From e1a5e47f197d72758eafc62b74a8478203166a8d Mon Sep 17 00:00:00 2001
From: Christopher Dryden <christopher.paul.dryden@gmail.com>
Date: Sat, 7 Feb 2026 18:30:21 +0000
Subject: [PATCH] tac: skip mmap for non-regular files to match GNU 9.9
 behavior

---
 src/uu/tac/src/tac.rs     | 15 +++++++--------
 tests/by-util/test_tac.rs | 40 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+), 8 deletions(-)

diff --git a/src/uu/tac/src/tac.rs b/src/uu/tac/src/tac.rs
index ec8ae450344..92277768645 100644
--- a/src/uu/tac/src/tac.rs
+++ b/src/uu/tac/src/tac.rs
@@ -362,20 +362,19 @@ fn tac(filenames: &[OsString], before: bool, regex: bool, separator: &str) -> UR
             }
         } else {
             let path = Path::new(filename);
-            if path.is_dir() {
-                let e: Box<dyn UError> =
-                    TacError::InvalidDirectoryArgument(filename.clone()).into();
-                show!(e);
+            let Ok(metadata) = path.metadata() else {
+                show!(TacError::FileNotFound(filename.clone()));
                 continue;
-            }
+            };
 
-            if path.metadata().is_err() {
-                let e: Box<dyn UError> = TacError::FileNotFound(filename.clone()).into();
+            if metadata.is_dir() {
+                let e: Box<dyn UError> =
+                    TacError::InvalidDirectoryArgument(filename.clone()).into();
                 show!(e);
                 continue;
             }
 
-            if let Some(mmap1) = try_mmap_path(path) {
+            if let Some(mmap1) = metadata.is_file().then(|| try_mmap_path(path)).flatten() {
                 mmap = mmap1;
                 &mmap
             } else {
diff --git a/tests/by-util/test_tac.rs b/tests/by-util/test_tac.rs
index 1fc42d1c83f..6ec45d91d91 100644
--- a/tests/by-util/test_tac.rs
+++ b/tests/by-util/test_tac.rs
@@ -416,3 +416,43 @@ fn test_regular_end_anchor() {
         .succeeds()
         .stdout_is("\nccc\nbbaaa\nb");
 }
+
+#[test]
+#[cfg(target_os = "linux")]
+fn test_non_regular_files() {
+    use std::process::Stdio;
+
+    new_ucmd!().arg("/dev/null").succeeds().no_output();
+
+    let content = std::fs::read_to_string("/proc/version").unwrap();
+    new_ucmd!()
+        .arg("/proc/version")
+        .succeeds()
+        .stdout_is(&content);
+
+    let scene = TestScenario::new(util_name!());
+    let at = &scene.fixtures;
+    at.mkfifo("fifo");
+    let fifo_path = at.plus("fifo");
+    let writer = std::thread::spawn(move || std::fs::write(fifo_path, b"a\nb\nc\n").unwrap());
+    scene.ucmd().arg("fifo").succeeds().stdout_is("c\nb\na\n");
+    writer.join().unwrap();
+
+    for dev in &["/dev/zero", "/dev/urandom"] {
+        let mut child = new_ucmd!()
+            .arg(dev)
+            .set_stdout(Stdio::piped())
+            .run_no_wait();
+        child.make_assertion_with_delay(500).is_alive();
+        child.kill();
+    }
+
+    at.symlink_file("/dev/urandom", "sym_urandom");
+    let mut child = scene
+        .ucmd()
+        .arg("sym_urandom")
+        .set_stdout(Stdio::piped())
+        .run_no_wait();
+    child.make_assertion_with_delay(500).is_alive();
+    child.kill();
+}