comm: check if the input file is a directory after opening it

This prevents a potential race condition where the path could be swapped
with a directory in between the `is_dir()` check and the `open()` call.

Author: andreacorbellini

src/uu/comm/src/comm.rs

@@ -285,10 +285,10 @@ fn open_file(name: &OsString, line_ending: LineEnding) -> io::Result<LineReader>
     if name == "-" {
         Ok(LineReader::new(Input::Stdin(stdin()), line_ending))
     } else {
-        if metadata(name)?.is_dir() {
+        let f = File::open(name)?;
+        if f.metadata()?.is_dir() {
             return Err(io::Error::other(translate!("comm-error-is-directory")));
         }
-        let f = File::open(name)?;
         Ok(LineReader::new(
             Input::FileIn(BufReader::new(f)),
             line_ending,