sync: add Semaphore (#1973)

Provide an asynchronous Semaphore implementation. This is useful for
synchronizing concurrent access to a shared resource.

1 files changed, 81 insertions, 0 deletions

diff --git a/tokio/tests/sync_semaphore.rs b/tokio/tests/sync_semaphore.rs
new file mode 100644
index 00000000..1cb0c749
--- /dev/null
+++ b/tokio/tests/sync_semaphore.rs
@@ -0,0 +1,81 @@
+#![cfg(feature = "full")]
+
+use std::sync::Arc;
+use tokio::sync::Semaphore;
+
+#[test]
+fn no_permits() {
+    // this should not panic
+    Semaphore::new(0);
+}
+
+#[test]
+fn try_acquire() {
+    let sem = Semaphore::new(1);
+    {
+        let p1 = sem.try_acquire();
+        assert!(p1.is_ok());
+        let p2 = sem.try_acquire();
+        assert!(p2.is_err());
+    }
+    let p3 = sem.try_acquire();
+    assert!(p3.is_ok());
+}
+
+#[tokio::test]
+async fn acquire() {
+    let sem = Arc::new(Semaphore::new(1));
+    let p1 = sem.try_acquire().unwrap();
+    let sem_clone = sem.clone();
+    let j = tokio::spawn(async move {
+        let _p2 = sem_clone.acquire().await;
+    });
+    drop(p1);
+    j.await.unwrap();
+}
+
+#[tokio::test]
+async fn add_permits() {
+    let sem = Arc::new(Semaphore::new(0));
+    let sem_clone = sem.clone();
+    let j = tokio::spawn(async move {
+        let _p2 = sem_clone.acquire().await;
+    });
+    sem.add_permits(1);
+    j.await.unwrap();
+}
+
+#[test]
+fn forget() {
+    let sem = Arc::new(Semaphore::new(1));
+    {
+        let p = sem.try_acquire().unwrap();
+        assert_eq!(sem.available_permits(), 0);
+        p.forget();
+        assert_eq!(sem.available_permits(), 0);
+    }
+    assert_eq!(sem.available_permits(), 0);
+    assert!(sem.try_acquire().is_err());
+}
+
+#[tokio::test]
+async fn stresstest() {
+    let sem = Arc::new(Semaphore::new(5));
+    let mut join_handles = Vec::new();
+    for _ in 0..1000 {
+        let sem_clone = sem.clone();
+        join_handles.push(tokio::spawn(async move {
+            let _p = sem_clone.acquire().await;
+        }));
+    }
+    for j in join_handles {
+        j.await.unwrap();
+    }
+    // there should be exactly 5 semaphores available now
+    let _p1 = sem.try_acquire().unwrap();
+    let _p2 = sem.try_acquire().unwrap();
+    let _p3 = sem.try_acquire().unwrap();
+    let _p4 = sem.try_acquire().unwrap();
+    let _p5 = sem.try_acquire().unwrap();
+    assert!(sem.try_acquire().is_err());
+}