comm: generate an error if the input is a directory

tested by tests/misc/read-errors

src/uu/comm/src/comm.rs

@@ -6,7 +6,7 @@
 // spell-checker:ignore (ToDO) delim mkdelim
 
 use std::cmp::Ordering;
-use std::fs::File;
+use std::fs::{metadata, File};
 use std::io::{self, stdin, BufRead, BufReader, Stdin};
 use std::path::Path;
 use uucore::error::{FromIo, UResult, USimpleError};
@@ -130,7 +130,11 @@ fn open_file(name: &str, line_ending: LineEnding) -> io::Result<LineReader> {
     if name == "-" {
         Ok(LineReader::new(Input::Stdin(stdin()), line_ending))
     } else {
-        let f = File::open(Path::new(name))?;
+        let path = Path::new(name);
+        if metadata(path)?.is_dir() {
+            return Err(io::Error::new(io::ErrorKind::Other, "Is a directory"));
+        }
+        let f = File::open(path)?;
         Ok(LineReader::new(
             Input::FileIn(BufReader::new(f)),
             line_ending,

tests/by-util/test_comm.rs

@@ -292,3 +292,36 @@ fn test_no_such_file() {
         .fails()
         .stderr_only("comm: bogus_file_1: No such file or directory\n");
 }
+
+#[test]
+fn test_is_dir() {
+    let scene = TestScenario::new(util_name!());
+    let at = &scene.fixtures;
+
+    scene
+        .ucmd()
+        .args(&[".", "."])
+        .fails()
+        .stderr_only("comm: .: Is a directory\n");
+    at.mkdir("dir");
+
+    scene
+        .ucmd()
+        .args(&["dir", "."])
+        .fails()
+        .stderr_only("comm: dir: Is a directory\n");
+
+    at.touch("file");
+    scene
+        .ucmd()
+        .args(&[".", "file"])
+        .fails()
+        .stderr_only("comm: .: Is a directory\n");
+
+    at.touch("file");
+    scene
+        .ucmd()
+        .args(&["file", "."])
+        .fails()
+        .stderr_only("comm: .: Is a directory\n");
+}