head: fix -c0 and -n0 on directories to match GNU behavior

When zero bytes or zero lines are requested, there is nothing to read,
so we should not check whether the path is a directory. GNU head
succeeds in this case because it never attempts to open/read the file.

Previously, uutils head would stat the file and reject directories
even when no reading was needed, causing a spurious 'Is a directory'
error.

Fixes #12215

Author: sylvestre

src/uu/head/src/head.rs

@@ -447,7 +447,11 @@ fn uu_head(options: &HeadOptions) -> UResult<()> {
 
             Ok(())
         } else {
-            if Path::new(file).is_dir() {
+            // Only check if the path is a directory when we would actually
+            // read from it. When 0 bytes or 0 lines are requested, there is
+            // nothing to read, so we should succeed just like GNU head does.
+            let zero_output = matches!(options.mode, Mode::FirstBytes(0) | Mode::FirstLines(0));
+            if !zero_output && Path::new(file).is_dir() {
                 show!(USimpleError::new(
                     1,
                     translate!("head-error-reading-file", "name" => file.quote(), "err" => "Is a directory")

tests/by-util/test_head.rs

@@ -933,3 +933,24 @@ fn test_do_not_attempt_to_read_a_directory() {
         .fails_with_code(1)
         .stderr_contains("error reading '.'");
 }
+
+/// Regression test for https://github.com/uutils/coreutils/issues/12215
+/// `head -c0 <directory>` should succeed (nothing to read), matching GNU.
+#[test]
+fn test_zero_bytes_on_directory_succeeds() {
+    new_ucmd!()
+        .args(&["-c", "0", "."])
+        .succeeds()
+        .no_stdout()
+        .no_stderr();
+}
+
+/// `head -n0 <directory>` should also succeed.
+#[test]
+fn test_zero_lines_on_directory_succeeds() {
+    new_ucmd!()
+        .args(&["-n", "0", "."])
+        .succeeds()
+        .no_stdout()
+        .no_stderr();
+}