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This commit is contained in:
Julien THILLARD
2026-03-17 23:40:39 +01:00
parent 10be5b301c
commit fd0770f813
15 changed files with 4526 additions and 16 deletions
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297
crates/std/src/io/buffered/linewritershim.rs Normal file
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use core::slice::memchr;
use crate::io::{self, BufWriter, IoSlice, Write};
/// Private helper struct for implementing the line-buffered writing logic.
///
/// This shim temporarily wraps a BufWriter, and uses its internals to
/// implement a line-buffered writer (specifically by using the internal
/// methods like write_to_buf and flush_buf). In this way, a more
/// efficient abstraction can be created than one that only had access to
/// `write` and `flush`, without needlessly duplicating a lot of the
/// implementation details of BufWriter. This also allows existing
/// `BufWriters` to be temporarily given line-buffering logic; this is what
/// enables Stdout to be alternately in line-buffered or block-buffered mode.
#[derive(Debug)]
pub struct LineWriterShim<'a, W: ?Sized + Write> {
buffer: &'a mut BufWriter<W>,
}
impl<'a, W: ?Sized + Write> LineWriterShim<'a, W> {
pub fn new(buffer: &'a mut BufWriter<W>) -> Self {
Self { buffer }
}
/// Gets a reference to the inner writer (that is, the writer
/// wrapped by the BufWriter).
fn inner(&self) -> &W {
self.buffer.get_ref()
}
/// Gets a mutable reference to the inner writer (that is, the writer
/// wrapped by the BufWriter). Be careful with this writer, as writes to
/// it will bypass the buffer.
fn inner_mut(&mut self) -> &mut W {
self.buffer.get_mut()
}
/// Gets the content currently buffered in self.buffer
fn buffered(&self) -> &[u8] {
self.buffer.buffer()
}
/// Flushes the buffer iff the last byte is a newline (indicating that an
/// earlier write only succeeded partially, and we want to retry flushing
/// the buffered line before continuing with a subsequent write).
fn flush_if_completed_line(&mut self) -> io::Result<()> {
match self.buffered().last().copied() {
Some(b'\n') => self.buffer.flush_buf(),
_ => Ok(()),
}
}
}
impl<'a, W: ?Sized + Write> Write for LineWriterShim<'a, W> {
/// Writes some data into this BufWriter with line buffering.
///
/// This means that, if any newlines are present in the data, the data up to
/// the last newline is sent directly to the underlying writer, and data
/// after it is buffered. Returns the number of bytes written.
///
/// This function operates on a "best effort basis"; in keeping with the
/// convention of `Write::write`, it makes at most one attempt to write
/// new data to the underlying writer. If that write only reports a partial
/// success, the remaining data will be buffered.
///
/// Because this function attempts to send completed lines to the underlying
/// writer, it will also flush the existing buffer if it ends with a
/// newline, even if the incoming data does not contain any newlines.
fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
let newline_idx = match memchr::memrchr(b'\n', buf) {
// If there are no new newlines (that is, if this write is less than
// one line), just do a regular buffered write (which may flush if
// we exceed the inner buffer's size)
None => {
self.flush_if_completed_line()?;
return self.buffer.write(buf);
}
// Otherwise, arrange for the lines to be written directly to the
// inner writer.
Some(newline_idx) => newline_idx + 1,
};
// Flush existing content to prepare for our write. We have to do this
// before attempting to write `buf` in order to maintain consistency;
// if we add `buf` to the buffer then try to flush it all at once,
// we're obligated to return Ok(), which would mean suppressing any
// errors that occur during flush.
self.buffer.flush_buf()?;
// This is what we're going to try to write directly to the inner
// writer. The rest will be buffered, if nothing goes wrong.
let lines = &buf[..newline_idx];
// Write `lines` directly to the inner writer. In keeping with the
// `write` convention, make at most one attempt to add new (unbuffered)
// data. Because this write doesn't touch the BufWriter state directly,
// and the buffer is known to be empty, we don't need to worry about
// self.buffer.panicked here.
let flushed = self.inner_mut().write(lines)?;
// If buffer returns Ok(0), propagate that to the caller without
// doing additional buffering; otherwise we're just guaranteeing
// an "ErrorKind::WriteZero" later.
if flushed == 0 {
return Ok(0);
}
// Now that the write has succeeded, buffer the rest (or as much of
// the rest as possible). If there were any unwritten newlines, we
// only buffer out to the last unwritten newline that fits in the
// buffer; this helps prevent flushing partial lines on subsequent
// calls to LineWriterShim::write.
// Handle the cases in order of most-common to least-common, under
// the presumption that most writes succeed in totality, and that most
// writes are smaller than the buffer.
// - Is this a partial line (ie, no newlines left in the unwritten tail)
// - If not, does the data out to the last unwritten newline fit in
// the buffer?
// - If not, scan for the last newline that *does* fit in the buffer
let tail = if flushed >= newline_idx {
let tail = &buf[flushed..];
// Avoid unnecessary short writes by not splitting the remaining
// bytes if they're larger than the buffer.
// They can be written in full by the next call to write.
if tail.len() >= self.buffer.capacity() {
return Ok(flushed);
}
tail
} else if newline_idx - flushed <= self.buffer.capacity() {
&buf[flushed..newline_idx]
} else {
let scan_area = &buf[flushed..];
let scan_area = &scan_area[..self.buffer.capacity()];
match memchr::memrchr(b'\n', scan_area) {
Some(newline_idx) => &scan_area[..newline_idx + 1],
None => scan_area,
}
};
let buffered = self.buffer.write_to_buf(tail);
Ok(flushed + buffered)
}
fn flush(&mut self) -> io::Result<()> {
self.buffer.flush()
}
/// Writes some vectored data into this BufWriter with line buffering.
///
/// This means that, if any newlines are present in the data, the data up to
/// and including the buffer containing the last newline is sent directly to
/// the inner writer, and the data after it is buffered. Returns the number
/// of bytes written.
///
/// This function operates on a "best effort basis"; in keeping with the
/// convention of `Write::write`, it makes at most one attempt to write
/// new data to the underlying writer.
///
/// Because this function attempts to send completed lines to the underlying
/// writer, it will also flush the existing buffer if it contains any
/// newlines.
///
/// Because sorting through an array of `IoSlice` can be a bit convoluted,
/// This method differs from write in the following ways:
///
/// - It attempts to write the full content of all the buffers up to and
/// including the one containing the last newline. This means that it
/// may attempt to write a partial line, that buffer has data past the
/// newline.
/// - If the write only reports partial success, it does not attempt to
/// find the precise location of the written bytes and buffer the rest.
///
/// If the underlying vector doesn't support vectored writing, we instead
/// simply write the first non-empty buffer with `write`. This way, we
/// get the benefits of more granular partial-line handling without losing
/// anything in efficiency
fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
// If there's no specialized behavior for write_vectored, just use
// write. This has the benefit of more granular partial-line handling.
if !self.is_write_vectored() {
return match bufs.iter().find(|buf| !buf.is_empty()) {
Some(buf) => self.write(buf),
None => Ok(0),
};
}
// Find the buffer containing the last newline
// FIXME: This is overly slow if there are very many bufs and none contain
// newlines. e.g. writev() on Linux only writes up to 1024 slices, so
// scanning the rest is wasted effort. This makes write_all_vectored()
// quadratic.
let last_newline_buf_idx = bufs
.iter()
.enumerate()
.rev()
.find_map(|(i, buf)| memchr::memchr(b'\n', buf).map(|_| i));
// If there are no new newlines (that is, if this write is less than
// one line), just do a regular buffered write
let last_newline_buf_idx = match last_newline_buf_idx {
// No newlines; just do a normal buffered write
None => {
self.flush_if_completed_line()?;
return self.buffer.write_vectored(bufs);
}
Some(i) => i,
};
// Flush existing content to prepare for our write
self.buffer.flush_buf()?;
// This is what we're going to try to write directly to the inner
// writer. The rest will be buffered, if nothing goes wrong.
let (lines, tail) = bufs.split_at(last_newline_buf_idx + 1);
// Write `lines` directly to the inner writer. In keeping with the
// `write` convention, make at most one attempt to add new (unbuffered)
// data. Because this write doesn't touch the BufWriter state directly,
// and the buffer is known to be empty, we don't need to worry about
// self.panicked here.
let flushed = self.inner_mut().write_vectored(lines)?;
// If inner returns Ok(0), propagate that to the caller without
// doing additional buffering; otherwise we're just guaranteeing
// an "ErrorKind::WriteZero" later.
if flushed == 0 {
return Ok(0);
}
// Don't try to reconstruct the exact amount written; just bail
// in the event of a partial write
let mut lines_len: usize = 0;
for buf in lines {
// With overlapping/duplicate slices the total length may in theory
// exceed usize::MAX
lines_len = lines_len.saturating_add(buf.len());
if flushed < lines_len {
return Ok(flushed);
}
}
// Now that the write has succeeded, buffer the rest (or as much of the
// rest as possible)
let buffered: usize = tail
.iter()
.filter(|buf| !buf.is_empty())
.map(|buf| self.buffer.write_to_buf(buf))
.take_while(|&n| n > 0)
.sum();
Ok(flushed + buffered)
}
fn is_write_vectored(&self) -> bool {
self.inner().is_write_vectored()
}
/// Writes some data into this BufWriter with line buffering.
///
/// This means that, if any newlines are present in the data, the data up to
/// the last newline is sent directly to the underlying writer, and data
/// after it is buffered.
///
/// Because this function attempts to send completed lines to the underlying
/// writer, it will also flush the existing buffer if it contains any
/// newlines, even if the incoming data does not contain any newlines.
fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
match memchr::memrchr(b'\n', buf) {
// If there are no new newlines (that is, if this write is less than
// one line), just do a regular buffered write (which may flush if
// we exceed the inner buffer's size)
None => {
self.flush_if_completed_line()?;
self.buffer.write_all(buf)
}
Some(newline_idx) => {
let (lines, tail) = buf.split_at(newline_idx + 1);
if self.buffered().is_empty() {
self.inner_mut().write_all(lines)?;
} else {
// If there is any buffered data, we add the incoming lines
// to that buffer before flushing, which saves us at least
// one write call. We can't really do this with `write`,
// since we can't do this *and* not suppress errors *and*
// report a consistent state to the caller in a return
// value, but here in write_all it's fine.
self.buffer.write_all(lines)?;
self.buffer.flush_buf()?;
}
self.buffer.write_all(tail)
}
}
}
}