1use crate::sync::batch_semaphore::Semaphore;
2use std::marker::PhantomData;
3use std::{fmt, mem, ops};
4
5/// RAII structure used to release the shared read access of a lock when
6/// dropped.
7///
8/// This structure is created by the [`read`] method on
9/// [`RwLock`].
10///
11/// [`read`]: method@crate::sync::RwLock::read
12/// [`RwLock`]: struct@crate::sync::RwLock
13#[clippy::has_significant_drop]
14#[must_use = "if unused the RwLock will immediately unlock"]
15pub struct RwLockReadGuard<'a, T: ?Sized> {
16 // When changing the fields in this struct, make sure to update the
17 // `skip_drop` method.
18 #[cfg(all(tokio_unstable, feature = "tracing"))]
19 pub(super) resource_span: tracing::Span,
20 pub(super) s: &'a Semaphore,
21 pub(super) data: *const T,
22 pub(super) marker: PhantomData<&'a T>,
23}
24
25#[allow(dead_code)] // Unused fields are still used in Drop.
26struct Inner<'a, T: ?Sized> {
27 #[cfg(all(tokio_unstable, feature = "tracing"))]
28 resource_span: tracing::Span,
29 s: &'a Semaphore,
30 data: *const T,
31}
32
33impl<'a, T: ?Sized> RwLockReadGuard<'a, T> {
34 fn skip_drop(self) -> Inner<'a, T> {
35 let me = mem::ManuallyDrop::new(self);
36 // SAFETY: This duplicates the values in every field of the guard, then
37 // forgets the originals, so in the end no value is duplicated.
38 Inner {
39 #[cfg(all(tokio_unstable, feature = "tracing"))]
40 resource_span: unsafe { std::ptr::read(&me.resource_span) },
41 s: me.s,
42 data: me.data,
43 }
44 }
45
46 /// Makes a new `RwLockReadGuard` for a component of the locked data.
47 ///
48 /// This operation cannot fail as the `RwLockReadGuard` passed in already
49 /// locked the data.
50 ///
51 /// This is an associated function that needs to be
52 /// used as `RwLockReadGuard::map(...)`. A method would interfere with
53 /// methods of the same name on the contents of the locked data.
54 ///
55 /// This is an asynchronous version of [`RwLockReadGuard::map`] from the
56 /// [`parking_lot` crate].
57 ///
58 /// [`RwLockReadGuard::map`]: https://docs.rs/lock_api/latest/lock_api/struct.RwLockReadGuard.html#method.map
59 /// [`parking_lot` crate]: https://crates.io/crates/parking_lot
60 ///
61 /// # Examples
62 ///
63 /// ```
64 /// use tokio::sync::{RwLock, RwLockReadGuard};
65 ///
66 /// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
67 /// struct Foo(u32);
68 ///
69 /// # #[tokio::main]
70 /// # async fn main() {
71 /// let lock = RwLock::new(Foo(1));
72 ///
73 /// let guard = lock.read().await;
74 /// let guard = RwLockReadGuard::map(guard, |f| &f.0);
75 ///
76 /// assert_eq!(1, *guard);
77 /// # }
78 /// ```
79 #[inline]
80 pub fn map<F, U: ?Sized>(this: Self, f: F) -> RwLockReadGuard<'a, U>
81 where
82 F: FnOnce(&T) -> &U,
83 {
84 let data = f(&*this) as *const U;
85 let this = this.skip_drop();
86
87 RwLockReadGuard {
88 s: this.s,
89 data,
90 marker: PhantomData,
91 #[cfg(all(tokio_unstable, feature = "tracing"))]
92 resource_span: this.resource_span,
93 }
94 }
95
96 /// Attempts to make a new [`RwLockReadGuard`] for a component of the
97 /// locked data. The original guard is returned if the closure returns
98 /// `None`.
99 ///
100 /// This operation cannot fail as the `RwLockReadGuard` passed in already
101 /// locked the data.
102 ///
103 /// This is an associated function that needs to be used as
104 /// `RwLockReadGuard::try_map(..)`. A method would interfere with methods of the
105 /// same name on the contents of the locked data.
106 ///
107 /// This is an asynchronous version of [`RwLockReadGuard::try_map`] from the
108 /// [`parking_lot` crate].
109 ///
110 /// [`RwLockReadGuard::try_map`]: https://docs.rs/lock_api/latest/lock_api/struct.RwLockReadGuard.html#method.try_map
111 /// [`parking_lot` crate]: https://crates.io/crates/parking_lot
112 ///
113 /// # Examples
114 ///
115 /// ```
116 /// use tokio::sync::{RwLock, RwLockReadGuard};
117 ///
118 /// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
119 /// struct Foo(u32);
120 ///
121 /// # #[tokio::main]
122 /// # async fn main() {
123 /// let lock = RwLock::new(Foo(1));
124 ///
125 /// let guard = lock.read().await;
126 /// let guard = RwLockReadGuard::try_map(guard, |f| Some(&f.0)).expect("should not fail");
127 ///
128 /// assert_eq!(1, *guard);
129 /// # }
130 /// ```
131 #[inline]
132 pub fn try_map<F, U: ?Sized>(this: Self, f: F) -> Result<RwLockReadGuard<'a, U>, Self>
133 where
134 F: FnOnce(&T) -> Option<&U>,
135 {
136 let data = match f(&*this) {
137 Some(data) => data as *const U,
138 None => return Err(this),
139 };
140 let this = this.skip_drop();
141
142 Ok(RwLockReadGuard {
143 s: this.s,
144 data,
145 marker: PhantomData,
146 #[cfg(all(tokio_unstable, feature = "tracing"))]
147 resource_span: this.resource_span,
148 })
149 }
150}
151
152impl<T: ?Sized> ops::Deref for RwLockReadGuard<'_, T> {
153 type Target = T;
154
155 fn deref(&self) -> &T {
156 unsafe { &*self.data }
157 }
158}
159
160impl<'a, T: ?Sized> fmt::Debug for RwLockReadGuard<'a, T>
161where
162 T: fmt::Debug,
163{
164 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
165 fmt::Debug::fmt(&**self, f)
166 }
167}
168
169impl<'a, T: ?Sized> fmt::Display for RwLockReadGuard<'a, T>
170where
171 T: fmt::Display,
172{
173 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
174 fmt::Display::fmt(&**self, f)
175 }
176}
177
178impl<'a, T: ?Sized> Drop for RwLockReadGuard<'a, T> {
179 fn drop(&mut self) {
180 self.s.release(1);
181
182 #[cfg(all(tokio_unstable, feature = "tracing"))]
183 self.resource_span.in_scope(|| {
184 tracing::trace!(
185 target: "runtime::resource::state_update",
186 current_readers = 1,
187 current_readers.op = "sub",
188 )
189 });
190 }
191}
192