library/alloc/src/io/buf_read.rs

use core::slice::memchr;

use crate::io::{ErrorKind, Lines, Read, Result, Split, append_to_string, lines, split};
use crate::string::String;
use crate::vec::Vec;

/// A `BufRead` is a type of [`Read`]er which has an internal buffer, allowing it
/// to perform extra ways of reading.
///
/// For example, reading line-by-line is inefficient without using a buffer, so
/// if you want to read by line, you'll need `BufRead`, which includes a
/// [`read_line`] method as well as a [`lines`] iterator.
///
/// # Examples
///
/// A locked standard input implements `BufRead`:
///
/// ```no_run
/// use std::io;
/// use std::io::prelude::*;
///
/// let stdin = io::stdin();
/// for line in stdin.lock().lines() {
///     println!("{}", line?);
/// }
/// # std::io::Result::Ok(())
/// ```
///
/// If you have something that implements [`Read`], you can use the `BufReader`
/// type to turn it into a `BufRead`.
///
/// For example, `File` implements [`Read`], but not `BufRead`.
/// `BufReader` to the rescue!
///
/// [`read_line`]: BufRead::read_line
/// [`lines`]: BufRead::lines
///
/// ```no_run
/// use std::io::{self, BufReader};
/// use std::io::prelude::*;
/// use std::fs::File;
///
/// fn main() -> io::Result<()> {
///     let f = File::open("foo.txt")?;
///     let f = BufReader::new(f);
///
///     for line in f.lines() {
///         let line = line?;
///         println!("{line}");
///     }
///
///     Ok(())
/// }
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), rustc_diagnostic_item = "IoBufRead")]
pub trait BufRead: Read {
    /// Returns the contents of the internal buffer, filling it with more data, via [`Read`] methods, if empty.
    ///
    /// This is a lower-level method and is meant to be used together with [`consume`],
    /// which can be used to mark bytes that should not be returned by subsequent calls to `read`.
    ///
    /// [`consume`]: BufRead::consume
    ///
    /// Returns an empty buffer when the stream has reached EOF.
    ///
    /// # Errors
    ///
    /// This function will return an I/O error if a [`Read`] method was called, but returned an error.
    ///
    /// # Examples
    ///
    /// A locked standard input implements `BufRead`:
    ///
    /// ```no_run
    /// use std::io;
    /// use std::io::prelude::*;
    ///
    /// let stdin = io::stdin();
    /// let mut stdin = stdin.lock();
    ///
    /// let buffer = stdin.fill_buf()?;
    ///
    /// // work with buffer
    /// println!("{buffer:?}");
    ///
    /// // mark the bytes we worked with as read
    /// let length = buffer.len();
    /// stdin.consume(length);
    /// # std::io::Result::Ok(())
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn fill_buf(&mut self) -> Result<&[u8]>;

    /// Marks the given `amount` of additional bytes from the internal buffer as having been read.
    /// Subsequent calls to `read` only return bytes that have not been marked as read.
    ///
    /// This is a lower-level method and is meant to be used together with [`fill_buf`],
    /// which can be used to fill the internal buffer via [`Read`] methods.
    ///
    /// It is a logic error if `amount` exceeds the number of unread bytes in the internal buffer, which is returned by [`fill_buf`].
    ///
    /// # Examples
    ///
    /// Since `consume()` is meant to be used with [`fill_buf`],
    /// that method's example includes an example of `consume()`.
    ///
    /// [`fill_buf`]: BufRead::fill_buf
    #[stable(feature = "rust1", since = "1.0.0")]
    fn consume(&mut self, amount: usize);

    /// Checks if there is any data left to be `read`.
    ///
    /// This function may fill the buffer to check for data,
    /// so this function returns `Result<bool>`, not `bool`.
    ///
    /// The default implementation calls `fill_buf` and checks that the
    /// returned slice is empty (which means that there is no data left,
    /// since EOF is reached).
    ///
    /// # Errors
    ///
    /// This function will return an I/O error if a [`Read`] method was called, but returned an error.
    ///
    /// Examples
    ///
    /// ```
    /// #![feature(buf_read_has_data_left)]
    /// use std::io;
    /// use std::io::prelude::*;
    ///
    /// let stdin = io::stdin();
    /// let mut stdin = stdin.lock();
    ///
    /// while stdin.has_data_left()? {
    ///     let mut line = String::new();
    ///     stdin.read_line(&mut line)?;
    ///     // work with line
    ///     println!("{line:?}");
    /// }
    /// # std::io::Result::Ok(())
    /// ```
    #[unstable(feature = "buf_read_has_data_left", issue = "86423")]
    fn has_data_left(&mut self) -> Result<bool> {
        self.fill_buf().map(|b| !b.is_empty())
    }

    /// Reads all bytes into `buf` until the delimiter `byte` or EOF is reached.
    ///
    /// This function will read bytes from the underlying stream until the
    /// delimiter or EOF is found. Once found, all bytes up to, and including,
    /// the delimiter (if found) will be appended to `buf`.
    ///
    /// If successful, this function will return the total number of bytes read.
    ///
    /// This function is blocking and should be used carefully: it is possible for
    /// an attacker to continuously send bytes without ever sending the delimiter
    /// or EOF.
    ///
    /// # Errors
    ///
    /// This function will ignore all instances of [`ErrorKind::Interrupted`] and
    /// will otherwise return any errors returned by [`fill_buf`].
    ///
    /// If an I/O error is encountered then all bytes read so far will be
    /// present in `buf` and its length will have been adjusted appropriately.
    ///
    /// [`fill_buf`]: BufRead::fill_buf
    ///
    /// # Examples
    ///
    /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
    /// this example, we use [`Cursor`] to read all the bytes in a byte slice
    /// in hyphen delimited segments:
    ///
    /// [`Cursor`]: crate::io::Cursor
    ///
    /// ```
    /// use std::io::{self, BufRead};
    ///
    /// let mut cursor = io::Cursor::new(b"lorem-ipsum");
    /// let mut buf = vec![];
    ///
    /// // cursor is at 'l'
    /// let num_bytes = cursor.read_until(b'-', &mut buf)
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 6);
    /// assert_eq!(buf, b"lorem-");
    /// buf.clear();
    ///
    /// // cursor is at 'i'
    /// let num_bytes = cursor.read_until(b'-', &mut buf)
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 5);
    /// assert_eq!(buf, b"ipsum");
    /// buf.clear();
    ///
    /// // cursor is at EOF
    /// let num_bytes = cursor.read_until(b'-', &mut buf)
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 0);
    /// assert_eq!(buf, b"");
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize> {
        default_read_until(self, byte, buf)
    }

    /// Skips all bytes until the delimiter `byte` or EOF is reached.
    ///
    /// This function will read (and discard) bytes from the underlying stream until the
    /// delimiter or EOF is found.
    ///
    /// If successful, this function will return the total number of bytes read,
    /// including the delimiter byte if found.
    ///
    /// This is useful for efficiently skipping data such as NUL-terminated strings
    /// in binary file formats without buffering.
    ///
    /// This function is blocking and should be used carefully: it is possible for
    /// an attacker to continuously send bytes without ever sending the delimiter
    /// or EOF.
    ///
    /// # Errors
    ///
    /// This function will ignore all instances of [`ErrorKind::Interrupted`] and
    /// will otherwise return any errors returned by [`fill_buf`].
    ///
    /// If an I/O error is encountered then all bytes read so far will be
    /// present in `buf` and its length will have been adjusted appropriately.
    ///
    /// [`fill_buf`]: BufRead::fill_buf
    ///
    /// # Examples
    ///
    /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
    /// this example, we use [`Cursor`] to read some NUL-terminated information
    /// about Ferris from a binary string, skipping the fun fact:
    ///
    /// [`Cursor`]: crate::io::Cursor
    ///
    /// ```
    /// use std::io::{self, BufRead};
    ///
    /// let mut cursor = io::Cursor::new(b"Ferris\0Likes long walks on the beach\0Crustacean\0!");
    ///
    /// // read name
    /// let mut name = Vec::new();
    /// let num_bytes = cursor.read_until(b'\0', &mut name)
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 7);
    /// assert_eq!(name, b"Ferris\0");
    ///
    /// // skip fun fact
    /// let num_bytes = cursor.skip_until(b'\0')
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 30);
    ///
    /// // read animal type
    /// let mut animal = Vec::new();
    /// let num_bytes = cursor.read_until(b'\0', &mut animal)
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 11);
    /// assert_eq!(animal, b"Crustacean\0");
    ///
    /// // reach EOF
    /// let num_bytes = cursor.skip_until(b'\0')
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 1);
    /// ```
    #[stable(feature = "bufread_skip_until", since = "1.83.0")]
    fn skip_until(&mut self, byte: u8) -> Result<usize> {
        default_skip_until(self, byte)
    }

    /// Reads all bytes until a newline (the `0xA` byte) is reached, and append
    /// them to the provided [`String`] buffer.
    ///
    /// Previous content of the buffer will be preserved. To avoid appending to
    /// the buffer, you need to [`clear`] it first.
    ///
    /// This function will read bytes from the underlying stream until the
    /// newline delimiter (the `0xA` byte) or EOF is found. Once found, all bytes
    /// up to, and including, the delimiter (if found) will be appended to
    /// `buf`.
    ///
    /// If successful, this function will return the total number of bytes read.
    ///
    /// If this function returns [`Ok(0)`], the stream has reached EOF.
    ///
    /// This function is blocking and should be used carefully: it is possible for
    /// an attacker to continuously send bytes without ever sending a newline
    /// or EOF. You can use [`take`] to limit the maximum number of bytes read.
    ///
    /// [`Ok(0)`]: Ok
    /// [`clear`]: String::clear
    /// [`take`]: crate::io::Read::take
    ///
    /// # Errors
    ///
    /// This function has the same error semantics as [`read_until`] and will
    /// also return an error if the read bytes are not valid UTF-8. If an I/O
    /// error is encountered then `buf` may contain some bytes already read in
    /// the event that all data read so far was valid UTF-8.
    ///
    /// [`read_until`]: BufRead::read_until
    ///
    /// # Examples
    ///
    /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
    /// this example, we use [`Cursor`] to read all the lines in a byte slice:
    ///
    /// [`Cursor`]: crate::io::Cursor
    ///
    /// ```
    /// use std::io::{self, BufRead};
    ///
    /// let mut cursor = io::Cursor::new(b"foo\nbar");
    /// let mut buf = String::new();
    ///
    /// // cursor is at 'f'
    /// let num_bytes = cursor.read_line(&mut buf)
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 4);
    /// assert_eq!(buf, "foo\n");
    /// buf.clear();
    ///
    /// // cursor is at 'b'
    /// let num_bytes = cursor.read_line(&mut buf)
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 3);
    /// assert_eq!(buf, "bar");
    /// buf.clear();
    ///
    /// // cursor is at EOF
    /// let num_bytes = cursor.read_line(&mut buf)
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 0);
    /// assert_eq!(buf, "");
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn read_line(&mut self, buf: &mut String) -> Result<usize> {
        // Note that we are not calling the `.read_until` method here, but
        // rather our hardcoded implementation. For more details as to why, see
        // the comments in `default_read_to_string`.
        // ignore-tidy-undocumented-unsafe
        unsafe { append_to_string(buf, |b| default_read_until(self, b'\n', b)) }
    }

    /// Returns an iterator over the contents of this reader split on the byte
    /// `byte`.
    ///
    /// The iterator returned from this function will return instances of
    /// <code>[io::Result]<[Vec]\<u8>></code>. Each vector returned will *not* have
    /// the delimiter byte at the end.
    ///
    /// This function will yield errors whenever [`read_until`] would have
    /// also yielded an error.
    ///
    /// [io::Result]: self::Result "io::Result"
    /// [`read_until`]: BufRead::read_until
    ///
    /// # Examples
    ///
    /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
    /// this example, we use [`Cursor`] to iterate over all hyphen delimited
    /// segments in a byte slice
    ///
    /// [`Cursor`]: crate::io::Cursor
    ///
    /// ```
    /// use std::io::{self, BufRead};
    ///
    /// let cursor = io::Cursor::new(b"lorem-ipsum-dolor");
    ///
    /// let mut split_iter = cursor.split(b'-').map(|l| l.unwrap());
    /// assert_eq!(split_iter.next(), Some(b"lorem".to_vec()));
    /// assert_eq!(split_iter.next(), Some(b"ipsum".to_vec()));
    /// assert_eq!(split_iter.next(), Some(b"dolor".to_vec()));
    /// assert_eq!(split_iter.next(), None);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn split(self, byte: u8) -> Split<Self>
    where
        Self: Sized,
    {
        split(self, byte)
    }

    /// Returns an iterator over the lines of this reader.
    ///
    /// The iterator returned from this function will yield instances of
    /// <code>[io::Result]<[String]></code>. Each string returned will *not* have a newline
    /// byte (the `0xA` byte) or `CRLF` (`0xD`, `0xA` bytes) at the end.
    ///
    /// [io::Result]: crate::io::Result "io::Result"
    ///
    /// # Examples
    ///
    /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
    /// this example, we use [`Cursor`] to iterate over all the lines in a byte
    /// slice.
    ///
    /// [`Cursor`]: crate::io::Cursor
    ///
    /// ```
    /// use std::io::{self, BufRead};
    ///
    /// let cursor = io::Cursor::new(b"lorem\nipsum\r\ndolor");
    ///
    /// let mut lines_iter = cursor.lines().map(|l| l.unwrap());
    /// assert_eq!(lines_iter.next(), Some(String::from("lorem")));
    /// assert_eq!(lines_iter.next(), Some(String::from("ipsum")));
    /// assert_eq!(lines_iter.next(), Some(String::from("dolor")));
    /// assert_eq!(lines_iter.next(), None);
    /// ```
    ///
    /// # Errors
    ///
    /// Each line of the iterator has the same error semantics as [`BufRead::read_line`].
    #[stable(feature = "rust1", since = "1.0.0")]
    fn lines(self) -> Lines<Self>
    where
        Self: Sized,
    {
        lines(self)
    }
}

fn default_read_until<R: BufRead + ?Sized>(
    r: &mut R,
    delim: u8,
    buf: &mut Vec<u8>,
) -> Result<usize> {
    let mut read = 0;
    loop {
        let (done, used) = {
            let available = match r.fill_buf() {
                Ok(n) => n,
                Err(ref e) if e.is_interrupted() => continue,
                Err(e) => return Err(e),
            };
            let (done, available) = match memchr::memchr(delim, available) {
                Some(i) => (true, &available[..=i]),
                None => (false, available),
            };

            cfg_select! {
                no_global_oom_handling => {
                    let count = available.len();
                    buf.try_reserve(count)?;

                    // SAFETY:
                    // * self and buf are non-overlapping
                    // * buf[..len] is already initialized
                    // * buf[len..len + count] is initialized by copy_nonoverlapping
                    // * len + count is within the capacity of buf based on the reservation completed above
                    unsafe {
                        let len = buf.len();
                        let src = available.as_ptr();
                        let dst = buf.as_mut_ptr().add(len);
                        core::ptr::copy_nonoverlapping(src, dst, count);
                        buf.set_len(len + count);
                    }
                }
                _ => {
                    buf.extend_from_slice(available);
                }
            }

            (done, available.len())
        };
        r.consume(used);
        read += used;
        if done || used == 0 {
            return Ok(read);
        }
    }
}

fn default_skip_until<R: BufRead + ?Sized>(r: &mut R, delim: u8) -> Result<usize> {
    let mut read = 0;
    loop {
        let (done, used) = {
            let available = match r.fill_buf() {
                Ok(n) => n,
                Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
                Err(e) => return Err(e),
            };
            match memchr::memchr(delim, available) {
                Some(i) => (true, i + 1),
                None => (false, available.len()),
            }
        };
        r.consume(used);
        read += used;
        if done || used == 0 {
            return Ok(read);
        }
    }
}