From 78e33b4d4f6ca6c2568d07c4d7f770ad80996624 Mon Sep 17 00:00:00 2001
From: Hegui Dai <natural_selection_@outlook.com>
Date: Tue, 31 Mar 2026 10:25:49 +0800
Subject: [PATCH] sync: fix `notify_waiters` priority in `Notify`

Previously, if a `notify_waiters()` was followed by a `notify_one()`,
an unpolled `Notified` future created before the `notify_waiters()`
call would consume the `NOTIFIED` permit created by the
`notify_one()` call.

This commit fixes this by verifying the `notify_waiters_calls` count
before optimistically attempting to acquire the `NOTIFIED` permit. If
the count indicates a `notify_waiters()` call has already happened, the
future transitions directly to `State::Done` and leaves the permit
intact for other waiters.

Fixes: #7965
---
 tokio/src/sync/notify.rs   |  5 +++++
 tokio/tests/sync_notify.rs | 12 ++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/tokio/src/sync/notify.rs b/tokio/src/sync/notify.rs
index 33bce0e4f66..68418c0c9f2 100644
--- a/tokio/src/sync/notify.rs
+++ b/tokio/src/sync/notify.rs
@@ -1118,6 +1118,11 @@ impl NotifiedProject<'_> {
                 State::Init => {
                     let curr = notify.state.load(SeqCst);
 
+                    if get_num_notify_waiters_calls(curr) != *notify_waiters_calls {
+                        *state = State::Done;
+                        continue 'outer_loop;
+                    }
+
                     // Optimistically try acquiring a pending notification
                     let res = notify.state.compare_exchange(
                         set_state(curr, NOTIFIED),
diff --git a/tokio/tests/sync_notify.rs b/tokio/tests/sync_notify.rs
index 13b3f921e98..9f8186794ea 100644
--- a/tokio/tests/sync_notify.rs
+++ b/tokio/tests/sync_notify.rs
@@ -301,3 +301,15 @@ fn test_waker_update() {
 
     assert!(future.is_woken());
 }
+
+#[test]
+fn notify_one_has_priority_over_notify_waiters() {
+    use futures::FutureExt;
+    let notify = tokio::sync::Notify::new();
+    let notified1 = notify.notified();
+    notify.notify_waiters();
+    notify.notify_one();
+    assert!(notified1.now_or_never().is_some());
+    let notified2 = notify.notified();
+    assert!(notified2.now_or_never().is_some());
+}