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
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
//! A collection of tasks spawned on a Tokio runtime.
//!
//! This module provides the [`JoinSet`] type, a collection which stores a set
//! of spawned tasks and allows asynchronously awaiting the output of those
//! tasks as they complete. See the documentation for the [`JoinSet`] type for
//! details.
use std::fmt;
use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};

use crate::runtime::Handle;
#[cfg(tokio_unstable)]
use crate::task::Id;
use crate::task::{AbortHandle, JoinError, JoinHandle, LocalSet};
use crate::util::IdleNotifiedSet;

/// A collection of tasks spawned on a Tokio runtime.
///
/// A `JoinSet` can be used to await the completion of some or all of the tasks
/// in the set. The set is not ordered, and the tasks will be returned in the
/// order they complete.
///
/// All of the tasks must have the same return type `T`.
///
/// When the `JoinSet` is dropped, all tasks in the `JoinSet` are immediately aborted.
///
/// # Examples
///
/// Spawn multiple tasks and wait for them.
///
/// ```
/// use tokio::task::JoinSet;
///
/// #[tokio::main]
/// async fn main() {
///     let mut set = JoinSet::new();
///
///     for i in 0..10 {
///         set.spawn(async move { i });
///     }
///
///     let mut seen = [false; 10];
///     while let Some(res) = set.join_next().await {
///         let idx = res.unwrap();
///         seen[idx] = true;
///     }
///
///     for i in 0..10 {
///         assert!(seen[i]);
///     }
/// }
/// ```
#[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
pub struct JoinSet<T> {
    inner: IdleNotifiedSet<JoinHandle<T>>,
}

/// A variant of [`task::Builder`] that spawns tasks on a [`JoinSet`] rather
/// than on the current default runtime.
///
/// [`task::Builder`]: crate::task::Builder
#[cfg(all(tokio_unstable, feature = "tracing"))]
#[cfg_attr(docsrs, doc(cfg(all(tokio_unstable, feature = "tracing"))))]
#[must_use = "builders do nothing unless used to spawn a task"]
pub struct Builder<'a, T> {
    joinset: &'a mut JoinSet<T>,
    builder: super::Builder<'a>,
}

impl<T> JoinSet<T> {
    /// Create a new `JoinSet`.
    pub fn new() -> Self {
        Self {
            inner: IdleNotifiedSet::new(),
        }
    }

    /// Returns the number of tasks currently in the `JoinSet`.
    pub fn len(&self) -> usize {
        self.inner.len()
    }

    /// Returns whether the `JoinSet` is empty.
    pub fn is_empty(&self) -> bool {
        self.inner.is_empty()
    }
}

impl<T: 'static> JoinSet<T> {
    /// Returns a [`Builder`] that can be used to configure a task prior to
    /// spawning it on this `JoinSet`.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::task::JoinSet;
    ///
    /// #[tokio::main]
    /// async fn main() -> std::io::Result<()> {
    ///     let mut set = JoinSet::new();
    ///
    ///     // Use the builder to configure a task's name before spawning it.
    ///     set.build_task()
    ///         .name("my_task")
    ///         .spawn(async { /* ... */ })?;
    ///
    ///     Ok(())
    /// }
    /// ```
    #[cfg(all(tokio_unstable, feature = "tracing"))]
    #[cfg_attr(docsrs, doc(cfg(all(tokio_unstable, feature = "tracing"))))]
    pub fn build_task(&mut self) -> Builder<'_, T> {
        Builder {
            builder: super::Builder::new(),
            joinset: self,
        }
    }

    /// Spawn the provided task on the `JoinSet`, returning an [`AbortHandle`]
    /// that can be used to remotely cancel the task.
    ///
    /// The provided future will start running in the background immediately
    /// when this method is called, even if you don't await anything on this
    /// `JoinSet`.
    ///
    /// # Panics
    ///
    /// This method panics if called outside of a Tokio runtime.
    ///
    /// [`AbortHandle`]: crate::task::AbortHandle
    #[track_caller]
    pub fn spawn<F>(&mut self, task: F) -> AbortHandle
    where
        F: Future<Output = T>,
        F: Send + 'static,
        T: Send,
    {
        self.insert(crate::spawn(task))
    }

    /// Spawn the provided task on the provided runtime and store it in this
    /// `JoinSet` returning an [`AbortHandle`] that can be used to remotely
    /// cancel the task.
    ///
    /// The provided future will start running in the background immediately
    /// when this method is called, even if you don't await anything on this
    /// `JoinSet`.
    ///
    /// [`AbortHandle`]: crate::task::AbortHandle
    #[track_caller]
    pub fn spawn_on<F>(&mut self, task: F, handle: &Handle) -> AbortHandle
    where
        F: Future<Output = T>,
        F: Send + 'static,
        T: Send,
    {
        self.insert(handle.spawn(task))
    }

    /// Spawn the provided task on the current [`LocalSet`] and store it in this
    /// `JoinSet`, returning an [`AbortHandle`] that can be used to remotely
    /// cancel the task.
    ///
    /// The provided future will start running in the background immediately
    /// when this method is called, even if you don't await anything on this
    /// `JoinSet`.
    ///
    /// # Panics
    ///
    /// This method panics if it is called outside of a `LocalSet`.
    ///
    /// [`LocalSet`]: crate::task::LocalSet
    /// [`AbortHandle`]: crate::task::AbortHandle
    #[track_caller]
    pub fn spawn_local<F>(&mut self, task: F) -> AbortHandle
    where
        F: Future<Output = T>,
        F: 'static,
    {
        self.insert(crate::task::spawn_local(task))
    }

    /// Spawn the provided task on the provided [`LocalSet`] and store it in
    /// this `JoinSet`, returning an [`AbortHandle`] that can be used to
    /// remotely cancel the task.
    ///
    /// Unlike the [`spawn_local`] method, this method may be used to spawn local
    /// tasks on a `LocalSet` that is _not_ currently running. The provided
    /// future will start running whenever the `LocalSet` is next started.
    ///
    /// [`LocalSet`]: crate::task::LocalSet
    /// [`AbortHandle`]: crate::task::AbortHandle
    /// [`spawn_local`]: Self::spawn_local
    #[track_caller]
    pub fn spawn_local_on<F>(&mut self, task: F, local_set: &LocalSet) -> AbortHandle
    where
        F: Future<Output = T>,
        F: 'static,
    {
        self.insert(local_set.spawn_local(task))
    }

    /// Spawn the blocking code on the blocking threadpool and store
    /// it in this `JoinSet`, returning an [`AbortHandle`] that can be
    /// used to remotely cancel the task.
    ///
    /// # Examples
    ///
    /// Spawn multiple blocking tasks and wait for them.
    ///
    /// ```
    /// use tokio::task::JoinSet;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let mut set = JoinSet::new();
    ///
    ///     for i in 0..10 {
    ///         set.spawn_blocking(move || { i });
    ///     }
    ///
    ///     let mut seen = [false; 10];
    ///     while let Some(res) = set.join_next().await {
    ///         let idx = res.unwrap();
    ///         seen[idx] = true;
    ///     }
    ///
    ///     for i in 0..10 {
    ///         assert!(seen[i]);
    ///     }
    /// }
    /// ```
    ///
    /// # Panics
    ///
    /// This method panics if called outside of a Tokio runtime.
    ///
    /// [`AbortHandle`]: crate::task::AbortHandle
    #[track_caller]
    pub fn spawn_blocking<F>(&mut self, f: F) -> AbortHandle
    where
        F: FnOnce() -> T,
        F: Send + 'static,
        T: Send,
    {
        self.insert(crate::runtime::spawn_blocking(f))
    }

    /// Spawn the blocking code on the blocking threadpool of the
    /// provided runtime and store it in this `JoinSet`, returning an
    /// [`AbortHandle`] that can be used to remotely cancel the task.
    ///
    /// [`AbortHandle`]: crate::task::AbortHandle
    #[track_caller]
    pub fn spawn_blocking_on<F>(&mut self, f: F, handle: &Handle) -> AbortHandle
    where
        F: FnOnce() -> T,
        F: Send + 'static,
        T: Send,
    {
        self.insert(handle.spawn_blocking(f))
    }

    fn insert(&mut self, jh: JoinHandle<T>) -> AbortHandle {
        let abort = jh.abort_handle();
        let mut entry = self.inner.insert_idle(jh);

        // Set the waker that is notified when the task completes.
        entry.with_value_and_context(|jh, ctx| jh.set_join_waker(ctx.waker()));
        abort
    }

    /// Waits until one of the tasks in the set completes and returns its output.
    ///
    /// Returns `None` if the set is empty.
    ///
    /// # Cancel Safety
    ///
    /// This method is cancel safe. If `join_next` is used as the event in a `tokio::select!`
    /// statement and some other branch completes first, it is guaranteed that no tasks were
    /// removed from this `JoinSet`.
    pub async fn join_next(&mut self) -> Option<Result<T, JoinError>> {
        crate::future::poll_fn(|cx| self.poll_join_next(cx)).await
    }

    /// Waits until one of the tasks in the set completes and returns its
    /// output, along with the [task ID] of the completed task.
    ///
    /// Returns `None` if the set is empty.
    ///
    /// When this method returns an error, then the id of the task that failed can be accessed
    /// using the [`JoinError::id`] method.
    ///
    /// # Cancel Safety
    ///
    /// This method is cancel safe. If `join_next_with_id` is used as the event in a `tokio::select!`
    /// statement and some other branch completes first, it is guaranteed that no tasks were
    /// removed from this `JoinSet`.
    ///
    /// [task ID]: crate::task::Id
    /// [`JoinError::id`]: fn@crate::task::JoinError::id
    #[cfg(tokio_unstable)]
    #[cfg_attr(docsrs, doc(cfg(tokio_unstable)))]
    pub async fn join_next_with_id(&mut self) -> Option<Result<(Id, T), JoinError>> {
        crate::future::poll_fn(|cx| self.poll_join_next_with_id(cx)).await
    }

    /// Aborts all tasks and waits for them to finish shutting down.
    ///
    /// Calling this method is equivalent to calling [`abort_all`] and then calling [`join_next`] in
    /// a loop until it returns `None`.
    ///
    /// This method ignores any panics in the tasks shutting down. When this call returns, the
    /// `JoinSet` will be empty.
    ///
    /// [`abort_all`]: fn@Self::abort_all
    /// [`join_next`]: fn@Self::join_next
    pub async fn shutdown(&mut self) {
        self.abort_all();
        while self.join_next().await.is_some() {}
    }

    /// Aborts all tasks on this `JoinSet`.
    ///
    /// This does not remove the tasks from the `JoinSet`. To wait for the tasks to complete
    /// cancellation, you should call `join_next` in a loop until the `JoinSet` is empty.
    pub fn abort_all(&mut self) {
        self.inner.for_each(|jh| jh.abort());
    }

    /// Removes all tasks from this `JoinSet` without aborting them.
    ///
    /// The tasks removed by this call will continue to run in the background even if the `JoinSet`
    /// is dropped.
    pub fn detach_all(&mut self) {
        self.inner.drain(drop);
    }

    /// Polls for one of the tasks in the set to complete.
    ///
    /// If this returns `Poll::Ready(Some(_))`, then the task that completed is removed from the set.
    ///
    /// When the method returns `Poll::Pending`, the `Waker` in the provided `Context` is scheduled
    /// to receive a wakeup when a task in the `JoinSet` completes. Note that on multiple calls to
    /// `poll_join_next`, only the `Waker` from the `Context` passed to the most recent call is
    /// scheduled to receive a wakeup.
    ///
    /// # Returns
    ///
    /// This function returns:
    ///
    ///  * `Poll::Pending` if the `JoinSet` is not empty but there is no task whose output is
    ///     available right now.
    ///  * `Poll::Ready(Some(Ok(value)))` if one of the tasks in this `JoinSet` has completed.
    ///     The `value` is the return value of one of the tasks that completed.
    ///  * `Poll::Ready(Some(Err(err)))` if one of the tasks in this `JoinSet` has panicked or been
    ///     aborted. The `err` is the `JoinError` from the panicked/aborted task.
    ///  * `Poll::Ready(None)` if the `JoinSet` is empty.
    ///
    /// Note that this method may return `Poll::Pending` even if one of the tasks has completed.
    /// This can happen if the [coop budget] is reached.
    ///
    /// [coop budget]: crate::task#cooperative-scheduling
    fn poll_join_next(&mut self, cx: &mut Context<'_>) -> Poll<Option<Result<T, JoinError>>> {
        // The call to `pop_notified` moves the entry to the `idle` list. It is moved back to
        // the `notified` list if the waker is notified in the `poll` call below.
        let mut entry = match self.inner.pop_notified(cx.waker()) {
            Some(entry) => entry,
            None => {
                if self.is_empty() {
                    return Poll::Ready(None);
                } else {
                    // The waker was set by `pop_notified`.
                    return Poll::Pending;
                }
            }
        };

        let res = entry.with_value_and_context(|jh, ctx| Pin::new(jh).poll(ctx));

        if let Poll::Ready(res) = res {
            let _entry = entry.remove();
            Poll::Ready(Some(res))
        } else {
            // A JoinHandle generally won't emit a wakeup without being ready unless
            // the coop limit has been reached. We yield to the executor in this
            // case.
            cx.waker().wake_by_ref();
            Poll::Pending
        }
    }

    /// Polls for one of the tasks in the set to complete.
    ///
    /// If this returns `Poll::Ready(Some(_))`, then the task that completed is removed from the set.
    ///
    /// When the method returns `Poll::Pending`, the `Waker` in the provided `Context` is scheduled
    /// to receive a wakeup when a task in the `JoinSet` completes. Note that on multiple calls to
    /// `poll_join_next`, only the `Waker` from the `Context` passed to the most recent call is
    /// scheduled to receive a wakeup.
    ///
    /// # Returns
    ///
    /// This function returns:
    ///
    ///  * `Poll::Pending` if the `JoinSet` is not empty but there is no task whose output is
    ///     available right now.
    ///  * `Poll::Ready(Some(Ok((id, value))))` if one of the tasks in this `JoinSet` has completed.
    ///     The `value` is the return value of one of the tasks that completed, and
    ///    `id` is the [task ID] of that task.
    ///  * `Poll::Ready(Some(Err(err)))` if one of the tasks in this `JoinSet` has panicked or been
    ///     aborted. The `err` is the `JoinError` from the panicked/aborted task.
    ///  * `Poll::Ready(None)` if the `JoinSet` is empty.
    ///
    /// Note that this method may return `Poll::Pending` even if one of the tasks has completed.
    /// This can happen if the [coop budget] is reached.
    ///
    /// [coop budget]: crate::task#cooperative-scheduling
    /// [task ID]: crate::task::Id
    #[cfg(tokio_unstable)]
    fn poll_join_next_with_id(
        &mut self,
        cx: &mut Context<'_>,
    ) -> Poll<Option<Result<(Id, T), JoinError>>> {
        // The call to `pop_notified` moves the entry to the `idle` list. It is moved back to
        // the `notified` list if the waker is notified in the `poll` call below.
        let mut entry = match self.inner.pop_notified(cx.waker()) {
            Some(entry) => entry,
            None => {
                if self.is_empty() {
                    return Poll::Ready(None);
                } else {
                    // The waker was set by `pop_notified`.
                    return Poll::Pending;
                }
            }
        };

        let res = entry.with_value_and_context(|jh, ctx| Pin::new(jh).poll(ctx));

        if let Poll::Ready(res) = res {
            let entry = entry.remove();
            // If the task succeeded, add the task ID to the output. Otherwise, the
            // `JoinError` will already have the task's ID.
            Poll::Ready(Some(res.map(|output| (entry.id(), output))))
        } else {
            // A JoinHandle generally won't emit a wakeup without being ready unless
            // the coop limit has been reached. We yield to the executor in this
            // case.
            cx.waker().wake_by_ref();
            Poll::Pending
        }
    }
}

impl<T> Drop for JoinSet<T> {
    fn drop(&mut self) {
        self.inner.drain(|join_handle| join_handle.abort());
    }
}

impl<T> fmt::Debug for JoinSet<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("JoinSet").field("len", &self.len()).finish()
    }
}

impl<T> Default for JoinSet<T> {
    fn default() -> Self {
        Self::new()
    }
}

// === impl Builder ===

#[cfg(all(tokio_unstable, feature = "tracing"))]
#[cfg_attr(docsrs, doc(cfg(all(tokio_unstable, feature = "tracing"))))]
impl<'a, T: 'static> Builder<'a, T> {
    /// Assigns a name to the task which will be spawned.
    pub fn name(self, name: &'a str) -> Self {
        let builder = self.builder.name(name);
        Self { builder, ..self }
    }

    /// Spawn the provided task with this builder's settings and store it in the
    /// [`JoinSet`], returning an [`AbortHandle`] that can be used to remotely
    /// cancel the task.
    ///
    /// # Returns
    ///
    /// An [`AbortHandle`] that can be used to remotely cancel the task.
    ///
    /// # Panics
    ///
    /// This method panics if called outside of a Tokio runtime.
    ///
    /// [`AbortHandle`]: crate::task::AbortHandle
    #[track_caller]
    pub fn spawn<F>(self, future: F) -> std::io::Result<AbortHandle>
    where
        F: Future<Output = T>,
        F: Send + 'static,
        T: Send,
    {
        Ok(self.joinset.insert(self.builder.spawn(future)?))
    }

    /// Spawn the provided task on the provided [runtime handle] with this
    /// builder's settings, and store it in the [`JoinSet`].
    ///
    /// # Returns
    ///
    /// An [`AbortHandle`] that can be used to remotely cancel the task.
    ///
    ///
    /// [`AbortHandle`]: crate::task::AbortHandle
    /// [runtime handle]: crate::runtime::Handle
    #[track_caller]
    pub fn spawn_on<F>(self, future: F, handle: &Handle) -> std::io::Result<AbortHandle>
    where
        F: Future<Output = T>,
        F: Send + 'static,
        T: Send,
    {
        Ok(self.joinset.insert(self.builder.spawn_on(future, handle)?))
    }

    /// Spawn the provided task on the current [`LocalSet`] with this builder's
    /// settings, and store it in the [`JoinSet`].
    ///
    /// # Returns
    ///
    /// An [`AbortHandle`] that can be used to remotely cancel the task.
    ///
    /// # Panics
    ///
    /// This method panics if it is called outside of a `LocalSet`.
    ///
    /// [`LocalSet`]: crate::task::LocalSet
    /// [`AbortHandle`]: crate::task::AbortHandle
    #[track_caller]
    pub fn spawn_local<F>(self, future: F) -> std::io::Result<AbortHandle>
    where
        F: Future<Output = T>,
        F: 'static,
    {
        Ok(self.joinset.insert(self.builder.spawn_local(future)?))
    }

    /// Spawn the provided task on the provided [`LocalSet`] with this builder's
    /// settings, and store it in the [`JoinSet`].
    ///
    /// # Returns
    ///
    /// An [`AbortHandle`] that can be used to remotely cancel the task.
    ///
    /// [`LocalSet`]: crate::task::LocalSet
    /// [`AbortHandle`]: crate::task::AbortHandle
    #[track_caller]
    pub fn spawn_local_on<F>(self, future: F, local_set: &LocalSet) -> std::io::Result<AbortHandle>
    where
        F: Future<Output = T>,
        F: 'static,
    {
        Ok(self
            .joinset
            .insert(self.builder.spawn_local_on(future, local_set)?))
    }
}

// Manual `Debug` impl so that `Builder` is `Debug` regardless of whether `T` is
// `Debug`.
#[cfg(all(tokio_unstable, feature = "tracing"))]
#[cfg_attr(docsrs, doc(cfg(all(tokio_unstable, feature = "tracing"))))]
impl<'a, T> fmt::Debug for Builder<'a, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("join_set::Builder")
            .field("joinset", &self.joinset)
            .field("builder", &self.builder)
            .finish()
    }
}