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
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
use core::fmt;
use core::future::Future;
use core::marker::{PhantomData, Unpin};
use core::mem;
use core::pin::Pin;
use core::ptr::NonNull;
use core::sync::atomic::Ordering;
use core::task::{Context, Poll};

use crate::header::Header;
use crate::state::*;

/// A spawned task.
///
/// A [`Task`] can be awaited to retrieve the output of its future.
///
/// Dropping a [`Task`] cancels it, which means its future won't be polled again. To drop the
/// [`Task`] handle without canceling it, use [`detach()`][`Task::detach()`] instead. To cancel a
/// task gracefully and wait until it is fully destroyed, use the [`cancel()`][Task::cancel()]
/// method.
///
/// Note that canceling a task actually wakes it and reschedules one last time. Then, the executor
/// can destroy the task by simply dropping its [`Runnable`][`super::Runnable`] or by invoking
/// [`run()`][`super::Runnable::run()`].
///
/// # Examples
///
/// ```
/// use smol::{future, Executor};
/// use std::thread;
///
/// let ex = Executor::new();
///
/// // Spawn a future onto the executor.
/// let task = ex.spawn(async {
///     println!("Hello from a task!");
///     1 + 2
/// });
///
/// // Run an executor thread.
/// thread::spawn(move || future::block_on(ex.run(future::pending::<()>())));
///
/// // Wait for the task's output.
/// assert_eq!(future::block_on(task), 3);
/// ```
#[must_use = "tasks get canceled when dropped, use `.detach()` to run them in the background"]
pub struct Task<T> {
    /// A raw task pointer.
    pub(crate) ptr: NonNull<()>,

    /// A marker capturing generic type `T`.
    pub(crate) _marker: PhantomData<T>,
}

unsafe impl<T: Send> Send for Task<T> {}
unsafe impl<T> Sync for Task<T> {}

impl<T> Unpin for Task<T> {}

#[cfg(feature = "std")]
impl<T> std::panic::UnwindSafe for Task<T> {}
#[cfg(feature = "std")]
impl<T> std::panic::RefUnwindSafe for Task<T> {}

impl<T> Task<T> {
    /// Detaches the task to let it keep running in the background.
    ///
    /// # Examples
    ///
    /// ```
    /// use smol::{Executor, Timer};
    /// use std::time::Duration;
    ///
    /// let ex = Executor::new();
    ///
    /// // Spawn a deamon future.
    /// ex.spawn(async {
    ///     loop {
    ///         println!("I'm a daemon task looping forever.");
    ///         Timer::after(Duration::from_secs(1)).await;
    ///     }
    /// })
    /// .detach();
    /// ```
    pub fn detach(self) {
        let mut this = self;
        let _out = this.set_detached();
        mem::forget(this);
    }

    /// Cancels the task and waits for it to stop running.
    ///
    /// Returns the task's output if it was completed just before it got canceled, or [`None`] if
    /// it didn't complete.
    ///
    /// While it's possible to simply drop the [`Task`] to cancel it, this is a cleaner way of
    /// canceling because it also waits for the task to stop running.
    ///
    /// # Examples
    ///
    /// ```
    /// use smol::{future, Executor, Timer};
    /// use std::thread;
    /// use std::time::Duration;
    ///
    /// let ex = Executor::new();
    ///
    /// // Spawn a deamon future.
    /// let task = ex.spawn(async {
    ///     loop {
    ///         println!("Even though I'm in an infinite loop, you can still cancel me!");
    ///         Timer::after(Duration::from_secs(1)).await;
    ///     }
    /// });
    ///
    /// // Run an executor thread.
    /// thread::spawn(move || future::block_on(ex.run(future::pending::<()>())));
    ///
    /// future::block_on(async {
    ///     Timer::after(Duration::from_secs(3)).await;
    ///     task.cancel().await;
    /// });
    /// ```
    pub async fn cancel(self) -> Option<T> {
        let mut this = self;
        this.set_canceled();

        struct Fut<T>(Task<T>);

        impl<T> Future for Fut<T> {
            type Output = Option<T>;

            fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
                self.0.poll_task(cx)
            }
        }

        Fut(this).await
    }

    /// Puts the task in canceled state.
    fn set_canceled(&mut self) {
        let ptr = self.ptr.as_ptr();
        let header = ptr as *const Header;

        unsafe {
            let mut state = (*header).state.load(Ordering::Acquire);

            loop {
                // If the task has been completed or closed, it can't be canceled.
                if state & (COMPLETED | CLOSED) != 0 {
                    break;
                }

                // If the task is not scheduled nor running, we'll need to schedule it.
                let new = if state & (SCHEDULED | RUNNING) == 0 {
                    (state | SCHEDULED | CLOSED) + REFERENCE
                } else {
                    state | CLOSED
                };

                // Mark the task as closed.
                match (*header).state.compare_exchange_weak(
                    state,
                    new,
                    Ordering::AcqRel,
                    Ordering::Acquire,
                ) {
                    Ok(_) => {
                        // If the task is not scheduled nor running, schedule it one more time so
                        // that its future gets dropped by the executor.
                        if state & (SCHEDULED | RUNNING) == 0 {
                            ((*header).vtable.schedule)(ptr);
                        }

                        // Notify the awaiter that the task has been closed.
                        if state & AWAITER != 0 {
                            (*header).notify(None);
                        }

                        break;
                    }
                    Err(s) => state = s,
                }
            }
        }
    }

    /// Puts the task in detached state.
    fn set_detached(&mut self) -> Option<T> {
        let ptr = self.ptr.as_ptr();
        let header = ptr as *const Header;

        unsafe {
            // A place where the output will be stored in case it needs to be dropped.
            let mut output = None;

            // Optimistically assume the `Task` is being detached just after creating the task.
            // This is a common case so if the `Task` is datached, the overhead of it is only one
            // compare-exchange operation.
            if let Err(mut state) = (*header).state.compare_exchange_weak(
                SCHEDULED | TASK | REFERENCE,
                SCHEDULED | REFERENCE,
                Ordering::AcqRel,
                Ordering::Acquire,
            ) {
                loop {
                    // If the task has been completed but not yet closed, that means its output
                    // must be dropped.
                    if state & COMPLETED != 0 && state & CLOSED == 0 {
                        // Mark the task as closed in order to grab its output.
                        match (*header).state.compare_exchange_weak(
                            state,
                            state | CLOSED,
                            Ordering::AcqRel,
                            Ordering::Acquire,
                        ) {
                            Ok(_) => {
                                // Read the output.
                                output =
                                    Some((((*header).vtable.get_output)(ptr) as *mut T).read());

                                // Update the state variable because we're continuing the loop.
                                state |= CLOSED;
                            }
                            Err(s) => state = s,
                        }
                    } else {
                        // If this is the last reference to the task and it's not closed, then
                        // close it and schedule one more time so that its future gets dropped by
                        // the executor.
                        let new = if state & (!(REFERENCE - 1) | CLOSED) == 0 {
                            SCHEDULED | CLOSED | REFERENCE
                        } else {
                            state & !TASK
                        };

                        // Unset the `TASK` flag.
                        match (*header).state.compare_exchange_weak(
                            state,
                            new,
                            Ordering::AcqRel,
                            Ordering::Acquire,
                        ) {
                            Ok(_) => {
                                // If this is the last reference to the task, we need to either
                                // schedule dropping its future or destroy it.
                                if state & !(REFERENCE - 1) == 0 {
                                    if state & CLOSED == 0 {
                                        ((*header).vtable.schedule)(ptr);
                                    } else {
                                        ((*header).vtable.destroy)(ptr);
                                    }
                                }

                                break;
                            }
                            Err(s) => state = s,
                        }
                    }
                }
            }

            output
        }
    }

    /// Polls the task to retrieve its output.
    ///
    /// Returns `Some` if the task has completed or `None` if it was closed.
    ///
    /// A task becomes closed in the following cases:
    ///
    /// 1. It gets canceled by `Runnable::drop()`, `Task::drop()`, or `Task::cancel()`.
    /// 2. Its output gets awaited by the `Task`.
    /// 3. It panics while polling the future.
    /// 4. It is completed and the `Task` gets dropped.
    fn poll_task(&mut self, cx: &mut Context<'_>) -> Poll<Option<T>> {
        let ptr = self.ptr.as_ptr();
        let header = ptr as *const Header;

        unsafe {
            let mut state = (*header).state.load(Ordering::Acquire);

            loop {
                // If the task has been closed, notify the awaiter and return `None`.
                if state & CLOSED != 0 {
                    // If the task is scheduled or running, we need to wait until its future is
                    // dropped.
                    if state & (SCHEDULED | RUNNING) != 0 {
                        // Replace the waker with one associated with the current task.
                        (*header).register(cx.waker());

                        // Reload the state after registering. It is possible changes occurred just
                        // before registration so we need to check for that.
                        state = (*header).state.load(Ordering::Acquire);

                        // If the task is still scheduled or running, we need to wait because its
                        // future is not dropped yet.
                        if state & (SCHEDULED | RUNNING) != 0 {
                            return Poll::Pending;
                        }
                    }

                    // Even though the awaiter is most likely the current task, it could also be
                    // another task.
                    (*header).notify(Some(cx.waker()));
                    return Poll::Ready(None);
                }

                // If the task is not completed, register the current task.
                if state & COMPLETED == 0 {
                    // Replace the waker with one associated with the current task.
                    (*header).register(cx.waker());

                    // Reload the state after registering. It is possible that the task became
                    // completed or closed just before registration so we need to check for that.
                    state = (*header).state.load(Ordering::Acquire);

                    // If the task has been closed, restart.
                    if state & CLOSED != 0 {
                        continue;
                    }

                    // If the task is still not completed, we're blocked on it.
                    if state & COMPLETED == 0 {
                        return Poll::Pending;
                    }
                }

                // Since the task is now completed, mark it as closed in order to grab its output.
                match (*header).state.compare_exchange(
                    state,
                    state | CLOSED,
                    Ordering::AcqRel,
                    Ordering::Acquire,
                ) {
                    Ok(_) => {
                        // Notify the awaiter. Even though the awaiter is most likely the current
                        // task, it could also be another task.
                        if state & AWAITER != 0 {
                            (*header).notify(Some(cx.waker()));
                        }

                        // Take the output from the task.
                        let output = ((*header).vtable.get_output)(ptr) as *mut T;
                        return Poll::Ready(Some(output.read()));
                    }
                    Err(s) => state = s,
                }
            }
        }
    }
}

impl<T> Drop for Task<T> {
    fn drop(&mut self) {
        self.set_canceled();
        self.set_detached();
    }
}

impl<T> Future for Task<T> {
    type Output = T;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        match self.poll_task(cx) {
            Poll::Ready(t) => Poll::Ready(t.expect("task has failed")),
            Poll::Pending => Poll::Pending,
        }
    }
}

impl<T> fmt::Debug for Task<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let ptr = self.ptr.as_ptr();
        let header = ptr as *const Header;

        f.debug_struct("Task")
            .field("header", unsafe { &(*header) })
            .finish()
    }
}