ln: add error handling for hard link creation on directories

src/uu/ln/locales/en-US.ftl

@@ -35,4 +35,5 @@ ln-prompt-replace = replace {$file}?
 ln-cannot-backup = cannot backup {$file}
 ln-failed-to-access = failed to access {$file}
 ln-failed-to-create-hard-link = failed to create hard link {$source} => {$dest}
+ln-failed-to-create-hard-link-dir = {$source}: hard link not allowed for directory
 ln-backup = backup: {$backup}

src/uu/ln/locales/fr-FR.ftl

@@ -36,4 +36,5 @@ ln-prompt-replace = remplacer {$file} ?
 ln-cannot-backup = impossible de sauvegarder {$file}
 ln-failed-to-access = échec d'accès à {$file}
 ln-failed-to-create-hard-link = échec de création du lien physique {$source} => {$dest}
+ln-failed-to-create-hard-link-dir = {$source} : lien physique non autorisé pour un répertoire
 ln-backup = sauvegarde : {$backup}

src/uu/ln/src/ln.rs

@@ -62,6 +62,9 @@ enum LnError {
 
     #[error("{}", translate!("ln-error-extra-operand", "operand" => _0.to_string_lossy(), "program" => _1.clone()))]
     ExtraOperand(OsString, String),
+
+    #[error("{}", translate!("ln-failed-to-create-hard-link-dir", "source" => _0.to_string_lossy()))]
+    FailedToCreateHardLinkDir(PathBuf),
 }
 
 impl UError for LnError {
@@ -431,6 +434,11 @@ fn link(src: &Path, dst: &Path, settings: &Settings) -> UResult<()> {
     if settings.symbolic {
         symlink(&source, dst)?;
     } else {
+        // Cannot create hard link to a directory
+        if src.is_dir() {
+            return Err(LnError::FailedToCreateHardLinkDir(source.to_path_buf()).into());
+        }
+
         let p = if settings.logical && source.is_symlink() {
             // if we want to have an hard link,
             // source is a symlink and -L is passed

tests/by-util/test_ln.rs

@@ -934,3 +934,19 @@ fn test_ln_non_utf8_paths() {
     let symlink_path = at.plus(symlink_name);
     assert!(symlink_path.is_symlink());
 }
+
+#[test]
+fn test_ln_hard_link_dir() {
+    let scene = TestScenario::new(util_name!());
+    let at = &scene.fixtures;
+
+    at.mkdir("dir");
+
+    let result = scene.ucmd().args(&["dir", "dir_link"]).fails();
+
+    assert!(
+        result
+            .stderr_str()
+            .contains("hard link not allowed for directory")
+    );
+}