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1 use crate::sync::batch_semaphore::Semaphore;
2 use std::fmt;
3 use std::marker;
4 use std::mem;
5 use std::ops;
6 
7 /// RAII structure used to release the shared read access of a lock when
8 /// dropped.
9 ///
10 /// This structure is created by the [`read`] method on
11 /// [`RwLock`].
12 ///
13 /// [`read`]: method@crate::sync::RwLock::read
14 /// [`RwLock`]: struct@crate::sync::RwLock
15 pub struct RwLockReadGuard<'a, T: ?Sized> {
16     pub(super) s: &'a Semaphore,
17     pub(super) data: *const T,
18     pub(super) marker: marker::PhantomData<&'a T>,
19 }
20 
21 impl<'a, T: ?Sized> RwLockReadGuard<'a, T> {
22     /// Make a new `RwLockReadGuard` for a component of the locked data.
23     ///
24     /// This operation cannot fail as the `RwLockReadGuard` passed in already
25     /// locked the data.
26     ///
27     /// This is an associated function that needs to be
28     /// used as `RwLockReadGuard::map(...)`. A method would interfere with
29     /// methods of the same name on the contents of the locked data.
30     ///
31     /// This is an asynchronous version of [`RwLockReadGuard::map`] from the
32     /// [`parking_lot` crate].
33     ///
34     /// [`RwLockReadGuard::map`]: https://docs.rs/lock_api/latest/lock_api/struct.RwLockReadGuard.html#method.map
35     /// [`parking_lot` crate]: https://crates.io/crates/parking_lot
36     ///
37     /// # Examples
38     ///
39     /// ```
40     /// use tokio::sync::{RwLock, RwLockReadGuard};
41     ///
42     /// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
43     /// struct Foo(u32);
44     ///
45     /// # #[tokio::main]
46     /// # async fn main() {
47     /// let lock = RwLock::new(Foo(1));
48     ///
49     /// let guard = lock.read().await;
50     /// let guard = RwLockReadGuard::map(guard, |f| &f.0);
51     ///
52     /// assert_eq!(1, *guard);
53     /// # }
54     /// ```
55     #[inline]
map<F, U: ?Sized>(this: Self, f: F) -> RwLockReadGuard<'a, U> where F: FnOnce(&T) -> &U,56     pub fn map<F, U: ?Sized>(this: Self, f: F) -> RwLockReadGuard<'a, U>
57     where
58         F: FnOnce(&T) -> &U,
59     {
60         let data = f(&*this) as *const U;
61         let s = this.s;
62         // NB: Forget to avoid drop impl from being called.
63         mem::forget(this);
64         RwLockReadGuard {
65             s,
66             data,
67             marker: marker::PhantomData,
68         }
69     }
70 
71     /// Attempts to make a new [`RwLockReadGuard`] for a component of the
72     /// locked data. The original guard is returned if the closure returns
73     /// `None`.
74     ///
75     /// This operation cannot fail as the `RwLockReadGuard` passed in already
76     /// locked the data.
77     ///
78     /// This is an associated function that needs to be used as
79     /// `RwLockReadGuard::try_map(..)`. A method would interfere with methods of the
80     /// same name on the contents of the locked data.
81     ///
82     /// This is an asynchronous version of [`RwLockReadGuard::try_map`] from the
83     /// [`parking_lot` crate].
84     ///
85     /// [`RwLockReadGuard::try_map`]: https://docs.rs/lock_api/latest/lock_api/struct.RwLockReadGuard.html#method.try_map
86     /// [`parking_lot` crate]: https://crates.io/crates/parking_lot
87     ///
88     /// # Examples
89     ///
90     /// ```
91     /// use tokio::sync::{RwLock, RwLockReadGuard};
92     ///
93     /// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
94     /// struct Foo(u32);
95     ///
96     /// # #[tokio::main]
97     /// # async fn main() {
98     /// let lock = RwLock::new(Foo(1));
99     ///
100     /// let guard = lock.read().await;
101     /// let guard = RwLockReadGuard::try_map(guard, |f| Some(&f.0)).expect("should not fail");
102     ///
103     /// assert_eq!(1, *guard);
104     /// # }
105     /// ```
106     #[inline]
try_map<F, U: ?Sized>(this: Self, f: F) -> Result<RwLockReadGuard<'a, U>, Self> where F: FnOnce(&T) -> Option<&U>,107     pub fn try_map<F, U: ?Sized>(this: Self, f: F) -> Result<RwLockReadGuard<'a, U>, Self>
108     where
109         F: FnOnce(&T) -> Option<&U>,
110     {
111         let data = match f(&*this) {
112             Some(data) => data as *const U,
113             None => return Err(this),
114         };
115         let s = this.s;
116         // NB: Forget to avoid drop impl from being called.
117         mem::forget(this);
118         Ok(RwLockReadGuard {
119             s,
120             data,
121             marker: marker::PhantomData,
122         })
123     }
124 }
125 
126 impl<T: ?Sized> ops::Deref for RwLockReadGuard<'_, T> {
127     type Target = T;
128 
deref(&self) -> &T129     fn deref(&self) -> &T {
130         unsafe { &*self.data }
131     }
132 }
133 
134 impl<'a, T: ?Sized> fmt::Debug for RwLockReadGuard<'a, T>
135 where
136     T: fmt::Debug,
137 {
fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result138     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
139         fmt::Debug::fmt(&**self, f)
140     }
141 }
142 
143 impl<'a, T: ?Sized> fmt::Display for RwLockReadGuard<'a, T>
144 where
145     T: fmt::Display,
146 {
fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result147     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
148         fmt::Display::fmt(&**self, f)
149     }
150 }
151 
152 impl<'a, T: ?Sized> Drop for RwLockReadGuard<'a, T> {
drop(&mut self)153     fn drop(&mut self) {
154         self.s.release(1);
155     }
156 }
157