rwlock.rs.html -- source
Description: Source to the Rust file `library/std/src/sync/rwlock.rs`.
Keywords: rust, rustlang, rust-lang

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    #[cfg(all(test, not(target_os = "emscripten")))]
    mod tests;

    use crate::cell::UnsafeCell;
    use crate::fmt;
    use crate::mem;
    use crate::ops::{Deref, DerefMut};
    use crate::ptr;
    use crate::sys_common::poison::{self, LockResult, TryLockError, TryLockResult};
    use crate::sys_common::rwlock as sys;

    /// A reader-writer lock
    ///
    /// This type of lock allows a number of readers or at most one writer at any
    /// point in time. The write portion of this lock typically allows modification
    /// of the underlying data (exclusive access) and the read portion of this lock
    /// typically allows for read-only access (shared access).
    ///
    /// In comparison, a [`Mutex`] does not distinguish between readers or writers
    /// that acquire the lock, therefore blocking any threads waiting for the lock to
    /// become available. An `RwLock` will allow any number of readers to acquire the
    /// lock as long as a writer is not holding the lock.
    ///
    /// The priority policy of the lock is dependent on the underlying operating
    /// system's implementation, and this type does not guarantee that any
    /// particular policy will be used.
    ///
    /// The type parameter `T` represents the data that this lock protects. It is
    /// required that `T` satisfies [`Send`] to be shared across threads and
    /// [`Sync`] to allow concurrent access through readers. The RAII guards
    /// returned from the locking methods implement [`Deref`] (and [`DerefMut`]
    /// for the `write` methods) to allow access to the content of the lock.
    ///
    /// # Poisoning
    ///
    /// An `RwLock`, like [`Mutex`], will become poisoned on a panic. Note, however,
    /// that an `RwLock` may only be poisoned if a panic occurs while it is locked
    /// exclusively (write mode). If a panic occurs in any reader, then the lock
    /// will not be poisoned.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::RwLock;
    ///
    /// let lock = RwLock::new(5);
    ///
    /// // many reader locks can be held at once
    /// {
    ///     let r1 = lock.read().unwrap();
    ///     let r2 = lock.read().unwrap();
    ///     assert_eq!(*r1, 5);
    ///     assert_eq!(*r2, 5);
    /// } // read locks are dropped at this point
    ///
    /// // only one write lock may be held, however
    /// {
    ///     let mut w = lock.write().unwrap();
    ///     *w += 1;
    ///     assert_eq!(*w, 6);
    /// } // write lock is dropped here
    /// ```
    ///
    /// [`Mutex`]: super::Mutex
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct RwLock<T: ?Sized> {
        inner: Box<sys::RWLock>,
        poison: poison::Flag,
        data: UnsafeCell<T>,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<T: ?Sized + Send> Send for RwLock<T> {}
    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}

    /// RAII structure used to release the shared read access of a lock when
    /// dropped.
    ///
    /// This structure is created by the [`read`] and [`try_read`] methods on
    /// [`RwLock`].
    ///
    /// [`read`]: RwLock::read
    /// [`try_read`]: RwLock::try_read
    #[must_use = "if unused the RwLock will immediately unlock"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct RwLockReadGuard<'a, T: ?Sized + 'a> {
        lock: &'a RwLock<T>,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> !Send for RwLockReadGuard<'_, T> {}

    #[stable(feature = "rwlock_guard_sync", since = "1.23.0")]
    unsafe impl<T: ?Sized + Sync> Sync for RwLockReadGuard<'_, T> {}

    /// RAII structure used to release the exclusive write access of a lock when
    /// dropped.
    ///
    /// This structure is created by the [`write`] and [`try_write`] methods
    /// on [`RwLock`].
    ///
    /// [`write`]: RwLock::write
    /// [`try_write`]: RwLock::try_write
    #[must_use = "if unused the RwLock will immediately unlock"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct RwLockWriteGuard<'a, T: ?Sized + 'a> {
        lock: &'a RwLock<T>,
        poison: poison::Guard,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> !Send for RwLockWriteGuard<'_, T> {}

    #[stable(feature = "rwlock_guard_sync", since = "1.23.0")]
    unsafe impl<T: ?Sized + Sync> Sync for RwLockWriteGuard<'_, T> {}

    impl<T> RwLock<T> {
        /// Creates a new instance of an `RwLock<T>` which is unlocked.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::RwLock;
        ///
        /// let lock = RwLock::new(5);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn new(t: T) -> RwLock<T> {
            RwLock {
                inner: box sys::RWLock::new(),
                poison: poison::Flag::new(),
                data: UnsafeCell::new(t),
            }
        }
    }

    impl<T: ?Sized> RwLock<T> {
        /// Locks this rwlock with shared read access, blocking the current thread
        /// until it can be acquired.
        ///
        /// The calling thread will be blocked until there are no more writers which
        /// hold the lock. There may be other readers currently inside the lock when
        /// this method returns. This method does not provide any guarantees with
        /// respect to the ordering of whether contentious readers or writers will
        /// acquire the lock first.
        ///
        /// Returns an RAII guard which will release this thread's shared access
        /// once it is dropped.
        ///
        /// # Errors
        ///
        /// This function will return an error if the RwLock is poisoned. An RwLock
        /// is poisoned whenever a writer panics while holding an exclusive lock.
        /// The failure will occur immediately after the lock has been acquired.
        ///
        /// # Panics
        ///
        /// This function might panic when called if the lock is already held by the current thread.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::{Arc, RwLock};
        /// use std::thread;
        ///
        /// let lock = Arc::new(RwLock::new(1));
        /// let c_lock = Arc::clone(&lock);
        ///
        /// let n = lock.read().unwrap();
        /// assert_eq!(*n, 1);
        ///
        /// thread::spawn(move || {
        ///     let r = c_lock.read();
        ///     assert!(r.is_ok());
        /// }).join().unwrap();
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn read(&self) -> LockResult<RwLockReadGuard<'_, T>> {
            unsafe {
                self.inner.read();
                RwLockReadGuard::new(self)
            }
        }

        /// Attempts to acquire this rwlock with shared read access.
        ///
        /// If the access could not be granted at this time, then `Err` is returned.
        /// Otherwise, an RAII guard is returned which will release the shared access
        /// when it is dropped.
        ///
        /// This function does not block.
        ///
        /// This function does not provide any guarantees with respect to the ordering
        /// of whether contentious readers or writers will acquire the lock first.
        ///
        /// # Errors
        ///
        /// This function will return an error if the RwLock is poisoned. An RwLock
        /// is poisoned whenever a writer panics while holding an exclusive lock. An
        /// error will only be returned if the lock would have otherwise been
        /// acquired.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::RwLock;
        ///
        /// let lock = RwLock::new(1);
        ///
        /// match lock.try_read() {
        ///     Ok(n) => assert_eq!(*n, 1),
        ///     Err(_) => unreachable!(),
        /// };
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn try_read(&self) -> TryLockResult<RwLockReadGuard<'_, T>> {
            unsafe {
                if self.inner.try_read() {
                    Ok(RwLockReadGuard::new(self)?)
                } else {
                    Err(TryLockError::WouldBlock)
                }
            }
        }

        /// Locks this rwlock with exclusive write access, blocking the current
        /// thread until it can be acquired.
        ///
        /// This function will not return while other writers or other readers
        /// currently have access to the lock.
        ///
        /// Returns an RAII guard which will drop the write access of this rwlock
        /// when dropped.
        ///
        /// # Errors
        ///
        /// This function will return an error if the RwLock is poisoned. An RwLock
        /// is poisoned whenever a writer panics while holding an exclusive lock.
        /// An error will be returned when the lock is acquired.
        ///
        /// # Panics
        ///
        /// This function might panic when called if the lock is already held by the current thread.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::RwLock;
        ///
        /// let lock = RwLock::new(1);
        ///
        /// let mut n = lock.write().unwrap();
        /// *n = 2;
        ///
        /// assert!(lock.try_read().is_err());
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn write(&self) -> LockResult<RwLockWriteGuard<'_, T>> {
            unsafe {
                self.inner.write();
                RwLockWriteGuard::new(self)
            }
        }

        /// Attempts to lock this rwlock with exclusive write access.
        ///
        /// If the lock could not be acquired at this time, then `Err` is returned.
        /// Otherwise, an RAII guard is returned which will release the lock when
        /// it is dropped.
        ///
        /// This function does not block.
        ///
        /// This function does not provide any guarantees with respect to the ordering
        /// of whether contentious readers or writers will acquire the lock first.
        ///
        /// # Errors
        ///
        /// This function will return an error if the RwLock is poisoned. An RwLock
        /// is poisoned whenever a writer panics while holding an exclusive lock. An
        /// error will only be returned if the lock would have otherwise been
        /// acquired.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::RwLock;
        ///
        /// let lock = RwLock::new(1);
        ///
        /// let n = lock.read().unwrap();
        /// assert_eq!(*n, 1);
        ///
        /// assert!(lock.try_write().is_err());
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn try_write(&self) -> TryLockResult<RwLockWriteGuard<'_, T>> {
            unsafe {
                if self.inner.try_write() {
                    Ok(RwLockWriteGuard::new(self)?)
                } else {
                    Err(TryLockError::WouldBlock)
                }
            }
        }

        /// Determines whether the lock is poisoned.
        ///
        /// If another thread is active, the lock can still become poisoned at any
        /// time. You should not trust a `false` value for program correctness
        /// without additional synchronization.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::{Arc, RwLock};
        /// use std::thread;
        ///
        /// let lock = Arc::new(RwLock::new(0));
        /// let c_lock = Arc::clone(&lock);
        ///
        /// let _ = thread::spawn(move || {
        ///     let _lock = c_lock.write().unwrap();
        ///     panic!(); // the lock gets poisoned
        /// }).join();
        /// assert_eq!(lock.is_poisoned(), true);
        /// ```
        #[inline]
        #[stable(feature = "sync_poison", since = "1.2.0")]
        pub fn is_poisoned(&self) -> bool {
            self.poison.get()
        }

        /// Consumes this `RwLock`, returning the underlying data.
        ///
        /// # Errors
        ///
        /// This function will return an error if the RwLock is poisoned. An RwLock
        /// is poisoned whenever a writer panics while holding an exclusive lock. An
        /// error will only be returned if the lock would have otherwise been
        /// acquired.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::RwLock;
        ///
        /// let lock = RwLock::new(String::new());
        /// {
        ///     let mut s = lock.write().unwrap();
        ///     *s = "modified".to_owned();
        /// }
        /// assert_eq!(lock.into_inner().unwrap(), "modified");
        /// ```
        #[stable(feature = "rwlock_into_inner", since = "1.6.0")]
        pub fn into_inner(self) -> LockResult<T>
        where
            T: Sized,
        {
            // We know statically that there are no outstanding references to
            // `self` so there's no need to lock the inner lock.
            //
            // To get the inner value, we'd like to call `data.into_inner()`,
            // but because `RwLock` impl-s `Drop`, we can't move out of it, so
            // we'll have to destructure it manually instead.
            unsafe {
                // Like `let RwLock { inner, poison, data } = self`.
                let (inner, poison, data) = {
                    let RwLock { ref inner, ref poison, ref data } = self;
                    (ptr::read(inner), ptr::read(poison), ptr::read(data))
                };
                mem::forget(self);
                inner.destroy(); // Keep in sync with the `Drop` impl.
                drop(inner);

                poison::map_result(poison.borrow(), |_| data.into_inner())
            }
        }

        /// Returns a mutable reference to the underlying data.
        ///
        /// Since this call borrows the `RwLock` mutably, no actual locking needs to
        /// take place -- the mutable borrow statically guarantees no locks exist.
        ///
        /// # Errors
        ///
        /// This function will return an error if the RwLock is poisoned. An RwLock
        /// is poisoned whenever a writer panics while holding an exclusive lock. An
        /// error will only be returned if the lock would have otherwise been
        /// acquired.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::RwLock;
        ///
        /// let mut lock = RwLock::new(0);
        /// *lock.get_mut().unwrap() = 10;
        /// assert_eq!(*lock.read().unwrap(), 10);
        /// ```
        #[stable(feature = "rwlock_get_mut", since = "1.6.0")]
        pub fn get_mut(&mut self) -> LockResult<&mut T> {
            let data = self.data.get_mut();
            poison::map_result(self.poison.borrow(), |_| data)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<#[may_dangle] T: ?Sized> Drop for RwLock<T> {
        fn drop(&mut self) {
            // IMPORTANT: This code needs to be kept in sync with `RwLock::into_inner`.
            unsafe { self.inner.destroy() }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLock<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            match self.try_read() {
                Ok(guard) => f.debug_struct("RwLock").field("data", &&*guard).finish(),
                Err(TryLockError::Poisoned(err)) => {
                    f.debug_struct("RwLock").field("data", &&**err.get_ref()).finish()
                }
                Err(TryLockError::WouldBlock) => {
                    struct LockedPlaceholder;
                    impl fmt::Debug for LockedPlaceholder {
                        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                            f.write_str("<locked>")
                        }
                    }

                    f.debug_struct("RwLock").field("data", &LockedPlaceholder).finish()
                }
            }
        }
    }

    #[stable(feature = "rw_lock_default", since = "1.10.0")]
    impl<T: Default> Default for RwLock<T> {
        /// Creates a new `RwLock<T>`, with the `Default` value for T.
        fn default() -> RwLock<T> {
            RwLock::new(Default::default())
        }
    }

    #[stable(feature = "rw_lock_from", since = "1.24.0")]
    impl<T> From<T> for RwLock<T> {
        /// Creates a new instance of an `RwLock<T>` which is unlocked.
        /// This is equivalent to [`RwLock::new`].
        fn from(t: T) -> Self {
            RwLock::new(t)
        }
    }

    impl<'rwlock, T: ?Sized> RwLockReadGuard<'rwlock, T> {
        unsafe fn new(lock: &'rwlock RwLock<T>) -> LockResult<RwLockReadGuard<'rwlock, T>> {
            poison::map_result(lock.poison.borrow(), |_| RwLockReadGuard { lock })
        }
    }

    impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
        unsafe fn new(lock: &'rwlock RwLock<T>) -> LockResult<RwLockWriteGuard<'rwlock, T>> {
            poison::map_result(lock.poison.borrow(), |guard| RwLockWriteGuard { lock, poison: guard })
        }
    }

    #[stable(feature = "std_debug", since = "1.16.0")]
    impl<T: fmt::Debug> fmt::Debug for RwLockReadGuard<'_, T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("RwLockReadGuard").field("lock", &self.lock).finish()
        }
    }

    #[stable(feature = "std_guard_impls", since = "1.20.0")]
    impl<T: ?Sized + fmt::Display> fmt::Display for RwLockReadGuard<'_, T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            (**self).fmt(f)
        }
    }

    #[stable(feature = "std_debug", since = "1.16.0")]
    impl<T: fmt::Debug> fmt::Debug for RwLockWriteGuard<'_, T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("RwLockWriteGuard").field("lock", &self.lock).finish()
        }
    }

    #[stable(feature = "std_guard_impls", since = "1.20.0")]
    impl<T: ?Sized + fmt::Display> fmt::Display for RwLockWriteGuard<'_, T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            (**self).fmt(f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Deref for RwLockReadGuard<'_, T> {
        type Target = T;

        fn deref(&self) -> &T {
            unsafe { &*self.lock.data.get() }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Deref for RwLockWriteGuard<'_, T> {
        type Target = T;

        fn deref(&self) -> &T {
            unsafe { &*self.lock.data.get() }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> DerefMut for RwLockWriteGuard<'_, T> {
        fn deref_mut(&mut self) -> &mut T {
            unsafe { &mut *self.lock.data.get() }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Drop for RwLockReadGuard<'_, T> {
        fn drop(&mut self) {
            unsafe {
                self.lock.inner.read_unlock();
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Drop for RwLockWriteGuard<'_, T> {
        fn drop(&mut self) {
            self.lock.poison.done(&self.poison);
            unsafe {
                self.lock.inner.write_unlock();
            }
        }
    }