From f085b6211b8ebb6aba21f1f1f91e7b8b243aa815 Mon Sep 17 00:00:00 2001
From: Alice Ryhl <aliceryhl@google.com>
Date: Thu, 7 May 2026 09:32:14 +0200
Subject: [PATCH] sync: notify receivers in mpsc `OwnedPermit::release()`
 method (#8075)

---
 tokio/src/sync/mpsc/bounded.rs | 19 ++------
 tokio/tests/sync_mpsc.rs       | 85 ++++++++++++++++++++++++++++++++++
 2 files changed, 88 insertions(+), 16 deletions(-)

diff --git a/tokio/src/sync/mpsc/bounded.rs b/tokio/src/sync/mpsc/bounded.rs
index 06eeffc3f..f7b0081c6 100644
--- a/tokio/src/sync/mpsc/bounded.rs
+++ b/tokio/src/sync/mpsc/bounded.rs
@@ -1844,14 +1844,12 @@ impl<T> OwnedPermit<T> {
     ///
     /// [`Sender`]: Sender
     pub fn release(mut self) -> Sender<T> {
-        use chan::Semaphore;
-
         let chan = self.chan.take().unwrap_or_else(|| {
             unreachable!("OwnedPermit channel is only taken when the permit is moved")
         });
 
         // Add the permit back to the semaphore
-        chan.semaphore().add_permit();
+        drop(Permit { chan: &chan });
         Sender { chan }
     }
 
@@ -1910,21 +1908,10 @@ impl<T> OwnedPermit<T> {
 
 impl<T> Drop for OwnedPermit<T> {
     fn drop(&mut self) {
-        use chan::Semaphore;
-
         // Are we still holding onto the sender?
         if let Some(chan) = self.chan.take() {
-            let semaphore = chan.semaphore();
-
-            // Add the permit back to the semaphore
-            semaphore.add_permit();
-
-            // If this `OwnedPermit` is holding the last sender for this
-            // channel, wake the receiver so that it can be notified that the
-            // channel is closed.
-            if semaphore.is_closed() && semaphore.is_idle() {
-                chan.wake_rx();
-            }
+            // Reuse Drop impl of non-owned Permit.
+            drop(Permit { chan: &chan });
         }
 
         // Otherwise, do nothing.
diff --git a/tokio/tests/sync_mpsc.rs b/tokio/tests/sync_mpsc.rs
index 27157288e..93804581b 100644
--- a/tokio/tests/sync_mpsc.rs
+++ b/tokio/tests/sync_mpsc.rs
@@ -788,6 +788,91 @@ async fn drop_permit_iterator_releases_permits() {
     }
 }
 
+#[test]
+fn dropping_last_permit_wakes_closed_receiver() {
+    let (tx, mut rx) = mpsc::channel::<()>(100);
+
+    let permit = tx.try_reserve().unwrap();
+    rx.close();
+
+    let mut recv = tokio_test::task::spawn(rx.recv());
+    assert_pending!(recv.poll());
+    drop(permit);
+    assert!(recv.is_woken());
+    assert_ready!(recv.poll());
+}
+
+#[test]
+fn dropping_last_owned_permit_wakes_closed_receiver() {
+    let (tx, mut rx) = mpsc::channel::<()>(100);
+
+    let permit = tx.try_reserve_owned().unwrap();
+    rx.close();
+
+    let mut recv = tokio_test::task::spawn(rx.recv());
+    assert_pending!(recv.poll());
+    drop(permit);
+    assert!(recv.is_woken());
+    assert_ready!(recv.poll());
+}
+
+#[test]
+fn dropping_last_permit_iterator_wakes_closed_receiver() {
+    let (tx, mut rx) = mpsc::channel::<()>(100);
+
+    let permits = tx.try_reserve_many(1).unwrap();
+    rx.close();
+
+    let mut recv = tokio_test::task::spawn(rx.recv());
+    assert_pending!(recv.poll());
+    drop(permits);
+    assert!(recv.is_woken());
+    assert_ready!(recv.poll());
+}
+
+#[test]
+fn sending_last_permit_wakes_closed_receiver() {
+    let (tx, mut rx) = mpsc::channel::<()>(100);
+
+    let permit = tx.try_reserve().unwrap();
+    rx.close();
+
+    let mut recv = tokio_test::task::spawn(rx.recv());
+    assert_pending!(recv.poll());
+    permit.send(());
+    assert!(recv.is_woken());
+    assert_ready!(recv.poll());
+}
+
+#[test]
+fn sending_last_owned_permit_wakes_closed_receiver() {
+    let (tx, mut rx) = mpsc::channel::<()>(100);
+
+    let permit = tx.try_reserve_owned().unwrap();
+    rx.close();
+
+    let mut recv = tokio_test::task::spawn(rx.recv());
+    assert_pending!(recv.poll());
+    permit.send(());
+    assert!(recv.is_woken());
+    assert_ready!(recv.poll());
+}
+
+#[test]
+fn releasing_last_owned_permit_wakes_closed_receiver() {
+    let (tx, mut rx) = mpsc::channel::<()>(100);
+
+    let permit = tx.try_reserve_owned().unwrap();
+    rx.close();
+
+    let mut recv = tokio_test::task::spawn(rx.recv());
+    assert_pending!(recv.poll());
+    let inert_sender = permit.release();
+    assert!(recv.is_woken());
+    assert_ready!(recv.poll());
+    drop(inert_sender);
+}
+
 #[maybe_tokio_test]
 async fn dropping_rx_closes_channel() {
     let (tx, rx) = mpsc::channel(100);