mkdir: do not ignore EEXIST on non-recursive creation

Fixes uutils#13970

Signed-off-by: Alex Chen <l46983284@gmail.com>

Author: alexchen-sys

src/uu/mkdir/src/mkdir.rs

@@ -353,7 +353,7 @@ fn create_single_dir(path: &Path, is_parent: bool, config: &Config) -> UResult<(
             Ok(())
         }
 
-        Err(_) if path.is_dir() => {
+        Err(_) if config.recursive && path.is_dir() => {
             // Directory already exists - check if this is a logical directory creation
             // (i.e., not just a parent reference like "test_dir/..")
             let ends_with_parent_dir = matches!(

tests/by-util/test_mkdir.rs

@@ -1134,3 +1134,55 @@ mod diagnostics {
         assert!(!stderr.contains(":1:"), "{stderr}");
     }
 }
+
+#[test]
+fn test_mkdir_concurrent_non_recursive() {
+    // Test concurrent mkdir operations without -p: exactly one process must succeed per round
+    use std::sync::Arc;
+    use std::sync::atomic::{AtomicUsize, Ordering};
+    use std::thread;
+
+    for round in 0..10 {
+        let scene = TestScenario::new(util_name!());
+        let target_dir = scene.fixtures.plus(format!("concurrent_target_{round}"));
+        let path_str = target_dir.to_string_lossy().to_string();
+        let bin_path = scene.bin_path.clone();
+
+        let winners = Arc::new(AtomicUsize::new(0));
+        let mut handles = vec![];
+
+        for _ in 0..16 {
+            let path_clone = path_str.clone();
+            let bin_path_clone = bin_path.clone();
+            let winners_clone = Arc::clone(&winners);
+
+            let handle = thread::spawn(move || {
+                let result = std::process::Command::new(&bin_path_clone)
+                    .arg("mkdir")
+                    .arg(&path_clone)
+                    .current_dir(std::env::current_dir().unwrap())
+                    .output()
+                    .expect("failed to run binary");
+                if result.status.success() {
+                    winners_clone.fetch_add(1, Ordering::SeqCst);
+                }
+            });
+            handles.push(handle);
+        }
+
+        for h in handles {
+            h.join().unwrap();
+        }
+
+        assert_eq!(
+            winners.load(Ordering::SeqCst),
+            1,
+            "round {round}: expected exactly 1 winner for concurrent non-recursive mkdir"
+        );
+        assert!(
+            scene
+                .fixtures
+                .dir_exists(format!("concurrent_target_{round}"))
+        );
+    }
+}