wasmer_types::lib::std::sync

Struct RwLockWriteGuard

1.0.0 · Source

pub struct RwLockWriteGuard<'a, T>where
    T: 'a + ?Sized,
{ /* private fields */ }

Available on crate feature std only.

Expand description

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.

Implementations§

Source §

impl<'a, T> RwLockWriteGuard<'a, T>
where T: ?Sized,

Source

pub fn map<U, F>( orig: RwLockWriteGuard<'a, T>, f: F, ) -> MappedRwLockWriteGuard<'a, U>
where F: FnOnce(&mut T) -> &mut U, U: ?Sized,

🔬This is a nightly-only experimental API. (mapped_lock_guards)

Makes a MappedRwLockWriteGuard for a component of the borrowed data, e.g. an enum variant.

The RwLock is already locked for writing, so this cannot fail.

This is an associated function that needs to be used as RwLockWriteGuard::map(...). A method would interfere with methods of the same name on the contents of the RwLockWriteGuard used through Deref.

§Panics

If the closure panics, the guard will be dropped (unlocked) and the RwLock will be poisoned.

Source

pub fn try_map<U, F>( orig: RwLockWriteGuard<'a, T>, f: F, ) -> Result<MappedRwLockWriteGuard<'a, U>, RwLockWriteGuard<'a, T>>
where F: FnOnce(&mut T) -> Option<&mut U>, U: ?Sized,

🔬This is a nightly-only experimental API. (mapped_lock_guards)

Makes a MappedRwLockWriteGuard for a component of the borrowed data. The original guard is returned as an Err(...) if the closure returns None.

The RwLock is already locked for writing, so this cannot fail.

This is an associated function that needs to be used as RwLockWriteGuard::try_map(...). A method would interfere with methods of the same name on the contents of the RwLockWriteGuard used through Deref.

§Panics

If the closure panics, the guard will be dropped (unlocked) and the RwLock will be poisoned.

Source

pub fn downgrade(s: RwLockWriteGuard<'a, T>) -> RwLockReadGuard<'a, T>

🔬This is a nightly-only experimental API. (rwlock_downgrade)

Downgrades a write-locked RwLockWriteGuard into a read-locked RwLockReadGuard.

This method will atomically change the state of the RwLock from exclusive mode into shared mode. This means that it is impossible for a writing thread to get in between a thread calling downgrade and the same thread reading whatever it wrote while it had the RwLock in write mode.

Note that since we have the RwLockWriteGuard, we know that the RwLock is already locked for writing, so this method cannot fail.

§Example

#![feature(rwlock_downgrade)]
use std::sync::{Arc, RwLock, RwLockWriteGuard};

// The inner value starts as 0.
let rw = Arc::new(RwLock::new(0));

// Put the lock in write mode.
let mut main_write_guard = rw.write().unwrap();

let evil = rw.clone();
let handle = std::thread::spawn(move || {
    // This will not return until the main thread drops the `main_read_guard`.
    let mut evil_guard = evil.write().unwrap();

    assert_eq!(*evil_guard, 1);
    *evil_guard = 2;
});

// After spawning the writer thread, set the inner value to 1.
*main_write_guard = 1;

// Atomically downgrade the write guard into a read guard.
let main_read_guard = RwLockWriteGuard::downgrade(main_write_guard);

// Since `downgrade` is atomic, the writer thread cannot have set the inner value to 2.
assert_eq!(*main_read_guard, 1, "`downgrade` was not atomic");

// Clean up everything now
drop(main_read_guard);
handle.join().unwrap();

let final_check = rw.read().unwrap();
assert_eq!(*final_check, 2);