pidof: Support '-t' flag

This flag makes pidof print thread ids of threads belonging to the
matching processes.

src/uu/pgrep/src/process.rs

@@ -186,6 +186,8 @@ pub struct ProcessInformation {
     cached_stat: Option<Rc<Vec<String>>>,
 
     cached_start_time: Option<u64>,
+
+    cached_thread_ids: Option<Rc<Vec<usize>>>,
 }
 
 impl ProcessInformation {
@@ -328,6 +330,31 @@ impl ProcessInformation {
 
         Teletype::Unknown
     }
+
+    pub fn thread_ids(&mut self) -> Rc<Vec<usize>> {
+        if let Some(c) = &self.cached_thread_ids {
+            return Rc::clone(c);
+        }
+
+        let tids_dir = format!("/proc/{}/task", self.pid);
+        let result = Rc::new(
+            WalkDir::new(tids_dir)
+                .min_depth(1)
+                .max_depth(1)
+                .follow_links(false)
+                .into_iter()
+                .flatten()
+                .flat_map(|it| {
+                    it.path()
+                        .file_name()
+                        .map(|it| it.to_str().unwrap().parse::<usize>().unwrap())
+                })
+                .collect::<Vec<_>>(),
+        );
+
+        self.cached_thread_ids = Some(Rc::clone(&result));
+        Rc::clone(&result)
+    }
 }
 impl TryFrom<DirEntry> for ProcessInformation {
     type Error = io::Error;

src/uu/pidof/src/pidof.rs

@@ -25,15 +25,23 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
     };
 
     let collected = collect_matched_pids(&matches);
+    let to_print = if matches.get_flag("t") {
+        collected
+            .into_iter()
+            .flat_map(|mut it| (*it.thread_ids()).clone())
+            .collect::<Vec<_>>()
+    } else {
+        collected.into_iter().map(|it| it.pid).collect::<Vec<_>>()
+    };
 
-    if collected.is_empty() {
+    if to_print.is_empty() {
         uucore::error::set_exit_code(1);
         return Ok(());
     };
 
-    let output = collected
+    let output = to_print
         .into_iter()
-        .map(|it| it.pid.to_string())
+        .map(|it| it.to_string())
         .collect::<Vec<_>>()
         .join(arg_separator);
 
@@ -164,6 +172,13 @@ pub fn uu_app() -> Command {
                 .help("Only return one PID")
                 .action(ArgAction::SetTrue),
         )
+        .arg(
+            Arg::new("t")
+                .short('t')
+                .long("lightweight")
+                .help("Show thread ids instead of process ids")
+                .action(ArgAction::SetTrue),
+        )
     // .arg(
     //     Arg::new("x")
     //         .short('x')