tokio/sync/rwlock/
write_guard.rs

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1use crate::sync::batch_semaphore::Semaphore;
2use crate::sync::rwlock::read_guard::RwLockReadGuard;
3use crate::sync::rwlock::write_guard_mapped::RwLockMappedWriteGuard;
4use std::marker::PhantomData;
5use std::{fmt, mem, ops};
6
7/// RAII structure used to release the exclusive write access of a lock when
8/// dropped.
9///
10/// This structure is created by the [`write`] method
11/// on [`RwLock`].
12///
13/// [`write`]: method@crate::sync::RwLock::write
14/// [`RwLock`]: struct@crate::sync::RwLock
15#[clippy::has_significant_drop]
16#[must_use = "if unused the RwLock will immediately unlock"]
17pub struct RwLockWriteGuard<'a, T: ?Sized> {
18    // When changing the fields in this struct, make sure to update the
19    // `skip_drop` method.
20    #[cfg(all(tokio_unstable, feature = "tracing"))]
21    pub(super) resource_span: tracing::Span,
22    pub(super) permits_acquired: u32,
23    pub(super) s: &'a Semaphore,
24    pub(super) data: *mut T,
25    pub(super) marker: PhantomData<&'a mut T>,
26}
27
28#[allow(dead_code)] // Unused fields are still used in Drop.
29struct Inner<'a, T: ?Sized> {
30    #[cfg(all(tokio_unstable, feature = "tracing"))]
31    resource_span: tracing::Span,
32    permits_acquired: u32,
33    s: &'a Semaphore,
34    data: *mut T,
35}
36
37impl<'a, T: ?Sized> RwLockWriteGuard<'a, T> {
38    fn skip_drop(self) -> Inner<'a, T> {
39        let me = mem::ManuallyDrop::new(self);
40        // SAFETY: This duplicates the values in every field of the guard, then
41        // forgets the originals, so in the end no value is duplicated.
42        Inner {
43            #[cfg(all(tokio_unstable, feature = "tracing"))]
44            resource_span: unsafe { std::ptr::read(&me.resource_span) },
45            permits_acquired: me.permits_acquired,
46            s: me.s,
47            data: me.data,
48        }
49    }
50
51    /// Makes a new [`RwLockMappedWriteGuard`] for a component of the locked data.
52    ///
53    /// This operation cannot fail as the `RwLockWriteGuard` passed in already
54    /// locked the data.
55    ///
56    /// This is an associated function that needs to be used as
57    /// `RwLockWriteGuard::map(..)`. A method would interfere with methods of
58    /// the same name on the contents of the locked data.
59    ///
60    /// This is an asynchronous version of [`RwLockWriteGuard::map`] from the
61    /// [`parking_lot` crate].
62    ///
63    /// [`RwLockMappedWriteGuard`]: struct@crate::sync::RwLockMappedWriteGuard
64    /// [`RwLockWriteGuard::map`]: https://docs.rs/lock_api/latest/lock_api/struct.RwLockWriteGuard.html#method.map
65    /// [`parking_lot` crate]: https://crates.io/crates/parking_lot
66    ///
67    /// # Examples
68    ///
69    /// ```
70    /// use tokio::sync::{RwLock, RwLockWriteGuard};
71    ///
72    /// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
73    /// struct Foo(u32);
74    ///
75    /// # #[tokio::main]
76    /// # async fn main() {
77    /// let lock = RwLock::new(Foo(1));
78    ///
79    /// {
80    ///     let mut mapped = RwLockWriteGuard::map(lock.write().await, |f| &mut f.0);
81    ///     *mapped = 2;
82    /// }
83    ///
84    /// assert_eq!(Foo(2), *lock.read().await);
85    /// # }
86    /// ```
87    #[inline]
88    pub fn map<F, U: ?Sized>(mut this: Self, f: F) -> RwLockMappedWriteGuard<'a, U>
89    where
90        F: FnOnce(&mut T) -> &mut U,
91    {
92        let data = f(&mut *this) as *mut U;
93        let this = this.skip_drop();
94
95        RwLockMappedWriteGuard {
96            permits_acquired: this.permits_acquired,
97            s: this.s,
98            data,
99            marker: PhantomData,
100            #[cfg(all(tokio_unstable, feature = "tracing"))]
101            resource_span: this.resource_span,
102        }
103    }
104
105    /// Makes a new [`RwLockReadGuard`] for a component of the locked data.
106    ///
107    /// This operation cannot fail as the `RwLockWriteGuard` passed in already
108    /// locked the data.
109    ///
110    /// This is an associated function that needs to be used as
111    /// `RwLockWriteGuard::downgrade_map(..)`. A method would interfere with methods of
112    /// the same name on the contents of the locked data.
113    ///
114    /// This is equivalent to a combination of asynchronous [`RwLockWriteGuard::map`] and [`RwLockWriteGuard::downgrade`]
115    /// from the [`parking_lot` crate].
116    ///
117    /// Inside of `f`, you retain exclusive access to the data, despite only being given a `&T`. Handing out a
118    /// `&mut T` would result in unsoundness, as you could use interior mutability.
119    ///
120    /// [`RwLockMappedWriteGuard`]: struct@crate::sync::RwLockMappedWriteGuard
121    /// [`RwLockWriteGuard::map`]: https://docs.rs/lock_api/latest/lock_api/struct.RwLockWriteGuard.html#method.map
122    /// [`RwLockWriteGuard::downgrade`]: https://docs.rs/lock_api/latest/lock_api/struct.RwLockWriteGuard.html#method.downgrade
123    /// [`parking_lot` crate]: https://crates.io/crates/parking_lot
124    ///
125    /// # Examples
126    ///
127    /// ```
128    /// use tokio::sync::{RwLock, RwLockWriteGuard};
129    ///
130    /// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
131    /// struct Foo(u32);
132    ///
133    /// # #[tokio::main]
134    /// # async fn main() {
135    /// let lock = RwLock::new(Foo(1));
136    ///
137    /// let mapped = RwLockWriteGuard::downgrade_map(lock.write().await, |f| &f.0);
138    /// let foo = lock.read().await;
139    /// assert_eq!(foo.0, *mapped);
140    /// # }
141    /// ```
142    #[inline]
143    pub fn downgrade_map<F, U: ?Sized>(this: Self, f: F) -> RwLockReadGuard<'a, U>
144    where
145        F: FnOnce(&T) -> &U,
146    {
147        let data = f(&*this) as *const U;
148        let this = this.skip_drop();
149        let guard = RwLockReadGuard {
150            s: this.s,
151            data,
152            marker: PhantomData,
153            #[cfg(all(tokio_unstable, feature = "tracing"))]
154            resource_span: this.resource_span,
155        };
156
157        // Release all but one of the permits held by the write guard
158        let to_release = (this.permits_acquired - 1) as usize;
159        this.s.release(to_release);
160
161        #[cfg(all(tokio_unstable, feature = "tracing"))]
162        guard.resource_span.in_scope(|| {
163            tracing::trace!(
164            target: "runtime::resource::state_update",
165            write_locked = false,
166            write_locked.op = "override",
167            )
168        });
169
170        #[cfg(all(tokio_unstable, feature = "tracing"))]
171        guard.resource_span.in_scope(|| {
172            tracing::trace!(
173            target: "runtime::resource::state_update",
174            current_readers = 1,
175            current_readers.op = "add",
176            )
177        });
178
179        guard
180    }
181
182    /// Attempts to make a new [`RwLockMappedWriteGuard`] for a component of
183    /// the locked data. The original guard is returned if the closure returns
184    /// `None`.
185    ///
186    /// This operation cannot fail as the `RwLockWriteGuard` passed in already
187    /// locked the data.
188    ///
189    /// This is an associated function that needs to be
190    /// used as `RwLockWriteGuard::try_map(...)`. A method would interfere with
191    /// methods of the same name on the contents of the locked data.
192    ///
193    /// This is an asynchronous version of [`RwLockWriteGuard::try_map`] from
194    /// the [`parking_lot` crate].
195    ///
196    /// [`RwLockMappedWriteGuard`]: struct@crate::sync::RwLockMappedWriteGuard
197    /// [`RwLockWriteGuard::try_map`]: https://docs.rs/lock_api/latest/lock_api/struct.RwLockWriteGuard.html#method.try_map
198    /// [`parking_lot` crate]: https://crates.io/crates/parking_lot
199    ///
200    /// # Examples
201    ///
202    /// ```
203    /// use tokio::sync::{RwLock, RwLockWriteGuard};
204    ///
205    /// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
206    /// struct Foo(u32);
207    ///
208    /// # #[tokio::main]
209    /// # async fn main() {
210    /// let lock = RwLock::new(Foo(1));
211    ///
212    /// {
213    ///     let guard = lock.write().await;
214    ///     let mut guard = RwLockWriteGuard::try_map(guard, |f| Some(&mut f.0)).expect("should not fail");
215    ///     *guard = 2;
216    /// }
217    ///
218    /// assert_eq!(Foo(2), *lock.read().await);
219    /// # }
220    /// ```
221    #[inline]
222    pub fn try_map<F, U: ?Sized>(
223        mut this: Self,
224        f: F,
225    ) -> Result<RwLockMappedWriteGuard<'a, U>, Self>
226    where
227        F: FnOnce(&mut T) -> Option<&mut U>,
228    {
229        let data = match f(&mut *this) {
230            Some(data) => data as *mut U,
231            None => return Err(this),
232        };
233        let this = this.skip_drop();
234
235        Ok(RwLockMappedWriteGuard {
236            permits_acquired: this.permits_acquired,
237            s: this.s,
238            data,
239            marker: PhantomData,
240            #[cfg(all(tokio_unstable, feature = "tracing"))]
241            resource_span: this.resource_span,
242        })
243    }
244
245    /// Attempts to make a new [`RwLockReadGuard`] for a component of
246    /// the locked data. The original guard is returned if the closure returns
247    /// `None`.
248    ///
249    /// This operation cannot fail as the `RwLockWriteGuard` passed in already
250    /// locked the data.
251    ///
252    /// This is an associated function that needs to be
253    /// used as `RwLockWriteGuard::try_downgrade_map(...)`. A method would interfere with
254    /// methods of the same name on the contents of the locked data.
255    ///
256    /// This is equivalent to a combination of asynchronous [`RwLockWriteGuard::try_map`] and [`RwLockWriteGuard::downgrade`]
257    /// from the [`parking_lot` crate].
258    ///
259    /// Inside of `f`, you retain exclusive access to the data, despite only being given a `&T`. Handing out a
260    /// `&mut T` would result in unsoundness, as you could use interior mutability.
261    ///
262    /// If this function returns `Err(...)`, the lock is never unlocked nor downgraded.
263    ///
264    /// [`RwLockMappedWriteGuard`]: struct@crate::sync::RwLockMappedWriteGuard
265    /// [`RwLockWriteGuard::map`]: https://docs.rs/lock_api/latest/lock_api/struct.RwLockWriteGuard.html#method.map
266    /// [`RwLockWriteGuard::downgrade`]: https://docs.rs/lock_api/latest/lock_api/struct.RwLockWriteGuard.html#method.downgrade
267    /// [`parking_lot` crate]: https://crates.io/crates/parking_lot
268    ///
269    /// # Examples
270    ///
271    /// ```
272    /// use tokio::sync::{RwLock, RwLockWriteGuard};
273    ///
274    /// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
275    /// struct Foo(u32);
276    ///
277    /// # #[tokio::main]
278    /// # async fn main() {
279    /// let lock = RwLock::new(Foo(1));
280    ///
281    /// let guard = RwLockWriteGuard::try_downgrade_map(lock.write().await, |f| Some(&f.0)).expect("should not fail");
282    /// let foo = lock.read().await;
283    /// assert_eq!(foo.0, *guard);
284    /// # }
285    /// ```
286    #[inline]
287    pub fn try_downgrade_map<F, U: ?Sized>(this: Self, f: F) -> Result<RwLockReadGuard<'a, U>, Self>
288    where
289        F: FnOnce(&T) -> Option<&U>,
290    {
291        let data = match f(&*this) {
292            Some(data) => data as *const U,
293            None => return Err(this),
294        };
295        let this = this.skip_drop();
296        let guard = RwLockReadGuard {
297            s: this.s,
298            data,
299            marker: PhantomData,
300            #[cfg(all(tokio_unstable, feature = "tracing"))]
301            resource_span: this.resource_span,
302        };
303
304        // Release all but one of the permits held by the write guard
305        let to_release = (this.permits_acquired - 1) as usize;
306        this.s.release(to_release);
307
308        #[cfg(all(tokio_unstable, feature = "tracing"))]
309        guard.resource_span.in_scope(|| {
310            tracing::trace!(
311            target: "runtime::resource::state_update",
312            write_locked = false,
313            write_locked.op = "override",
314            )
315        });
316
317        #[cfg(all(tokio_unstable, feature = "tracing"))]
318        guard.resource_span.in_scope(|| {
319            tracing::trace!(
320            target: "runtime::resource::state_update",
321            current_readers = 1,
322            current_readers.op = "add",
323            )
324        });
325
326        Ok(guard)
327    }
328
329    /// Converts this `RwLockWriteGuard` into an `RwLockMappedWriteGuard`. This
330    /// method can be used to store a non-mapped guard in a struct field that
331    /// expects a mapped guard.
332    ///
333    /// This is equivalent to calling `RwLockWriteGuard::map(guard, |me| me)`.
334    #[inline]
335    pub fn into_mapped(this: Self) -> RwLockMappedWriteGuard<'a, T> {
336        RwLockWriteGuard::map(this, |me| me)
337    }
338
339    /// Atomically downgrades a write lock into a read lock without allowing
340    /// any writers to take exclusive access of the lock in the meantime.
341    ///
342    /// **Note:** This won't *necessarily* allow any additional readers to acquire
343    /// locks, since [`RwLock`] is fair and it is possible that a writer is next
344    /// in line.
345    ///
346    /// Returns an RAII guard which will drop this read access of the `RwLock`
347    /// when dropped.
348    ///
349    /// # Examples
350    ///
351    /// ```
352    /// # use tokio::sync::RwLock;
353    /// # use std::sync::Arc;
354    /// #
355    /// # #[tokio::main]
356    /// # async fn main() {
357    /// let lock = Arc::new(RwLock::new(1));
358    ///
359    /// let n = lock.write().await;
360    ///
361    /// let cloned_lock = lock.clone();
362    /// let handle = tokio::spawn(async move {
363    ///     *cloned_lock.write().await = 2;
364    /// });
365    ///
366    /// let n = n.downgrade();
367    /// assert_eq!(*n, 1, "downgrade is atomic");
368    ///
369    /// drop(n);
370    /// handle.await.unwrap();
371    /// assert_eq!(*lock.read().await, 2, "second writer obtained write lock");
372    /// # }
373    /// ```
374    ///
375    /// [`RwLock`]: struct@crate::sync::RwLock
376    pub fn downgrade(self) -> RwLockReadGuard<'a, T> {
377        let this = self.skip_drop();
378        let guard = RwLockReadGuard {
379            s: this.s,
380            data: this.data,
381            marker: PhantomData,
382            #[cfg(all(tokio_unstable, feature = "tracing"))]
383            resource_span: this.resource_span,
384        };
385
386        // Release all but one of the permits held by the write guard
387        let to_release = (this.permits_acquired - 1) as usize;
388        this.s.release(to_release);
389
390        #[cfg(all(tokio_unstable, feature = "tracing"))]
391        guard.resource_span.in_scope(|| {
392            tracing::trace!(
393            target: "runtime::resource::state_update",
394            write_locked = false,
395            write_locked.op = "override",
396            )
397        });
398
399        #[cfg(all(tokio_unstable, feature = "tracing"))]
400        guard.resource_span.in_scope(|| {
401            tracing::trace!(
402            target: "runtime::resource::state_update",
403            current_readers = 1,
404            current_readers.op = "add",
405            )
406        });
407
408        guard
409    }
410}
411
412impl<T: ?Sized> ops::Deref for RwLockWriteGuard<'_, T> {
413    type Target = T;
414
415    fn deref(&self) -> &T {
416        unsafe { &*self.data }
417    }
418}
419
420impl<T: ?Sized> ops::DerefMut for RwLockWriteGuard<'_, T> {
421    fn deref_mut(&mut self) -> &mut T {
422        unsafe { &mut *self.data }
423    }
424}
425
426impl<'a, T: ?Sized> fmt::Debug for RwLockWriteGuard<'a, T>
427where
428    T: fmt::Debug,
429{
430    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
431        fmt::Debug::fmt(&**self, f)
432    }
433}
434
435impl<'a, T: ?Sized> fmt::Display for RwLockWriteGuard<'a, T>
436where
437    T: fmt::Display,
438{
439    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
440        fmt::Display::fmt(&**self, f)
441    }
442}
443
444impl<'a, T: ?Sized> Drop for RwLockWriteGuard<'a, T> {
445    fn drop(&mut self) {
446        self.s.release(self.permits_acquired as usize);
447
448        #[cfg(all(tokio_unstable, feature = "tracing"))]
449        self.resource_span.in_scope(|| {
450            tracing::trace!(
451            target: "runtime::resource::state_update",
452            write_locked = false,
453            write_locked.op = "override",
454            )
455        });
456    }
457}
tokio/sync/rwlock/write_guard.rs

tokio/sync/rwlock/
write_guard.rs
