sync: add OwnedRwLockReadGuard and OwnedRwLockWriteGuard (#3340)

Author: Kai Jewson

tokio/src/sync/mod.rs

@@ -451,6 +451,9 @@ cfg_sync! {
 
     mod rwlock;
     pub use rwlock::RwLock;
+    pub use rwlock::owned_read_guard::OwnedRwLockReadGuard;
+    pub use rwlock::owned_write_guard::OwnedRwLockWriteGuard;
+    pub use rwlock::owned_write_guard_mapped::OwnedRwLockMappedWriteGuard;
     pub use rwlock::read_guard::RwLockReadGuard;
     pub use rwlock::write_guard::RwLockWriteGuard;
     pub use rwlock::write_guard_mapped::RwLockMappedWriteGuard;

tokio/src/sync/rwlock.rs

@@ -2,10 +2,19 @@ use crate::sync::batch_semaphore::{Semaphore, TryAcquireError};
 use crate::sync::mutex::TryLockError;
 use std::cell::UnsafeCell;
 use std::marker;
+use std::marker::PhantomData;
+use std::mem::ManuallyDrop;
+use std::sync::Arc;
 
+pub(crate) mod owned_read_guard;
+pub(crate) mod owned_write_guard;
+pub(crate) mod owned_write_guard_mapped;
 pub(crate) mod read_guard;
 pub(crate) mod write_guard;
 pub(crate) mod write_guard_mapped;
+pub(crate) use owned_read_guard::OwnedRwLockReadGuard;
+pub(crate) use owned_write_guard::OwnedRwLockWriteGuard;
+pub(crate) use owned_write_guard_mapped::OwnedRwLockMappedWriteGuard;
 pub(crate) use read_guard::RwLockReadGuard;
 pub(crate) use write_guard::RwLockWriteGuard;
 pub(crate) use write_guard_mapped::RwLockMappedWriteGuard;
@@ -101,13 +110,31 @@ fn bounds() {
     check_sync::<RwLockReadGuard<'_, u32>>();
     check_unpin::<RwLockReadGuard<'_, u32>>();
 
+    check_send::<OwnedRwLockReadGuard<u32, i32>>();
+    check_sync::<OwnedRwLockReadGuard<u32, i32>>();
+    check_unpin::<OwnedRwLockReadGuard<u32, i32>>();
+
     check_send::<RwLockWriteGuard<'_, u32>>();
     check_sync::<RwLockWriteGuard<'_, u32>>();
     check_unpin::<RwLockWriteGuard<'_, u32>>();
 
-    let rwlock = RwLock::new(0);
+    check_send::<RwLockMappedWriteGuard<'_, u32>>();
+    check_sync::<RwLockMappedWriteGuard<'_, u32>>();
+    check_unpin::<RwLockMappedWriteGuard<'_, u32>>();
+
+    check_send::<OwnedRwLockWriteGuard<u32>>();
+    check_sync::<OwnedRwLockWriteGuard<u32>>();
+    check_unpin::<OwnedRwLockWriteGuard<u32>>();
+
+    check_send::<OwnedRwLockMappedWriteGuard<u32, i32>>();
+    check_sync::<OwnedRwLockMappedWriteGuard<u32, i32>>();
+    check_unpin::<OwnedRwLockMappedWriteGuard<u32, i32>>();
+
+    let rwlock = Arc::new(RwLock::new(0));
     check_send_sync_val(rwlock.read());
+    check_send_sync_val(Arc::clone(&rwlock).read_owned());
     check_send_sync_val(rwlock.write());
+    check_send_sync_val(Arc::clone(&rwlock).write_owned());
 }
 
 // As long as T: Send + Sync, it's fine to send and share RwLock<T> between threads.
@@ -120,14 +147,42 @@ unsafe impl<T> Sync for RwLock<T> where T: ?Sized + Send + Sync {}
 // `T` is `Send`.
 unsafe impl<T> Send for RwLockReadGuard<'_, T> where T: ?Sized + Sync {}
 unsafe impl<T> Sync for RwLockReadGuard<'_, T> where T: ?Sized + Send + Sync {}
+// T is required to be `Send` because an OwnedRwLockReadGuard can be used to drop the value held in
+// the RwLock, unlike RwLockReadGuard.
+unsafe impl<T, U> Send for OwnedRwLockReadGuard<T, U>
+where
+    T: ?Sized + Send + Sync,
+    U: ?Sized + Sync,
+{
+}
+unsafe impl<T, U> Sync for OwnedRwLockReadGuard<T, U>
+where
+    T: ?Sized + Send + Sync,
+    U: ?Sized + Send + Sync,
+{
+}
 unsafe impl<T> Sync for RwLockWriteGuard<'_, T> where T: ?Sized + Send + Sync {}
+unsafe impl<T> Sync for OwnedRwLockWriteGuard<T> where T: ?Sized + Send + Sync {}
 unsafe impl<T> Sync for RwLockMappedWriteGuard<'_, T> where T: ?Sized + Send + Sync {}
+unsafe impl<T, U> Sync for OwnedRwLockMappedWriteGuard<T, U>
+where
+    T: ?Sized + Send + Sync,
+    U: ?Sized + Send + Sync,
+{
+}
 // Safety: Stores a raw pointer to `T`, so if `T` is `Sync`, the lock guard over
 // `T` is `Send` - but since this is also provides mutable access, we need to
 // make sure that `T` is `Send` since its value can be sent across thread
 // boundaries.
 unsafe impl<T> Send for RwLockWriteGuard<'_, T> where T: ?Sized + Send + Sync {}
+unsafe impl<T> Send for OwnedRwLockWriteGuard<T> where T: ?Sized + Send + Sync {}
 unsafe impl<T> Send for RwLockMappedWriteGuard<'_, T> where T: ?Sized + Send + Sync {}
+unsafe impl<T, U> Send for OwnedRwLockMappedWriteGuard<T, U>
+where
+    T: ?Sized + Send + Sync,
+    U: ?Sized + Send + Sync,
+{
+}
 
 impl<T: ?Sized> RwLock<T> {
     /// Creates a new instance of an `RwLock<T>` which is unlocked.
@@ -222,6 +277,64 @@ impl<T: ?Sized> RwLock<T> {
         }
     }
 
+    /// Locks this `RwLock` with shared read access, causing the current task
+    /// to yield until the lock has been acquired.
+    ///
+    /// The calling task will yield until there are no writers which hold the
+    /// lock. There may be other readers inside the lock when the task resumes.
+    ///
+    /// This method is identical to [`RwLock::read`], except that the returned
+    /// guard references the `RwLock` with an [`Arc`] rather than by borrowing
+    /// it. Therefore, the `RwLock` must be wrapped in an `Arc` to call this
+    /// method, and the guard will live for the `'static` lifetime, as it keeps
+    /// the `RwLock` alive by holding an `Arc`.
+    ///
+    /// Note that under the priority policy of [`RwLock`], read locks are not
+    /// granted until prior write locks, to prevent starvation. Therefore
+    /// deadlock may occur if a read lock is held by the current task, a write
+    /// lock attempt is made, and then a subsequent read lock attempt is made
+    /// by the current task.
+    ///
+    /// Returns an RAII guard which will drop this read access of the `RwLock`
+    /// when dropped.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::sync::Arc;
+    /// use tokio::sync::RwLock;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let lock = Arc::new(RwLock::new(1));
+    ///     let c_lock = lock.clone();
+    ///
+    ///     let n = lock.read_owned().await;
+    ///     assert_eq!(*n, 1);
+    ///
+    ///     tokio::spawn(async move {
+    ///         // While main has an active read lock, we acquire one too.
+    ///         let r = c_lock.read_owned().await;
+    ///         assert_eq!(*r, 1);
+    ///     }).await.expect("The spawned task has panicked");
+    ///
+    ///     // Drop the guard after the spawned task finishes.
+    ///     drop(n);
+    ///}
+    /// ```
+    pub async fn read_owned(self: Arc<Self>) -> OwnedRwLockReadGuard<T> {
+        self.s.acquire(1).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!()
+        });
+        OwnedRwLockReadGuard {
+            data: self.c.get(),
+            lock: ManuallyDrop::new(self),
+            _p: PhantomData,
+        }
+    }
+
     /// Attempts to acquire this `RwLock` with shared read access.
     ///
     /// If the access couldn't be acquired immediately, returns [`TryLockError`].
@@ -268,6 +381,58 @@ impl<T: ?Sized> RwLock<T> {
         })
     }
 
+    /// Attempts to acquire this `RwLock` with shared read access.
+    ///
+    /// If the access couldn't be acquired immediately, returns [`TryLockError`].
+    /// Otherwise, an RAII guard is returned which will release read access
+    /// when dropped.
+    ///
+    /// This method is identical to [`RwLock::try_read`], except that the
+    /// returned guard references the `RwLock` with an [`Arc`] rather than by
+    /// borrowing it. Therefore, the `RwLock` must be wrapped in an `Arc` to
+    /// call this method, and the guard will live for the `'static` lifetime,
+    /// as it keeps the `RwLock` alive by holding an `Arc`.
+    ///
+    /// [`TryLockError`]: TryLockError
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::sync::Arc;
+    /// use tokio::sync::RwLock;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let lock = Arc::new(RwLock::new(1));
+    ///     let c_lock = lock.clone();
+    ///
+    ///     let v = lock.try_read_owned().unwrap();
+    ///     assert_eq!(*v, 1);
+    ///
+    ///     tokio::spawn(async move {
+    ///         // While main has an active read lock, we acquire one too.
+    ///         let n = c_lock.read_owned().await;
+    ///         assert_eq!(*n, 1);
+    ///     }).await.expect("The spawned task has panicked");
+    ///
+    ///     // Drop the guard when spawned task finishes.
+    ///     drop(v);
+    /// }
+    /// ```
+    pub fn try_read_owned(self: Arc<Self>) -> Result<OwnedRwLockReadGuard<T>, TryLockError> {
+        match self.s.try_acquire(1) {
+            Ok(permit) => permit,
+            Err(TryAcquireError::NoPermits) => return Err(TryLockError(())),
+            Err(TryAcquireError::Closed) => unreachable!(),
+        }
+
+        Ok(OwnedRwLockReadGuard {
+            data: self.c.get(),
+            lock: ManuallyDrop::new(self),
+            _p: PhantomData,
+        })
+    }
+
     /// Locks this `RwLock` with exclusive write access, causing the current
     /// task to yield until the lock has been acquired.
     ///
@@ -303,6 +468,48 @@ impl<T: ?Sized> RwLock<T> {
         }
     }
 
+    /// Locks this `RwLock` with exclusive write access, causing the current
+    /// task to yield until the lock has been acquired.
+    ///
+    /// The calling task will yield while other writers or readers currently
+    /// have access to the lock.
+    ///
+    /// This method is identical to [`RwLock::write`], except that the returned
+    /// guard references the `RwLock` with an [`Arc`] rather than by borrowing
+    /// it. Therefore, the `RwLock` must be wrapped in an `Arc` to call this
+    /// method, and the guard will live for the `'static` lifetime, as it keeps
+    /// the `RwLock` alive by holding an `Arc`.
+    ///
+    /// Returns an RAII guard which will drop the write access of this `RwLock`
+    /// when dropped.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::sync::Arc;
+    /// use tokio::sync::RwLock;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///   let lock = Arc::new(RwLock::new(1));
+    ///
+    ///   let mut n = lock.write_owned().await;
+    ///   *n = 2;
+    ///}
+    /// ```
+    pub async fn write_owned(self: Arc<Self>) -> OwnedRwLockWriteGuard<T> {
+        self.s.acquire(MAX_READS as u32).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!()
+        });
+        OwnedRwLockWriteGuard {
+            data: self.c.get(),
+            lock: ManuallyDrop::new(self),
+            _p: PhantomData,
+        }
+    }
+
     /// Attempts to acquire this `RwLock` with exclusive write access.
     ///
     /// If the access couldn't be acquired immediately, returns [`TryLockError`].
@@ -340,6 +547,50 @@ impl<T: ?Sized> RwLock<T> {
         })
     }
 
+    /// Attempts to acquire this `RwLock` with exclusive write access.
+    ///
+    /// If the access couldn't be acquired immediately, returns [`TryLockError`].
+    /// Otherwise, an RAII guard is returned which will release write access
+    /// when dropped.
+    ///
+    /// This method is identical to [`RwLock::try_write`], except that the
+    /// returned guard references the `RwLock` with an [`Arc`] rather than by
+    /// borrowing it. Therefore, the `RwLock` must be wrapped in an `Arc` to
+    /// call this method, and the guard will live for the `'static` lifetime,
+    /// as it keeps the `RwLock` alive by holding an `Arc`.
+    ///
+    /// [`TryLockError`]: TryLockError
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::sync::Arc;
+    /// use tokio::sync::RwLock;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let rw = Arc::new(RwLock::new(1));
+    ///
+    ///     let v = Arc::clone(&rw).read_owned().await;
+    ///     assert_eq!(*v, 1);
+    ///
+    ///     assert!(rw.try_write_owned().is_err());
+    /// }
+    /// ```
+    pub fn try_write_owned(self: Arc<Self>) -> Result<OwnedRwLockWriteGuard<T>, TryLockError> {
+        match self.s.try_acquire(MAX_READS as u32) {
+            Ok(permit) => permit,
+            Err(TryAcquireError::NoPermits) => return Err(TryLockError(())),
+            Err(TryAcquireError::Closed) => unreachable!(),
+        }
+
+        Ok(OwnedRwLockWriteGuard {
+            data: self.c.get(),
+            lock: ManuallyDrop::new(self),
+            _p: PhantomData,
+        })
+    }
+
     /// Returns a mutable reference to the underlying data.
     ///
     /// Since this call borrows the `RwLock` mutably, no actual locking needs to