sync: drop wakers outside lock in semaphore (tokio-rs#5475)

tokio/src/sync/batch_semaphore.rs

@@ -444,6 +444,7 @@ impl Semaphore {
         }
 
         assert_eq!(acquired, 0);
+        let mut old_waker = None;
 
         // Otherwise, register the waker & enqueue the node.
         node.waker.with_mut(|waker| {
@@ -455,7 +456,7 @@ impl Semaphore {
                 .map(|waker| !waker.will_wake(cx.waker()))
                 .unwrap_or(true)
             {
-                *waker = Some(cx.waker().clone());
+                old_waker = std::mem::replace(waker, Some(cx.waker().clone()));
             }
         });
 
@@ -468,6 +469,8 @@ impl Semaphore {
 
             waiters.queue.push_front(node);
         }
+        drop(waiters);
+        drop(old_waker);
 
         Pending
     }

tokio/src/sync/tests/semaphore_batch.rs

@@ -252,3 +252,32 @@ fn cancel_acquire_releases_permits() {
     assert_eq!(6, s.available_permits());
     assert_ok!(s.try_acquire(6));
 }
+
+#[test]
+fn release_permits_at_drop() {
+    use crate::sync::semaphore::*;
+    use futures::task::ArcWake;
+    use std::future::Future;
+    use std::sync::Arc;
+
+    let sem = Arc::new(Semaphore::new(1));
+
+    struct ReleaseOnDrop(Option<OwnedSemaphorePermit>);
+
+    impl ArcWake for ReleaseOnDrop {
+        fn wake_by_ref(_arc_self: &Arc<Self>) {}
+    }
+
+    let mut fut = Box::pin(async {
+        let _permit = sem.acquire().await.unwrap();
+    });
+
+    // Second iteration shouldn't deadlock.
+    for _ in 0..=1 {
+        let waker = futures::task::waker(Arc::new(ReleaseOnDrop(
+            sem.clone().try_acquire_owned().ok(),
+        )));
+        let mut cx = std::task::Context::from_waker(&waker);
+        assert!(fut.as_mut().poll(&mut cx).is_pending());
+    }
+}