linewritershim.rs - source
Description: Source of the Rust file `library/std/src/io/buffered/linewritershim.rs`.
Keywords: rust, rustlang, rust-lang

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    use crate::io::{self, BufWriter, IoSlice, Write};
    use crate::memchr;

    /// 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: Write> {
        buffer: &'a mut BufWriter<W>,
    }

    impl<'a, W: Write> LineWriterShim<'a, W> {
        pub fn new(buffer: &'a mut BufWriter<W>) -> Self {
            Self { buffer }
        }

        /// Get 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()
        }

        /// Get the content currently buffered in self.buffer
        fn buffered(&self) -> &[u8] {
            self.buffer.buffer()
        }

        /// Flush 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: Write> Write for LineWriterShim<'a, W> {
        /// Write some data into this BufReader 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 {
                &buf[flushed..]
            } 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()
        }

        /// Write some vectored data into this BufReader 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
            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 lines_len = lines.iter().map(|buf| buf.len()).sum();
            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.buffer.is_write_vectored()
        }

        /// Write some data into this BufReader 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)
                }
            }
        }
    }