tail: fix -F to properly track symlinks with changing targets (#10158)

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

src/uu/tail/src/follow/watch.rs

@@ -277,6 +277,10 @@ impl Observer {
                             // If `path` is not a tailable file, add its parent to `Watcher`.
                             watcher_rx
                                 .watch(path.parent().unwrap(), RecursiveMode::NonRecursive)?;
+                            // Add symlinks to orphans for retry polling (target may not exist)
+                            if path.is_symlink() {
+                                self.orphans.push(path);
+                            }
                         } else {
                             // If there is no parent, add `path` to `orphans`.
                             self.orphans.push(path);
@@ -374,6 +378,9 @@ impl Observer {
                         }
                     }
                     self.files.update_metadata(event_path, Some(new_md));
+                } else if event_path.is_symlink() && settings.retry {
+                    self.files.reset_reader(event_path);
+                    self.orphans.push(event_path.clone());
                 }
             }
             EventKind::Remove(RemoveKind::File | RemoveKind::Any)

tests/by-util/test_tail.rs

@@ -5149,3 +5149,39 @@ fn test_debug_flag_with_inotify() {
         .with_all_output()
         .stderr_contains("tail: using notification mode");
 }
+
+#[test]
+#[cfg(target_os = "linux")]
+fn test_follow_dangling_symlink() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    at.symlink_file("target", "link");
+    let mut p = ucmd
+        .args(&["-s.1", "--max-unchanged-stats=1", "-F", "link"])
+        .run_no_wait();
+    p.delay(500);
+    at.write("target", "X\n");
+    p.delay(500);
+    p.kill().make_assertion().with_all_output().stdout_is("X\n");
+}
+
+#[test]
+#[cfg(target_os = "linux")]
+fn test_follow_symlink_target_change() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    at.write("t1", "A\n");
+    at.symlink_file("t1", "link");
+    let mut p = ucmd
+        .args(&["-s.1", "--max-unchanged-stats=1", "-F", "link"])
+        .run_no_wait();
+    p.delay(500);
+    at.remove("link");
+    at.symlink_file("t2", "link");
+    p.delay(500);
+    at.write("t2", "B\n");
+    p.delay(500);
+    p.kill()
+        .make_assertion()
+        .with_all_output()
+        .stdout_contains("A\n")
+        .stdout_contains("B\n");
+}