tail: add --debug flag to show follow implementation mode

src/uu/tail/locales/en-US.ftl

@@ -6,6 +6,7 @@ tail-usage = tail [FLAG]... [FILE]...
 
 # Help messages
 tail-help-bytes = Number of bytes to print
+tail-help-debug = indicate which --follow implementation is used
 tail-help-follow = Print the file as it grows
 tail-help-lines = Number of lines to print
 tail-help-pid = With -f, terminate after process ID, PID dies
@@ -70,3 +71,7 @@ tail-giving-up-on-this-name = ; giving up on this name
 tail-stdin-header = standard input
 tail-no-files-remaining = no files remaining
 tail-become-inaccessible = has become inaccessible
+
+# Debug messages
+tail-debug-using-notification-mode = using notification mode
+tail-debug-using-polling-mode = using polling mode

src/uu/tail/src/args.rs

@@ -38,6 +38,7 @@ pub mod options {
     pub const MAX_UNCHANGED_STATS: &str = "max-unchanged-stats";
     pub const ARG_FILES: &str = "files";
     pub const PRESUME_INPUT_PIPE: &str = "-presume-input-pipe"; // NOTE: three hyphens is correct
+    pub const DEBUG: &str = "debug";
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
@@ -139,6 +140,7 @@ pub struct Settings {
     pub use_polling: bool,
     pub verbose: bool,
     pub presume_input_pipe: bool,
+    pub debug: bool,
     /// `FILE(s)` positional arguments
     pub inputs: Vec<Input>,
 }
@@ -155,6 +157,7 @@ impl Default for Settings {
             use_polling: Default::default(),
             verbose: Default::default(),
             presume_input_pipe: Default::default(),
+            debug: Default::default(),
             inputs: Vec::default(),
         }
     }
@@ -223,6 +226,7 @@ impl Settings {
             mode: FilterMode::from(matches)?,
             verbose: matches.get_flag(options::verbosity::VERBOSE),
             presume_input_pipe: matches.get_flag(options::PRESUME_INPUT_PIPE),
+            debug: matches.get_flag(options::DEBUG),
             ..Default::default()
         };
 
@@ -543,6 +547,12 @@ pub fn uu_app() -> Command {
                 .overrides_with(options::FOLLOW_RETRY)
                 .action(ArgAction::SetTrue),
         )
+        .arg(
+            Arg::new(options::DEBUG)
+                .long(options::DEBUG)
+                .help(translate!("tail-help-debug"))
+                .action(ArgAction::SetTrue),
+        )
         .arg(
             Arg::new(options::PRESUME_INPUT_PIPE)
                 .long("presume-input-pipe")

src/uu/tail/src/tail.rs

@@ -72,6 +72,16 @@ fn uu_tail(settings: &Settings) -> UResult<()> {
     let mut observer = Observer::from(settings);
 
     observer.start(settings)?;
+
+    // Print debug info about the follow implementation being used
+    if settings.debug && settings.follow.is_some() {
+        if observer.use_polling {
+            show_error!("{}", translate!("tail-debug-using-polling-mode"));
+        } else {
+            show_error!("{}", translate!("tail-debug-using-notification-mode"));
+        }
+    }
+
     // Do an initial tail print of each path's content.
     // Add `path` and `reader` to `files` map if `--follow` is selected.
     for input in &settings.inputs.clone() {

tests/by-util/test_tail.rs

@@ -5065,3 +5065,39 @@ fn test_follow_stdout_pipe_close() {
     child.close_stdout();
     child.delay(2000).make_assertion().is_not_alive();
 }
+
+#[test]
+fn test_debug_flag_with_polling() {
+    let ts = TestScenario::new(util_name!());
+    let at = &ts.fixtures;
+    at.touch("f");
+
+    let mut child = ts
+        .ucmd()
+        .args(&["--debug", "-f", "--use-polling", "f"])
+        .run_no_wait();
+
+    child.make_assertion_with_delay(500).is_alive();
+    child
+        .kill()
+        .make_assertion()
+        .with_all_output()
+        .stderr_contains("tail: using polling mode");
+}
+
+#[test]
+#[cfg(target_os = "linux")]
+fn test_debug_flag_with_inotify() {
+    let ts = TestScenario::new(util_name!());
+    let at = &ts.fixtures;
+    at.touch("f");
+
+    let mut child = ts.ucmd().args(&["--debug", "-f", "f"]).run_no_wait();
+
+    child.make_assertion_with_delay(500).is_alive();
+    child
+        .kill()
+        .make_assertion()
+        .with_all_output()
+        .stderr_contains("tail: using notification mode");
+}