From 72d4b7ede4daacef406bb2f187ac314d2e70e37f Mon Sep 17 00:00:00 2001
From: Navaneeth Yadamreddy <navaneeth@Navaneeths-MacBook-Air.local>
Date: Thu, 27 Aug 2026 17:55:42 +0530
Subject: [PATCH] =?UTF-8?q?mkdir:=20fix=20EEXIST=20race=20condition=20in?=
 =?UTF-8?q?=20non-recursive=20mode=20When=20mkdir(2)=20returns=20EEXIST,?=
 =?UTF-8?q?=20the=20previous=20code=20checked=20path.is=5Fdir()=20and=20re?=
 =?UTF-8?q?turned=20Ok(())=20if=20true=20=E2=80=94=20even=20for=20plain=20?=
 =?UTF-8?q?`mkdir=20dir`=20(non-recursive).=20This=20meant=20two=20concurr?=
 =?UTF-8?q?ent=20processes=20racing=20to=20create=20the=20same=20directory?=
 =?UTF-8?q?=20could=20both=20exit=200,=20breaking=20the=20classic=20mkdir-?=
 =?UTF-8?q?as-mutex=20pattern.=20Fix=20by=20only=20treating=20EEXIST=20+?=
 =?UTF-8?q?=20is=5Fdir()=20as=20success=20when=20creating=20parent=20direc?=
 =?UTF-8?q?tories=20(is=5Fparent)=20or=20when=20-p=20was=20given=20(recurs?=
 =?UTF-8?q?ive).=20In=20the=20plain=20`mkdir=20dir`=20case,=20EEXIST=20now?=
 =?UTF-8?q?=20always=20returns=20an=20error,=20matching=20GNU=20behavior.?=
 =?UTF-8?q?=20Fixes=20#13970?=
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---
 src/uu/mkdir/src/mkdir.rs   | 14 +++++++--
 tests/by-util/test_mkdir.rs | 62 +++++++++++++++++++++++++++++++++++++
 2 files changed, 74 insertions(+), 2 deletions(-)

diff --git a/src/uu/mkdir/src/mkdir.rs b/src/uu/mkdir/src/mkdir.rs
index 1d34d62bb7e..36be5a7dbfe 100644
--- a/src/uu/mkdir/src/mkdir.rs
+++ b/src/uu/mkdir/src/mkdir.rs
@@ -354,8 +354,18 @@ fn create_single_dir(path: &Path, is_parent: bool, config: &Config) -> UResult<(
         }
 
         Err(_) if path.is_dir() => {
-            // Directory already exists - check if this is a logical directory creation
-            // (i.e., not just a parent reference like "test_dir/..")
+            // Directory already exists. Only treat this as success when we
+            // are creating parent directories (is_parent) or when -p was
+            // given (recursive). In the plain `mkdir dir` case, EEXIST must
+            // be an error — even if the directory was created by a concurrent
+            // process — to preserve the mkdir-as-mutex pattern.
+            if !is_parent && !config.recursive {
+                return Err(USimpleError::new(
+                    1,
+                    translate!("mkdir-error-file-exists", "path" => path.maybe_quote()),
+                ));
+            }
+
             let ends_with_parent_dir = matches!(
                 path.components().next_back(),
                 Some(std::path::Component::ParentDir)
diff --git a/tests/by-util/test_mkdir.rs b/tests/by-util/test_mkdir.rs
index e335a3f0566..82db044daa2 100644
--- a/tests/by-util/test_mkdir.rs
+++ b/tests/by-util/test_mkdir.rs
@@ -1134,3 +1134,65 @@ mod diagnostics {
         assert!(!stderr.contains(":1:"), "{stderr}");
     }
 }
+
+/// Regression test: plain `mkdir` on an existing directory must fail with
+/// exit 1, not 0.  Previously the EEXIST path checked `path.is_dir()` and
+/// returned Ok(()), which broke the mkdir-as-mutex pattern under
+/// concurrency (issue #13970).
+#[test]
+fn test_mkdir_eexist_returns_error() {
+    let scene = TestScenario::new(util_name!());
+    let at = &scene.fixtures;
+    let dir = at.plus("existing_dir");
+
+    let dir_str = dir.to_str().unwrap();
+
+    // Create the directory first
+    new_ucmd!().args(&[dir_str]).succeeds();
+
+    // Trying again without -p must fail
+    new_ucmd!()
+        .args(&[dir_str])
+        .fails_with_code(1)
+        .stderr_contains("File exists");
+}
+
+/// Regression test: concurrent `mkdir` (no -p) on the same path must
+/// produce exactly one winner (issue #13970).
+#[test]
+fn test_mkdir_concurrent_eexist() {
+    use std::thread;
+
+    let scene = TestScenario::new(util_name!());
+    let at = &scene.fixtures;
+    let dir = at.plus("race_dir");
+    let path_str = dir.to_string_lossy().to_string();
+    let bin_path = scene.bin_path.clone();
+
+    for _ in 0..10 {
+        // Remove directory before each round
+        let _ = std::fs::remove_dir(&path_str);
+
+        let mut handles = vec![];
+        for _ in 0..20 {
+            let path = path_str.clone();
+            let bin = bin_path.clone();
+            handles.push(thread::spawn(move || {
+                std::process::Command::new(&bin)
+                    .arg("mkdir")
+                    .arg(&path)
+                    .stdout(std::process::Stdio::null())
+                    .stderr(std::process::Stdio::null())
+                    .status()
+                    .map(|s| s.success())
+                    .unwrap_or(false)
+            }));
+        }
+
+        let winners: usize = handles
+            .into_iter()
+            .map(|h| h.join().unwrap() as usize)
+            .sum();
+        assert_eq!(winners, 1, "Expected exactly 1 winner, got {winners}");
+    }
+}