iter()
.fold(<span class="number">0usize</span>, |acc, b| acc.saturating_add(b.len()));
<span class="kw">if </span>total_len &gt; <span class="self">self</span>.buf.capacity() - <span class="self">self</span>.buf.len() {
<span class="macro">ready!</span>(<span class="self">self</span>.as_mut().flush_buf(cx))<span class="question-mark">?</span>;
}
<span class="kw">let </span>me = <span class="self">self</span>.as_mut().project();
<span class="kw">if </span>total_len &gt;= me.buf.capacity() {
<span class="comment">// It&#39;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.
</span>me.inner.poll_write_vectored(cx, bufs)
} <span class="kw">else </span>{
bufs.iter().for_each(|b| me.buf.extend_from_slice(b));
Poll::Ready(<span class="prelude-val">Ok</span>(total_len))
}
} <span class="kw">else </span>{
<span class="comment">// Remove empty buffers at the beginning of bufs.
</span><span class="kw">while </span>bufs.first().map(|buf| buf.len()) == <span class="prelude-val">Some</span>(<span class="number">0</span>) {
bufs = <span class="kw-2">&amp;</span>bufs[<span class="number">1</span>..];
}
<span class="kw">if </span>bufs.is_empty() {
<span class="kw">return </span>Poll::Ready(<span class="prelude-val">Ok</span>(<span class="number">0</span>));
}
<span class="comment">// Flush if the first buffer doesn&#39;t fit.
</span><span class="kw">let </span>first_len = bufs[<span class="number">0</span>].len();
<span class="kw">if </span>first_len &gt; <span class="self">self</span>.buf.capacity() - <span class="self">self</span>.buf.len() {
<span class="macro">ready!</span>(<span class="self">self</span>.as_mut().flush_buf(cx))<span class="question-mark">?</span>;
<span class="macro">debug_assert!</span>(<span class="self">self</span>.buf.is_empty());
}
<span class="kw">let </span>me = <span class="self">self</span>.as_mut().project();
<span class="kw">if </span>first_len &gt;= me.buf.capacity() {
<span class="comment">// The slice is at least as large as the buffering capacity,
// so it&#39;s better to write it directly, bypassing the buffer.
</span><span class="macro">debug_assert!</span>(me.buf.is_empty());
<span class="kw">return </span>me.inner.poll_write(cx, <span class="kw-2">&amp;</span>bufs[<span class="number">0</span>]);
} <span class="kw">else </span>{
me.buf.extend_from_slice(<span class="kw-2">&amp;</span>bufs[<span class="number">0</span>]);
bufs = <span class="kw-2">&amp;</span>bufs[<span class="number">1</span>..];
}
<span class="kw">let </span><span class="kw-2">mut </span>total_written = first_len;
<span class="macro">debug_assert!</span>(total_written != <span class="number">0</span>);
<span class="comment">// Append the buffers that fit in the internal buffer.
</span><span class="kw">for </span>buf <span class="kw">in </span>bufs {
<span class="kw">if </span>buf.len() &gt; me.buf.capacity() - me.buf.len() {
<span class="kw">break</span>;
} <span class="kw">else </span>{
me.buf.extend_from_slice(buf);
total_written += buf.len();
}
}
Poll::Ready(<span class="prelude-val">Ok</span>(total_written))
}
}
<span class="kw">fn </span>is_write_vectored(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
<span class="bool-val">true
</span>}
<span class="kw">fn </span>poll_flush(<span class="kw-2">mut </span><span class="self">self</span>: Pin&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>&gt;, cx: <span class="kw-2">&amp;mut </span>Context&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; Poll&lt;io::Result&lt;()&gt;&gt; {
<span class="macro">ready!</span>(<span class="self">self</span>.as_mut().flush_buf(cx))<span class="question-mark">?</span>;
<span class="self">self</span>.get_pin_mut().poll_flush(cx)
}
<span class="kw">fn </span>poll_shutdown(<span class="kw-2">mut </span><span class="self">self</span>: Pin&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>&gt;, cx: <span class="kw-2">&amp;mut </span>Context&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; Poll&lt;io::Result&lt;()&gt;&gt; {
<span class="macro">ready!</span>(<span class="self">self</span>.as_mut().flush_buf(cx))<span class="question-mark">?</span>;
<span class="self">self</span>.get_pin_mut().poll_shutdown(cx)
}
}
<span class="attribute">#[derive(Debug, Clone, Copy)]
</span><span class="kw">pub</span>(<span class="kw">super</span>) <span class="kw">enum </span>SeekState {
<span class="doccomment">/// start_seek has not been called.
</span>Init,
<span class="doccomment">/// start_seek has been called, but poll_complete has not yet been called.
</span>Start(SeekFrom),
<span class="doccomment">/// Waiting for completion of poll_complete.
</span>Pending,
}
<span class="doccomment">/// Seek to the offset, in bytes, in the underlying writer.
///
/// Seeking always writes out the internal buffer before seeking.
</span><span class="kw">impl</span>&lt;W: AsyncWrite + AsyncSeek&gt; AsyncSeek <span class="kw">for </span>BufWriter&lt;W&gt; {
<span class="kw">fn </span>start_seek(<span class="self">self</span>: Pin&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>&gt;, pos: SeekFrom) -&gt; io::Result&lt;()&gt; {
<span class="comment">// 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`.
</span><span class="kw-2">*</span><span class="self">self</span>.project().seek_state = SeekState::Start(pos);
<span class="prelude-val">Ok</span>(())
}
<span class="kw">fn </span>poll_complete(<span class="kw-2">mut </span><span class="self">self</span>: Pin&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>&gt;, cx: <span class="kw-2">&amp;mut </span>Context&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; Poll&lt;io::Result&lt;u64&gt;&gt; {
<span class="kw">let </span>pos = <span class="kw">match </span><span class="self">self</span>.seek_state {
SeekState::Init =&gt; {
<span class="kw">return </span><span class="self">self</span>.project().inner.poll_complete(cx);
}
SeekState::Start(pos) =&gt; <span class="prelude-val">Some</span>(pos),
SeekState::Pending =&gt; <span class="prelude-val">None</span>,
};
<span class="comment">// Flush the internal buffer before seeking.
</span><span class="macro">ready!</span>(<span class="self">self</span>.as_mut().flush_buf(cx))<span class="question-mark">?</span>;
<span class="kw">let </span><span class="kw-2">mut </span>me = <span class="self">self</span>.project();
<span class="kw">if let </span><span class="prelude-val">Some</span>(pos) = pos {
<span class="comment">// Ensure previous seeks have finished before starting a new one
</span><span class="macro">ready!</span>(me.inner.as_mut().poll_complete(cx))<span class="question-mark">?</span>;
<span class="kw">if let </span><span class="prelude-val">Err</span>(e) = me.inner.as_mut().start_seek(pos) {
<span class="kw-2">*</span>me.seek_state = SeekState::Init;
<span class="kw">return </span>Poll::Ready(<span class="prelude-val">Err</span>(e));
}
}
<span class="kw">match </span>me.inner.poll_complete(cx) {
Poll::Ready(res) =&gt; {
<span class="kw-2">*</span>me.seek_state = SeekState::Init;
Poll::Ready(res)
}
Poll::Pending =&gt; {
<span class="kw-2">*</span>me.seek_state = SeekState::Pending;
Poll::Pending
}
}
}
}
<span class="kw">impl</span>&lt;W: AsyncWrite + AsyncRead&gt; AsyncRead <span class="kw">for </span>BufWriter&lt;W&gt; {
<span class="kw">fn </span>poll_read(
<span class="self">self</span>: Pin&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>&gt;,
cx: <span class="kw-2">&amp;mut </span>Context&lt;<span class="lifetime">&#39;_</span>&gt;,
buf: <span class="kw-2">&amp;mut </span>ReadBuf&lt;<span class="lifetime">&#39;_</span>&gt;,
) -&gt; Poll&lt;io::Result&lt;()&gt;&gt; {
<span class="self">self</span>.get_pin_mut().poll_read(cx, buf)
}
}
<span class="kw">impl</span>&lt;W: AsyncWrite + AsyncBufRead&gt; AsyncBufRead <span class="kw">for </span>BufWriter&lt;W&gt; {
<span class="kw">fn </span>poll_fill_buf(<span class="self">self</span>: Pin&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>&gt;, cx: <span class="kw-2">&amp;mut </span>Context&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; Poll&lt;io::Result&lt;<span class="kw-2">&amp;</span>[u8]&gt;&gt; {
<span class="self">self</span>.get_pin_mut().poll_fill_buf(cx)
}
<span class="kw">fn </span>consume(<span class="self">self</span>: Pin&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>&gt;, amt: usize) {
<span class="self">self</span>.get_pin_mut().consume(amt)
}
}
<span class="kw">impl</span>&lt;W: fmt::Debug&gt; fmt::Debug <span class="kw">for </span>BufWriter&lt;W&gt; {
<span class="kw">fn </span>fmt(<span class="kw-2">&amp;</span><span class="self">self</span>, f: <span class="kw-2">&amp;mut </span>fmt::Formatter&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; fmt::Result {
f.debug_struct(<span class="string">&quot;BufWriter&quot;</span>)
.field(<span class="string">&quot;writer&quot;</span>, <span class="kw-2">&amp;</span><span class="self">self</span>.inner)
.field(
<span class="string">&quot;buffer&quot;</span>,
<span class="kw-2">&amp;</span><span class="macro">format_args!</span>(<span class="string">&quot;{}/{}&quot;</span>, <span class="self">self</span>.buf.len(), <span class="self">self</span>.buf.capacity()),
)
.field(<span class="string">&quot;written&quot;</span>, <span class="kw-2">&amp;</span><span class="self">self</span>.written)
.finish()
}
}
<span class="attribute">#[cfg(test)]
</span><span class="kw">mod </span>tests {
<span class="kw">use super</span>::<span class="kw-2">*</span>;
<span class="attribute">#[test]
</span><span class="kw">fn </span>assert_unpin() {
<span class="kw">crate</span>::is_unpin::&lt;BufWriter&lt;()&gt;&gt;();
}
}
</code></pre></div>
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