1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
use crate::sync::rwlock::RwLock;
use std::marker::PhantomData;
use std::sync::Arc;
use std::{fmt, mem, ops, ptr};

/// Owned RAII structure used to release the shared read access of a lock when
/// dropped.
///
/// This structure is created by the [`read_owned`] method on
/// [`RwLock`].
///
/// [`read_owned`]: method@crate::sync::RwLock::read_owned
/// [`RwLock`]: struct@crate::sync::RwLock
#[clippy::has_significant_drop]
pub struct OwnedRwLockReadGuard<T: ?Sized, U: ?Sized = T> {
    // When changing the fields in this struct, make sure to update the
    // `skip_drop` method.
    #[cfg(all(tokio_unstable, feature = "tracing"))]
    pub(super) resource_span: tracing::Span,
    pub(super) lock: Arc<RwLock<T>>,
    pub(super) data: *const U,
    pub(super) _p: PhantomData<T>,
}

#[allow(dead_code)] // Unused fields are still used in Drop.
struct Inner<T: ?Sized, U: ?Sized> {
    #[cfg(all(tokio_unstable, feature = "tracing"))]
    resource_span: tracing::Span,
    lock: Arc<RwLock<T>>,
    data: *const U,
}

impl<T: ?Sized, U: ?Sized> OwnedRwLockReadGuard<T, U> {
    fn skip_drop(self) -> Inner<T, U> {
        let me = mem::ManuallyDrop::new(self);
        // SAFETY: This duplicates the values in every field of the guard, then
        // forgets the originals, so in the end no value is duplicated.
        unsafe {
            Inner {
                #[cfg(all(tokio_unstable, feature = "tracing"))]
                resource_span: ptr::read(&me.resource_span),
                lock: ptr::read(&me.lock),
                data: me.data,
            }
        }
    }

    /// Makes a new `OwnedRwLockReadGuard` for a component of the locked data.
    /// This operation cannot fail as the `OwnedRwLockReadGuard` passed in
    /// already locked the data.
    ///
    /// This is an associated function that needs to be
    /// used as `OwnedRwLockReadGuard::map(...)`. A method would interfere with
    /// methods of the same name on the contents of the locked data.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    /// use tokio::sync::{RwLock, OwnedRwLockReadGuard};
    ///
    /// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
    /// struct Foo(u32);
    ///
    /// # #[tokio::main]
    /// # async fn main() {
    /// let lock = Arc::new(RwLock::new(Foo(1)));
    ///
    /// let guard = lock.read_owned().await;
    /// let guard = OwnedRwLockReadGuard::map(guard, |f| &f.0);
    ///
    /// assert_eq!(1, *guard);
    /// # }
    /// ```
    #[inline]
    pub fn map<F, V: ?Sized>(this: Self, f: F) -> OwnedRwLockReadGuard<T, V>
    where
        F: FnOnce(&U) -> &V,
    {
        let data = f(&*this) as *const V;
        let this = this.skip_drop();

        OwnedRwLockReadGuard {
            lock: this.lock,
            data,
            _p: PhantomData,
            #[cfg(all(tokio_unstable, feature = "tracing"))]
            resource_span: this.resource_span,
        }
    }

    /// Attempts to make a new [`OwnedRwLockReadGuard`] for a component of the
    /// locked data. The original guard is returned if the closure returns
    /// `None`.
    ///
    /// This operation cannot fail as the `OwnedRwLockReadGuard` passed in
    /// already locked the data.
    ///
    /// This is an associated function that needs to be used as
    /// `OwnedRwLockReadGuard::try_map(..)`. A method would interfere with
    /// methods of the same name on the contents of the locked data.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    /// use tokio::sync::{RwLock, OwnedRwLockReadGuard};
    ///
    /// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
    /// struct Foo(u32);
    ///
    /// # #[tokio::main]
    /// # async fn main() {
    /// let lock = Arc::new(RwLock::new(Foo(1)));
    ///
    /// let guard = lock.read_owned().await;
    /// let guard = OwnedRwLockReadGuard::try_map(guard, |f| Some(&f.0)).expect("should not fail");
    ///
    /// assert_eq!(1, *guard);
    /// # }
    /// ```
    #[inline]
    pub fn try_map<F, V: ?Sized>(this: Self, f: F) -> Result<OwnedRwLockReadGuard<T, V>, Self>
    where
        F: FnOnce(&U) -> Option<&V>,
    {
        let data = match f(&*this) {
            Some(data) => data as *const V,
            None => return Err(this),
        };
        let this = this.skip_drop();

        Ok(OwnedRwLockReadGuard {
            lock: this.lock,
            data,
            _p: PhantomData,
            #[cfg(all(tokio_unstable, feature = "tracing"))]
            resource_span: this.resource_span,
        })
    }
}

impl<T: ?Sized, U: ?Sized> ops::Deref for OwnedRwLockReadGuard<T, U> {
    type Target = U;

    fn deref(&self) -> &U {
        unsafe { &*self.data }
    }
}

impl<T: ?Sized, U: ?Sized> fmt::Debug for OwnedRwLockReadGuard<T, U>
where
    U: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&**self, f)
    }
}

impl<T: ?Sized, U: ?Sized> fmt::Display for OwnedRwLockReadGuard<T, U>
where
    U: fmt::Display,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(&**self, f)
    }
}

impl<T: ?Sized, U: ?Sized> Drop for OwnedRwLockReadGuard<T, U> {
    fn drop(&mut self) {
        self.lock.s.release(1);

        #[cfg(all(tokio_unstable, feature = "tracing"))]
        self.resource_span.in_scope(|| {
            tracing::trace!(
            target: "runtime::resource::state_update",
            current_readers = 1,
            current_readers.op = "sub",
            )
        });
    }
}