tail: fix behaviour of tail -n0 in follow mode (#9114)

* tail: fix behaviour of -n0 in follow mode

* tests: added test for checking -n0 in follow mode

---------

Co-authored-by: Sylvestre Ledru <sylvestre@debian.org>

src/uu/tail/src/tail.rs

@@ -470,7 +470,7 @@ fn bounded_tail(file: &mut File, settings: &Settings) {
             file.seek(SeekFrom::Start(i as u64)).unwrap();
         }
         FilterMode::Lines(Signum::MinusZero, _) => {
-            return;
+            file.seek(SeekFrom::End(0)).unwrap();
         }
         FilterMode::Bytes(Signum::Negative(count)) => {
             if file.seek(SeekFrom::End(-(*count as i64))).is_err() {
@@ -484,7 +484,7 @@ fn bounded_tail(file: &mut File, settings: &Settings) {
             file.seek(SeekFrom::Start(*count - 1)).unwrap();
         }
         FilterMode::Bytes(Signum::MinusZero) => {
-            return;
+            file.seek(SeekFrom::End(0)).unwrap();
         }
         _ => {}
     }
@@ -524,6 +524,11 @@ fn unbounded_tail<T: Read>(reader: &mut BufReader<T>, settings: &Settings) -> UR
             chunks.fill(reader)?;
             chunks.print(&mut writer)?;
         }
+        FilterMode::Lines(Signum::MinusZero, sep) => {
+            let mut chunks = chunks::LinesChunkBuffer::new(*sep, 0);
+            chunks.fill(reader)?;
+            chunks.print(&mut writer)?;
+        }
         FilterMode::Bytes(Signum::PlusZero | Signum::Positive(1)) => {
             io::copy(reader, &mut writer)?;
         }

tests/by-util/test_tail.rs

@@ -233,6 +233,28 @@ fn test_nc_0_wo_follow2() {
         .no_output();
 }
 
+#[test]
+#[cfg(not(target_os = "windows"))]
+fn test_n0_with_follow() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    let test_file = "test.txt";
+    // Create file with multiple lines
+    at.write(test_file, "line1\nline2\nline3\n");
+
+    let mut child = ucmd.arg("-n0").arg("-f").arg(test_file).run_no_wait();
+    child.make_assertion_with_delay(500).is_alive();
+
+    // Append a new line
+    at.append(test_file, "new\n");
+
+    // Should only print the newly appended line
+    child
+        .make_assertion_with_delay(DEFAULT_SLEEP_INTERVAL_MILLIS)
+        .with_current_output()
+        .stdout_only("new\n");
+    child.kill();
+}
+
 // TODO: Add similar test for windows
 #[test]
 #[cfg(unix)]