tsort: returns error when input is dir - same as GNU tsort (#5860)

* fix: return error when input is dir

* test: when tsort is given a dir

* fix: do not need to mention tsort in error message

* test: using concrete directory name

* tsort: fix formatting in test

---------

Co-authored-by: Daniel Hofstetter <daniel.hofstetter@42dh.com>

src/uu/tsort/src/tsort.rs

@@ -32,7 +32,14 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
         stdin_buf = stdin();
         &mut stdin_buf as &mut dyn Read
     } else {
-        file_buf = File::open(Path::new(&input)).map_err_context(|| input.to_string())?;
+        let path = Path::new(&input);
+        if path.is_dir() {
+            return Err(USimpleError::new(
+                1,
+                format!("{}: read error: Is a directory", input),
+            ));
+        }
+        file_buf = File::open(path).map_err_context(|| input.to_string())?;
         &mut file_buf as &mut dyn Read
     });
 

tests/by-util/test_tsort.rs

@@ -64,3 +64,12 @@ fn test_multiple_arguments() {
         .fails()
         .stderr_contains("unexpected argument 'invalid_file' found");
 }
+
+#[test]
+fn test_error_on_dir() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    at.mkdir("tsort_test_dir");
+    ucmd.arg("tsort_test_dir")
+        .fails()
+        .stderr_contains("tsort: tsort_test_dir: read error: Is a directory");
+}