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
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
use pin_project_lite::pin_project;
use std::cell::RefCell;
use std::error::Error;
use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};
use std::{fmt, thread};

/// Declares a new task-local key of type [`tokio::task::LocalKey`].
///
/// # Syntax
///
/// The macro wraps any number of static declarations and makes them local to the current task.
/// Publicity and attributes for each static is preserved. For example:
///
/// # Examples
///
/// ```
/// # use tokio::task_local;
/// task_local! {
///     pub static ONE: u32;
///
///     #[allow(unused)]
///     static TWO: f32;
/// }
/// # fn main() {}
/// ```
///
/// See [LocalKey documentation][`tokio::task::LocalKey`] for more
/// information.
///
/// [`tokio::task::LocalKey`]: struct@crate::task::LocalKey
#[macro_export]
macro_rules! task_local {
     // empty (base case for the recursion)
    () => {};

    ($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty; $($rest:tt)*) => {
        $crate::__task_local_inner!($(#[$attr])* $vis $name, $t);
        $crate::task_local!($($rest)*);
    };

    ($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty) => {
        $crate::__task_local_inner!($(#[$attr])* $vis $name, $t);
    }
}

#[doc(hidden)]
#[macro_export]
macro_rules! __task_local_inner {
    ($(#[$attr:meta])* $vis:vis $name:ident, $t:ty) => {
        $vis static $name: $crate::task::LocalKey<$t> = {
            std::thread_local! {
                static __KEY: std::cell::RefCell<Option<$t>> = std::cell::RefCell::new(None);
            }

            $crate::task::LocalKey { inner: __KEY }
        };
    };
}

/// A key for task-local data.
///
/// This type is generated by the `task_local!` macro.
///
/// Unlike [`std::thread::LocalKey`], `tokio::task::LocalKey` will
/// _not_ lazily initialize the value on first access. Instead, the
/// value is first initialized when the future containing
/// the task-local is first polled by a futures executor, like Tokio.
///
/// # Examples
///
/// ```
/// # async fn dox() {
/// tokio::task_local! {
///     static NUMBER: u32;
/// }
///
/// NUMBER.scope(1, async move {
///     assert_eq!(NUMBER.get(), 1);
/// }).await;
///
/// NUMBER.scope(2, async move {
///     assert_eq!(NUMBER.get(), 2);
///
///     NUMBER.scope(3, async move {
///         assert_eq!(NUMBER.get(), 3);
///     }).await;
/// }).await;
/// # }
/// ```
/// [`std::thread::LocalKey`]: struct@std::thread::LocalKey
pub struct LocalKey<T: 'static> {
    #[doc(hidden)]
    pub inner: thread::LocalKey<RefCell<Option<T>>>,
}

impl<T: 'static> LocalKey<T> {
    /// Sets a value `T` as the task-local value for the future `F`.
    ///
    /// On completion of `scope`, the task-local will be dropped.
    ///
    /// ### Examples
    ///
    /// ```
    /// # async fn dox() {
    /// tokio::task_local! {
    ///     static NUMBER: u32;
    /// }
    ///
    /// NUMBER.scope(1, async move {
    ///     println!("task local value: {}", NUMBER.get());
    /// }).await;
    /// # }
    /// ```
    pub async fn scope<F>(&'static self, value: T, f: F) -> F::Output
    where
        F: Future,
    {
        TaskLocalFuture {
            local: &self,
            slot: Some(value),
            future: f,
        }
        .await
    }

    /// Accesses the current task-local and runs the provided closure.
    ///
    /// # Panics
    ///
    /// This function will panic if not called within the context
    /// of a future containing a task-local with the corresponding key.
    pub fn with<F, R>(&'static self, f: F) -> R
    where
        F: FnOnce(&T) -> R,
    {
        self.try_with(f).expect(
            "cannot access a Task Local Storage value \
             without setting it via `LocalKey::set`",
        )
    }

    /// Accesses the current task-local and runs the provided closure.
    ///
    /// If the task-local with the accociated key is not present, this
    /// method will return an `AccessError`. For a panicking variant,
    /// see `with`.
    pub fn try_with<F, R>(&'static self, f: F) -> Result<R, AccessError>
    where
        F: FnOnce(&T) -> R,
    {
        self.inner.with(|v| {
            if let Some(val) = v.borrow().as_ref() {
                Ok(f(val))
            } else {
                Err(AccessError { _private: () })
            }
        })
    }
}

impl<T: Copy + 'static> LocalKey<T> {
    /// Returns a copy of the task-local value
    /// if the task-local value implements `Copy`.
    pub fn get(&'static self) -> T {
        self.with(|v| *v)
    }
}

impl<T: 'static> fmt::Debug for LocalKey<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.pad("LocalKey { .. }")
    }
}

pin_project! {
    struct TaskLocalFuture<T: StaticLifetime, F> {
        local: &'static LocalKey<T>,
        slot: Option<T>,
        #[pin]
        future: F,
    }
}

impl<T: 'static, F: Future> Future for TaskLocalFuture<T, F> {
    type Output = F::Output;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        struct Guard<'a, T: 'static> {
            local: &'static LocalKey<T>,
            slot: &'a mut Option<T>,
            prev: Option<T>,
        }

        impl<T> Drop for Guard<'_, T> {
            fn drop(&mut self) {
                let value = self.local.inner.with(|c| c.replace(self.prev.take()));
                *self.slot = value;
            }
        }

        let mut project = self.project();
        let val = project.slot.take();

        let prev = project.local.inner.with(|c| c.replace(val));

        let _guard = Guard {
            prev,
            slot: &mut project.slot,
            local: *project.local,
        };

        project.future.poll(cx)
    }
}

// Required to make `pin_project` happy.
trait StaticLifetime: 'static {}
impl<T: 'static> StaticLifetime for T {}

/// An error returned by [`LocalKey::try_with`](method@LocalKey::try_with).
#[derive(Clone, Copy, Eq, PartialEq)]
pub struct AccessError {
    _private: (),
}

impl fmt::Debug for AccessError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("AccessError").finish()
    }
}

impl fmt::Display for AccessError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt("task-local value not set", f)
    }
}

impl Error for AccessError {}