sync: add Mutex::blocking_lock_owned (#5130)

Author: smessmer

tokio/src/sync/mutex.rs

@@ -418,6 +418,62 @@ impl<T: ?Sized> Mutex<T> {
         crate::future::block_on(self.lock())
     }
 
+    /// Blockingly locks this `Mutex`. When the lock has been acquired, function returns an
+    /// [`OwnedMutexGuard`].
+    ///
+    /// This method is identical to [`Mutex::blocking_lock`], except that the returned
+    /// guard references the `Mutex` with an [`Arc`] rather than by borrowing
+    /// it. Therefore, the `Mutex` must be wrapped in an `Arc` to call this
+    /// method, and the guard will live for the `'static` lifetime, as it keeps
+    /// the `Mutex` alive by holding an `Arc`.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if called within an asynchronous execution context.
+    ///
+    ///   - If you find yourself in an asynchronous execution context and needing
+    ///     to call some (synchronous) function which performs one of these
+    ///     `blocking_` operations, then consider wrapping that call inside
+    ///     [`spawn_blocking()`][crate::runtime::Handle::spawn_blocking]
+    ///     (or [`block_in_place()`][crate::task::block_in_place]).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::sync::Arc;
+    /// use tokio::sync::Mutex;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let mutex =  Arc::new(Mutex::new(1));
+    ///     let lock = mutex.lock().await;
+    ///
+    ///     let mutex1 = Arc::clone(&mutex);
+    ///     let blocking_task = tokio::task::spawn_blocking(move || {
+    ///         // This shall block until the `lock` is released.
+    ///         let mut n = mutex1.blocking_lock_owned();
+    ///         *n = 2;
+    ///     });
+    ///
+    ///     assert_eq!(*lock, 1);
+    ///     // Release the lock.
+    ///     drop(lock);
+    ///
+    ///     // Await the completion of the blocking task.
+    ///     blocking_task.await.unwrap();
+    ///
+    ///     // Assert uncontended.
+    ///     let n = mutex.try_lock().unwrap();
+    ///     assert_eq!(*n, 2);
+    /// }
+    ///
+    /// ```
+    #[track_caller]
+    #[cfg(feature = "sync")]
+    pub fn blocking_lock_owned(self: Arc<Self>) -> OwnedMutexGuard<T> {
+        crate::future::block_on(self.lock_owned())
+    }
+
     /// Locks this mutex, causing the current task to yield until the lock has
     /// been acquired. When the lock has been acquired, this returns an
     /// [`OwnedMutexGuard`].