Rollup merge of #130042 - lolbinarycat:bufreaker_peek_eof, r=Amanieu

properly handle EOF in BufReader::peek

previously this would cause an infinite loop due to it being unable to read `n` bytes.

library/std/src/io/buffered/bufreader.rs

@@ -99,7 +99,10 @@ impl<R: Read> BufReader<R> {
 impl<R: Read + ?Sized> BufReader<R> {
  /// Attempt to look ahead `n` bytes.
  ///
- /// `n` must be less than `capacity`.
+ /// `n` must be less than or equal to `capacity`.
+ ///
+ /// the returned slice may be less than `n` bytes long if
+ /// end of file is reached.
  ///
  /// ## Examples
  ///
@@ -117,6 +120,7 @@ impl<R: Read + ?Sized> BufReader<R> {
  /// let mut s = String::new();
  /// rdr.read_to_string(&mut s).unwrap();
  /// assert_eq!(&s, "hello");
+ /// assert_eq!(rdr.peek(1).unwrap().len(), 0);
  /// ```
  #[unstable(feature = "bufreader_peek", issue = "128405")]
  pub fn peek(&mut self, n: usize) -> io::Result<&[u8]> {
@@ -125,7 +129,11 @@ impl<R: Read + ?Sized> BufReader<R> {
  if self.buf.pos() > 0 {
  self.buf.backshift();
  }
- self.buf.read_more(&mut self.inner)?;
+ let new = self.buf.read_more(&mut self.inner)?;
+ if new == 0 {
+ // end of file, no more bytes to read
+ return Ok(&self.buf.buffer()[..]);
+ }
  debug_assert_eq!(self.buf.pos(), 0);
  }
  Ok(&self.buf.buffer()[..n])

library/std/src/io/buffered/bufreader/buffer.rs

@@ -98,7 +98,7 @@ impl Buffer {
  }
 
  /// Read more bytes into the buffer without discarding any of its contents
- pub fn read_more(&mut self, mut reader: impl Read) -> io::Result<()> {
+ pub fn read_more(&mut self, mut reader: impl Read) -> io::Result<usize> {
  let mut buf = BorrowedBuf::from(&mut self.buf[self.pos..]);
  let old_init = self.initialized - self.pos;
  unsafe {
@@ -107,7 +107,7 @@ impl Buffer {
  reader.read_buf(buf.unfilled())?;
  self.filled += buf.len();
  self.initialized += buf.init_len() - old_init;
- Ok(())
+ Ok(buf.len())
  }
 
  /// Remove bytes that have already been read from the buffer.