sync: Add forget_permits method to semaphore

tokio/src/sync/batch_semaphore.rs

@@ -368,6 +368,31 @@ impl Semaphore {
         assert_eq!(rem, 0);
     }
 
+    /// Decrease a semaphore's permits by a maximum of `n`.
+    ///
+    /// If there are insufficient permits and it's not possible to reduce by `n`,
+    /// return the number of permits that were actually reduced.
+    pub(crate) fn forget_permits(&self, n: usize) -> usize {
+        if n == 0 {
+            return 0;
+        }
+
+        let mut curr_bits = self.permits.load(Acquire);
+        loop {
+            let curr = curr_bits >> Self::PERMIT_SHIFT;
+            let new = curr.saturating_sub(n);
+            match self.permits.compare_exchange_weak(
+                curr_bits,
+                new << Self::PERMIT_SHIFT,
+                AcqRel,
+                Acquire,
+            ) {
+                Ok(_) => return std::cmp::min(curr, n),
+                Err(actual) => curr_bits = actual,
+            };
+        }
+    }
+
     fn poll_acquire(
         &self,
         cx: &mut Context<'_>,

tokio/src/sync/semaphore.rs

@@ -481,6 +481,14 @@ impl Semaphore {
         self.ll_sem.release(n);
     }
 
+    /// Decrease a semaphore's permits by a maximum of `n`.
+    ///
+    /// If there are insufficient permits and it's not possible to reduce by `n`,
+    /// return the number of permits that were actually reduced.
+    pub fn forget_permits(&self, n: usize) -> usize {
+        self.ll_sem.forget_permits(n)
+    }
+
     /// Acquires a permit from the semaphore.
     ///
     /// If the semaphore has been closed, this returns an [`AcquireError`].

tokio/src/sync/tests/loom_semaphore_batch.rs

@@ -213,3 +213,31 @@ fn release_during_acquire() {
         assert_eq!(10, semaphore.available_permits());
     })
 }
+
+#[test]
+fn concurrent_permit_updates() {
+    loom::model(move || {
+        let semaphore = Arc::new(Semaphore::new(5));
+        let t1 = {
+            let semaphore = semaphore.clone();
+            thread::spawn(move || semaphore.release(3))
+        };
+        let t2 = {
+            let semaphore = semaphore.clone();
+            thread::spawn(move || {
+                semaphore
+                    .try_acquire(1)
+                    .expect("try_acquire should succeed")
+            })
+        };
+        let t3 = {
+            let semaphore = semaphore.clone();
+            thread::spawn(move || semaphore.forget_permits(2))
+        };
+
+        t1.join().unwrap();
+        t2.join().unwrap();
+        t3.join().unwrap();
+        assert_eq!(semaphore.available_permits(), 5);
+    })
+}

tokio/src/sync/tests/semaphore_batch.rs

@@ -287,3 +287,24 @@ fn release_permits_at_drop() {
         assert!(fut.as_mut().poll(&mut cx).is_pending());
     }
 }
+
+#[test]
+fn forget_permits_basic() {
+    let s = Semaphore::new(10);
+    assert_eq!(s.forget_permits(4), 4);
+    assert_eq!(s.available_permits(), 6);
+    assert_eq!(s.forget_permits(10), 6);
+    assert_eq!(s.available_permits(), 0);
+}
+
+#[test]
+fn update_permits_many_times() {
+    let s = Semaphore::new(5);
+    let mut acquire = task::spawn(s.acquire(7));
+    assert_pending!(acquire.poll());
+    s.release(5);
+    assert_ready_ok!(acquire.poll());
+    assert_eq!(s.available_permits(), 3);
+    assert_eq!(s.forget_permits(3), 3);
+    assert_eq!(s.available_permits(), 0);
+}