sync: Add LockFuture for Lock (#1184)

tokio-sync/tests/lock.rs

@@ -1,5 +1,7 @@
 #![deny(warnings, rust_2018_idioms)]
 
+use pin_utils::pin_mut;
+use std::future::Future;
 use tokio_sync::lock::Lock;
 use tokio_test::task::MockTask;
 use tokio_test::{assert_pending, assert_ready};
@@ -48,3 +50,29 @@ fn readiness() {
     assert!(t2.is_woken());
     assert_ready!(t2.enter(|cx| l.poll_lock(cx)));
 }
+
+#[test]
+fn lock() {
+    let mut lock = Lock::new(false);
+
+    let mut lock2 = lock.clone();
+    std::thread::spawn(move || {
+        let l = lock2.lock();
+        pin_mut!(l);
+
+        let mut task = MockTask::new();
+        let mut g = task.enter(|cx| assert_ready!(l.poll(cx)));
+        std::thread::sleep(std::time::Duration::from_millis(500));
+        *g = true;
+        drop(g);
+    });
+
+    std::thread::sleep(std::time::Duration::from_millis(50));
+    let mut task = MockTask::new();
+    let l = lock.lock();
+    pin_mut!(l);
+
+    task.enter(|cx| assert_pending!(l.poll(cx)));
+    std::thread::sleep(std::time::Duration::from_millis(500));
+    assert!(task.is_woken());
+}