runtime: fix double increment of num_idle_threads on shutdown (#7910)

tokio/src/runtime/blocking/pool.rs

@@ -561,12 +561,6 @@ impl Inner {
                     shared = self.shared.lock();
                 }
 
-                // Work was produced, and we "took" it (by decrementing num_notify).
-                // This means that num_idle was decremented once for our wakeup.
-                // But, since we are exiting, we need to "undo" that, as we'll stay idle.
-                self.metrics.inc_num_idle_threads();
-                // NOTE: Technically we should also do num_notify++ and notify again,
-                // but since we're shutting down anyway, that won't be necessary.
                 break;
             }
         }

tokio/tests/rt_unstable_metrics.rs

@@ -62,6 +62,38 @@ fn num_idle_blocking_threads() {
     assert_eq!(1, rt.metrics().num_idle_blocking_threads());
 }
 
+#[test]
+fn num_idle_blocking_threads_is_zero_after_shutdown() {
+    let rt = current_thread();
+    let handle = rt.handle().clone();
+
+    // Spawn a blocking task to create a worker thread.
+    let _ = rt.block_on(rt.spawn_blocking(move || {}));
+
+    // Wait for the thread to become idle.
+    rt.block_on(async {
+        time::sleep(Duration::from_millis(5)).await;
+    });
+    if handle.metrics().num_idle_blocking_threads() == 0 {
+        rt.block_on(async {
+            time::sleep(Duration::from_secs(1)).await;
+        });
+    }
+    assert_eq!(1, handle.metrics().num_idle_blocking_threads());
+
+    // Drop the runtime, which triggers shutdown and joins all blocking
+    // threads. Before the fix for #6439, the shutdown path incremented
+    // num_idle_threads a second time for each idle worker, so this
+    // counter stayed at 1 instead of going back to 0.
+    drop(rt);
+
+    assert_eq!(
+        0,
+        handle.metrics().num_idle_blocking_threads(),
+        "num_idle_blocking_threads should be 0 after shutdown (see #6439)"
+    );
+}
+
 #[test]
 fn blocking_queue_depth() {
     let rt = tokio::runtime::Builder::new_current_thread()