DropHelper: fix num_tasks gauge (#15048)

crates/aptos-drop-helper/src/async_concurrent_dropper.rs

@@ -27,7 +27,7 @@ impl AsyncConcurrentDropper {
     pub fn new(name: &'static str, max_tasks: usize, num_threads: usize) -> Self {
         Self {
             name,
-            num_tasks_tracker: Arc::new(NumTasksTracker::new(max_tasks)),
+            num_tasks_tracker: Arc::new(NumTasksTracker::new(name, max_tasks)),
             thread_pool: ThreadPool::with_name(format!("{}_conc_dropper", name), num_threads),
         }
     }
@@ -49,8 +49,7 @@ impl AsyncConcurrentDropper {
 
     fn schedule_drop_impl<V: Send + 'static>(&self, v: V, notif_sender_opt: Option<Sender<()>>) {
         let _timer = TIMER.timer_with(&[self.name, "enqueue_drop"]);
-        let num_tasks = self.num_tasks_tracker.inc();
-        GAUGE.set_with(&[self.name, "num_tasks"], num_tasks as i64);
+        self.num_tasks_tracker.inc();
 
         let name = self.name;
         let num_tasks_tracker = self.num_tasks_tracker.clone();
@@ -70,32 +69,35 @@ impl AsyncConcurrentDropper {
 }
 
 struct NumTasksTracker {
+    name: &'static str,
     lock: Mutex<usize>,
     cvar: Condvar,
     max_tasks: usize,
 }
 
 impl NumTasksTracker {
-    fn new(max_tasks: usize) -> Self {
+    fn new(name: &'static str, max_tasks: usize) -> Self {
         Self {
+            name,
             lock: Mutex::new(0),
             cvar: Condvar::new(),
             max_tasks,
         }
     }
 
-    fn inc(&self) -> usize {
+    fn inc(&self) {
         let mut num_tasks = self.lock.lock();
         while *num_tasks >= self.max_tasks {
             num_tasks = self.cvar.wait(num_tasks).expect("lock poisoned.");
         }
         *num_tasks += 1;
-        *num_tasks
+        GAUGE.set_with(&[self.name, "num_tasks"], *num_tasks as i64);
     }
 
     fn dec(&self) {
         let mut num_tasks = self.lock.lock();
         *num_tasks -= 1;
+        GAUGE.set_with(&[self.name, "num_tasks"], *num_tasks as i64);
         self.cvar.notify_all();
     }