Head: find_nth_line_from_end function is buggy

[karlmcdowall]

find_nth_line_from_end will generate the wrong offset if it has to search backwards more than 64K into the file (i.e. any searches bigger than the BUF_SIZE used in the code). Specifically, the problem is here...

fn find_nth_line_from_end<R>(input: &mut R, n: u64, separator: u8) -> std::io::Result<u64>
where
    R: Read + Seek,
{
    let size = input.seek(SeekFrom::End(0))?;

    let mut buffer = [0u8; BUF_SIZE];
    let buf_size: usize = (BUF_SIZE as u64).min(size).try_into().unwrap();
    let buffer = &mut buffer[..buf_size];

    let mut i = 0u64;
    let mut lines = 0u64;

    loop {
        // the casts here are ok, `buffer.len()` should never be above a few k
  **// The bug is here as it neglects that the input index is adjusted again at (*) below...**
        input.seek(SeekFrom::Current(
            -((buffer.len() as i64).min((size - i) as i64)),    
        ))?;
        input.read_exact(buffer)?;    // (*)
        for byte in buffer.iter().rev() {
            if byte == &separator {
                lines += 1;
            }
            // if it were just `n`,
            if lines == n + 1 {
                input.rewind()?;
                return Ok(size - i);
            }
            i += 1;
        }
        if size - i == 0 {
            input.rewind()?;
            return Ok(0);
        }
    }
}

I've got a fix on a dev branch, will put it up for PR shortly.

[karlmcdowall]

Fixed with change here