sync: fix Mutex when lock future dropped before complete (#1902)

The bug caused the mutex to reach a state where it is locked and cannot be unlocked.

Fixes #1898

tokio/src/sync/mutex.rs

@@ -86,29 +86,24 @@ impl<T> Mutex<T> {
 
     /// A future that resolves on acquiring the lock and returns the `MutexGuard`.
     pub async fn lock(&self) -> MutexGuard<'_, T> {
-        let mut permit = semaphore::Permit::new();
-        poll_fn(|cx| permit.poll_acquire(cx, &self.s))
+        let mut guard = MutexGuard {
+            lock: self,
+            permit: semaphore::Permit::new(),
+        };
+        poll_fn(|cx| guard.permit.poll_acquire(cx, &self.s))
             .await
             .unwrap_or_else(|_| {
                 // The semaphore was closed. but, we never explicitly close it, and we have a
                 // handle to it through the Arc, which means that this can never happen.
                 unreachable!()
             });
-
-        MutexGuard { lock: self, permit }
+        guard
     }
 }
 
 impl<'a, T> Drop for MutexGuard<'a, T> {
     fn drop(&mut self) {
-        if self.permit.is_acquired() {
-            self.permit.release(&self.lock.s);
-        } else if ::std::thread::panicking() {
-            // A guard _should_ always hold its permit, but if the thread is already panicking,
-            // we don't want to generate a panic-while-panicing, since that's just unhelpful!
-        } else {
-            unreachable!("Permit not held when MutexGuard was dropped")
-        }
+        self.permit.release(&self.lock.s);
     }
 }
 

tokio/tests/sync_mutex.rs

@@ -2,10 +2,12 @@
 #![cfg(feature = "full")]
 
 use tokio::sync::Mutex;
+use tokio::time::{interval, timeout};
 use tokio_test::task::spawn;
 use tokio_test::{assert_pending, assert_ready};
 
 use std::sync::Arc;
+use std::time::Duration;
 
 #[test]
 fn straight_execution() {
@@ -79,3 +81,56 @@ fn lock() {
     assert!(*result);
 }
 */
+
+#[tokio::main]
+#[test]
+/// Ensure a mutex is unlocked if a future holding the lock
+/// is aborted prematurely.
+async fn aborted_future_1() {
+    let m1: Arc<Mutex<usize>> = Arc::new(Mutex::new(0));
+    {
+        let m2 = m1.clone();
+        // Try to lock mutex in a future that is aborted prematurely
+        timeout(Duration::from_millis(1u64), async move {
+            let mut iv = interval(Duration::from_millis(1000));
+            m2.lock().await;
+            iv.tick().await;
+            iv.tick().await;
+        })
+        .await
+        .unwrap_err();
+    }
+    // This should succeed as there is no lock left for the mutex.
+    timeout(Duration::from_millis(1u64), async move {
+        m1.lock().await;
+    })
+    .await
+    .expect("Mutex is locked");
+}
+
+#[tokio::main]
+#[test]
+/// This test is similar to `aborted_future_1` but this time the
+/// aborted future is waiting for the lock.
+async fn aborted_future_2() {
+    let m1: Arc<Mutex<usize>> = Arc::new(Mutex::new(0));
+    {
+        // Lock mutex
+        let _lock = m1.lock().await;
+        {
+            let m2 = m1.clone();
+            // Try to lock mutex in a future that is aborted prematurely
+            timeout(Duration::from_millis(1u64), async move {
+                m2.lock().await;
+            })
+            .await
+            .unwrap_err();
+        }
+    }
+    // This should succeed as there is no lock left for the mutex.
+    timeout(Duration::from_millis(1u64), async move {
+        m1.lock().await;
+    })
+    .await
+    .expect("Mutex is locked");
+}