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.

Author: Zalathar

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")]
 pubfnpeek(&mutself,n:usize) -> io::Result<&[u8]>{
@@ -125,7 +129,11 @@ impl<R: Read + ?Sized> BufReader<R> {
 ifself.buf.pos() > 0{
 self.buf.backshift();
 }
-self.buf.read_more(&mutself.inner)?;
+let new = self.buf.read_more(&mutself.inner)?;
+if new == 0{
+// end of file, no more bytes to read
+returnOk(&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
-pubfnread_more(&mutself,mutreader:implRead) -> io::Result<()>{
+pubfnread_more(&mutself,mutreader:implRead) -> io::Result<usize>{
 letmut buf = BorrowedBuf::from(&mutself.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.