tokio/runtime/blocking/
pool.rs

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
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
//! Thread pool for blocking operations

use crate::loom::sync::{Arc, Condvar, Mutex};
use crate::loom::thread;
use crate::runtime::blocking::schedule::BlockingSchedule;
use crate::runtime::blocking::{shutdown, BlockingTask};
use crate::runtime::builder::ThreadNameFn;
use crate::runtime::task::{self, JoinHandle};
use crate::runtime::{Builder, Callback, Handle, BOX_FUTURE_THRESHOLD};
use crate::util::metric_atomics::MetricAtomicUsize;
use crate::util::trace::{blocking_task, SpawnMeta};

use std::collections::{HashMap, VecDeque};
use std::fmt;
use std::io;
use std::sync::atomic::Ordering;
use std::time::Duration;

pub(crate) struct BlockingPool {
    spawner: Spawner,
    shutdown_rx: shutdown::Receiver,
}

#[derive(Clone)]
pub(crate) struct Spawner {
    inner: Arc<Inner>,
}

#[derive(Default)]
pub(crate) struct SpawnerMetrics {
    num_threads: MetricAtomicUsize,
    num_idle_threads: MetricAtomicUsize,
    queue_depth: MetricAtomicUsize,
}

impl SpawnerMetrics {
    fn num_threads(&self) -> usize {
        self.num_threads.load(Ordering::Relaxed)
    }

    fn num_idle_threads(&self) -> usize {
        self.num_idle_threads.load(Ordering::Relaxed)
    }

    cfg_unstable_metrics! {
        fn queue_depth(&self) -> usize {
            self.queue_depth.load(Ordering::Relaxed)
        }
    }

    fn inc_num_threads(&self) {
        self.num_threads.increment();
    }

    fn dec_num_threads(&self) {
        self.num_threads.decrement();
    }

    fn inc_num_idle_threads(&self) {
        self.num_idle_threads.increment();
    }

    fn dec_num_idle_threads(&self) -> usize {
        self.num_idle_threads.decrement()
    }

    fn inc_queue_depth(&self) {
        self.queue_depth.increment();
    }

    fn dec_queue_depth(&self) {
        self.queue_depth.decrement();
    }
}

struct Inner {
    /// State shared between worker threads.
    shared: Mutex<Shared>,

    /// Pool threads wait on this.
    condvar: Condvar,

    /// Spawned threads use this name.
    thread_name: ThreadNameFn,

    /// Spawned thread stack size.
    stack_size: Option<usize>,

    /// Call after a thread starts.
    after_start: Option<Callback>,

    /// Call before a thread stops.
    before_stop: Option<Callback>,

    // Maximum number of threads.
    thread_cap: usize,

    // Customizable wait timeout.
    keep_alive: Duration,

    // Metrics about the pool.
    metrics: SpawnerMetrics,
}

struct Shared {
    queue: VecDeque<Task>,
    num_notify: u32,
    shutdown: bool,
    shutdown_tx: Option<shutdown::Sender>,
    /// Prior to shutdown, we clean up `JoinHandles` by having each timed-out
    /// thread join on the previous timed-out thread. This is not strictly
    /// necessary but helps avoid Valgrind false positives, see
    /// <https://github.com/tokio-rs/tokio/commit/646fbae76535e397ef79dbcaacb945d4c829f666>
    /// for more information.
    last_exiting_thread: Option<thread::JoinHandle<()>>,
    /// This holds the `JoinHandles` for all running threads; on shutdown, the thread
    /// calling shutdown handles joining on these.
    worker_threads: HashMap<usize, thread::JoinHandle<()>>,
    /// This is a counter used to iterate `worker_threads` in a consistent order (for loom's
    /// benefit).
    worker_thread_index: usize,
}

pub(crate) struct Task {
    task: task::UnownedTask<BlockingSchedule>,
    mandatory: Mandatory,
}

#[derive(PartialEq, Eq)]
pub(crate) enum Mandatory {
    #[cfg_attr(not(fs), allow(dead_code))]
    Mandatory,
    NonMandatory,
}

pub(crate) enum SpawnError {
    /// Pool is shutting down and the task was not scheduled
    ShuttingDown,
    /// There are no worker threads available to take the task
    /// and the OS failed to spawn a new one
    NoThreads(io::Error),
}

impl From<SpawnError> for io::Error {
    fn from(e: SpawnError) -> Self {
        match e {
            SpawnError::ShuttingDown => {
                io::Error::new(io::ErrorKind::Other, "blocking pool shutting down")
            }
            SpawnError::NoThreads(e) => e,
        }
    }
}

impl Task {
    pub(crate) fn new(task: task::UnownedTask<BlockingSchedule>, mandatory: Mandatory) -> Task {
        Task { task, mandatory }
    }

    fn run(self) {
        self.task.run();
    }

    fn shutdown_or_run_if_mandatory(self) {
        match self.mandatory {
            Mandatory::NonMandatory => self.task.shutdown(),
            Mandatory::Mandatory => self.task.run(),
        }
    }
}

const KEEP_ALIVE: Duration = Duration::from_secs(10);

/// Runs the provided function on an executor dedicated to blocking operations.
/// Tasks will be scheduled as non-mandatory, meaning they may not get executed
/// in case of runtime shutdown.
#[track_caller]
#[cfg_attr(target_os = "wasi", allow(dead_code))]
pub(crate) fn spawn_blocking<F, R>(func: F) -> JoinHandle<R>
where
    F: FnOnce() -> R + Send + 'static,
    R: Send + 'static,
{
    let rt = Handle::current();
    rt.spawn_blocking(func)
}

cfg_fs! {
    #[cfg_attr(any(
        all(loom, not(test)), // the function is covered by loom tests
        test
    ), allow(dead_code))]
    /// Runs the provided function on an executor dedicated to blocking
    /// operations. Tasks will be scheduled as mandatory, meaning they are
    /// guaranteed to run unless a shutdown is already taking place. In case a
    /// shutdown is already taking place, `None` will be returned.
    pub(crate) fn spawn_mandatory_blocking<F, R>(func: F) -> Option<JoinHandle<R>>
    where
        F: FnOnce() -> R + Send + 'static,
        R: Send + 'static,
    {
        let rt = Handle::current();
        rt.inner.blocking_spawner().spawn_mandatory_blocking(&rt, func)
    }
}

// ===== impl BlockingPool =====

impl BlockingPool {
    pub(crate) fn new(builder: &Builder, thread_cap: usize) -> BlockingPool {
        let (shutdown_tx, shutdown_rx) = shutdown::channel();
        let keep_alive = builder.keep_alive.unwrap_or(KEEP_ALIVE);

        BlockingPool {
            spawner: Spawner {
                inner: Arc::new(Inner {
                    shared: Mutex::new(Shared {
                        queue: VecDeque::new(),
                        num_notify: 0,
                        shutdown: false,
                        shutdown_tx: Some(shutdown_tx),
                        last_exiting_thread: None,
                        worker_threads: HashMap::new(),
                        worker_thread_index: 0,
                    }),
                    condvar: Condvar::new(),
                    thread_name: builder.thread_name.clone(),
                    stack_size: builder.thread_stack_size,
                    after_start: builder.after_start.clone(),
                    before_stop: builder.before_stop.clone(),
                    thread_cap,
                    keep_alive,
                    metrics: SpawnerMetrics::default(),
                }),
            },
            shutdown_rx,
        }
    }

    pub(crate) fn spawner(&self) -> &Spawner {
        &self.spawner
    }

    pub(crate) fn shutdown(&mut self, timeout: Option<Duration>) {
        let mut shared = self.spawner.inner.shared.lock();

        // The function can be called multiple times. First, by explicitly
        // calling `shutdown` then by the drop handler calling `shutdown`. This
        // prevents shutting down twice.
        if shared.shutdown {
            return;
        }

        shared.shutdown = true;
        shared.shutdown_tx = None;
        self.spawner.inner.condvar.notify_all();

        let last_exited_thread = std::mem::take(&mut shared.last_exiting_thread);
        let workers = std::mem::take(&mut shared.worker_threads);

        drop(shared);

        if self.shutdown_rx.wait(timeout) {
            let _ = last_exited_thread.map(thread::JoinHandle::join);

            // Loom requires that execution be deterministic, so sort by thread ID before joining.
            // (HashMaps use a randomly-seeded hash function, so the order is nondeterministic)
            #[cfg(loom)]
            let workers: Vec<(usize, thread::JoinHandle<()>)> = {
                let mut workers: Vec<_> = workers.into_iter().collect();
                workers.sort_by_key(|(id, _)| *id);
                workers
            };

            for (_id, handle) in workers {
                let _ = handle.join();
            }
        }
    }
}

impl Drop for BlockingPool {
    fn drop(&mut self) {
        self.shutdown(None);
    }
}

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

// ===== impl Spawner =====

impl Spawner {
    #[track_caller]
    pub(crate) fn spawn_blocking<F, R>(&self, rt: &Handle, func: F) -> JoinHandle<R>
    where
        F: FnOnce() -> R + Send + 'static,
        R: Send + 'static,
    {
        let fn_size = std::mem::size_of::<F>();
        let (join_handle, spawn_result) = if fn_size > BOX_FUTURE_THRESHOLD {
            self.spawn_blocking_inner(
                Box::new(func),
                Mandatory::NonMandatory,
                SpawnMeta::new_unnamed(fn_size),
                rt,
            )
        } else {
            self.spawn_blocking_inner(
                func,
                Mandatory::NonMandatory,
                SpawnMeta::new_unnamed(fn_size),
                rt,
            )
        };

        match spawn_result {
            Ok(()) => join_handle,
            // Compat: do not panic here, return the join_handle even though it will never resolve
            Err(SpawnError::ShuttingDown) => join_handle,
            Err(SpawnError::NoThreads(e)) => {
                panic!("OS can't spawn worker thread: {}", e)
            }
        }
    }

    cfg_fs! {
        #[track_caller]
        #[cfg_attr(any(
            all(loom, not(test)), // the function is covered by loom tests
            test
        ), allow(dead_code))]
        pub(crate) fn spawn_mandatory_blocking<F, R>(&self, rt: &Handle, func: F) -> Option<JoinHandle<R>>
        where
            F: FnOnce() -> R + Send + 'static,
            R: Send + 'static,
        {
            let fn_size = std::mem::size_of::<F>();
            let (join_handle, spawn_result) = if fn_size > BOX_FUTURE_THRESHOLD {
                self.spawn_blocking_inner(
                    Box::new(func),
                    Mandatory::Mandatory,
                    SpawnMeta::new_unnamed(fn_size),
                    rt,
                )
            } else {
                self.spawn_blocking_inner(
                    func,
                    Mandatory::Mandatory,
                    SpawnMeta::new_unnamed(fn_size),
                    rt,
                )
            };

            if spawn_result.is_ok() {
                Some(join_handle)
            } else {
                None
            }
        }
    }

    #[track_caller]
    pub(crate) fn spawn_blocking_inner<F, R>(
        &self,
        func: F,
        is_mandatory: Mandatory,
        spawn_meta: SpawnMeta<'_>,
        rt: &Handle,
    ) -> (JoinHandle<R>, Result<(), SpawnError>)
    where
        F: FnOnce() -> R + Send + 'static,
        R: Send + 'static,
    {
        let id = task::Id::next();
        let fut =
            blocking_task::<F, BlockingTask<F>>(BlockingTask::new(func), spawn_meta, id.as_u64());

        let (task, handle) = task::unowned(fut, BlockingSchedule::new(rt), id);

        let spawned = self.spawn_task(Task::new(task, is_mandatory), rt);
        (handle, spawned)
    }

    fn spawn_task(&self, task: Task, rt: &Handle) -> Result<(), SpawnError> {
        let mut shared = self.inner.shared.lock();

        if shared.shutdown {
            // Shutdown the task: it's fine to shutdown this task (even if
            // mandatory) because it was scheduled after the shutdown of the
            // runtime began.
            task.task.shutdown();

            // no need to even push this task; it would never get picked up
            return Err(SpawnError::ShuttingDown);
        }

        shared.queue.push_back(task);
        self.inner.metrics.inc_queue_depth();

        if self.inner.metrics.num_idle_threads() == 0 {
            // No threads are able to process the task.

            if self.inner.metrics.num_threads() == self.inner.thread_cap {
                // At max number of threads
            } else {
                assert!(shared.shutdown_tx.is_some());
                let shutdown_tx = shared.shutdown_tx.clone();

                if let Some(shutdown_tx) = shutdown_tx {
                    let id = shared.worker_thread_index;

                    match self.spawn_thread(shutdown_tx, rt, id) {
                        Ok(handle) => {
                            self.inner.metrics.inc_num_threads();
                            shared.worker_thread_index += 1;
                            shared.worker_threads.insert(id, handle);
                        }
                        Err(ref e)
                            if is_temporary_os_thread_error(e)
                                && self.inner.metrics.num_threads() > 0 =>
                        {
                            // OS temporarily failed to spawn a new thread.
                            // The task will be picked up eventually by a currently
                            // busy thread.
                        }
                        Err(e) => {
                            // The OS refused to spawn the thread and there is no thread
                            // to pick up the task that has just been pushed to the queue.
                            return Err(SpawnError::NoThreads(e));
                        }
                    }
                }
            }
        } else {
            // Notify an idle worker thread. The notification counter
            // is used to count the needed amount of notifications
            // exactly. Thread libraries may generate spurious
            // wakeups, this counter is used to keep us in a
            // consistent state.
            self.inner.metrics.dec_num_idle_threads();
            shared.num_notify += 1;
            self.inner.condvar.notify_one();
        }

        Ok(())
    }

    fn spawn_thread(
        &self,
        shutdown_tx: shutdown::Sender,
        rt: &Handle,
        id: usize,
    ) -> io::Result<thread::JoinHandle<()>> {
        let mut builder = thread::Builder::new().name((self.inner.thread_name)());

        if let Some(stack_size) = self.inner.stack_size {
            builder = builder.stack_size(stack_size);
        }

        let rt = rt.clone();

        builder.spawn(move || {
            // Only the reference should be moved into the closure
            let _enter = rt.enter();
            rt.inner.blocking_spawner().inner.run(id);
            drop(shutdown_tx);
        })
    }
}

cfg_unstable_metrics! {
    impl Spawner {
        pub(crate) fn num_threads(&self) -> usize {
            self.inner.metrics.num_threads()
        }

        pub(crate) fn num_idle_threads(&self) -> usize {
            self.inner.metrics.num_idle_threads()
        }

        pub(crate) fn queue_depth(&self) -> usize {
            self.inner.metrics.queue_depth()
        }
    }
}

// Tells whether the error when spawning a thread is temporary.
#[inline]
fn is_temporary_os_thread_error(error: &io::Error) -> bool {
    matches!(error.kind(), io::ErrorKind::WouldBlock)
}

impl Inner {
    fn run(&self, worker_thread_id: usize) {
        if let Some(f) = &self.after_start {
            f();
        }

        let mut shared = self.shared.lock();
        let mut join_on_thread = None;

        'main: loop {
            // BUSY
            while let Some(task) = shared.queue.pop_front() {
                self.metrics.dec_queue_depth();
                drop(shared);
                task.run();

                shared = self.shared.lock();
            }

            // IDLE
            self.metrics.inc_num_idle_threads();

            while !shared.shutdown {
                let lock_result = self.condvar.wait_timeout(shared, self.keep_alive).unwrap();

                shared = lock_result.0;
                let timeout_result = lock_result.1;

                if shared.num_notify != 0 {
                    // We have received a legitimate wakeup,
                    // acknowledge it by decrementing the counter
                    // and transition to the BUSY state.
                    shared.num_notify -= 1;
                    break;
                }

                // Even if the condvar "timed out", if the pool is entering the
                // shutdown phase, we want to perform the cleanup logic.
                if !shared.shutdown && timeout_result.timed_out() {
                    // We'll join the prior timed-out thread's JoinHandle after dropping the lock.
                    // This isn't done when shutting down, because the thread calling shutdown will
                    // handle joining everything.
                    let my_handle = shared.worker_threads.remove(&worker_thread_id);
                    join_on_thread = std::mem::replace(&mut shared.last_exiting_thread, my_handle);

                    break 'main;
                }

                // Spurious wakeup detected, go back to sleep.
            }

            if shared.shutdown {
                // Drain the queue
                while let Some(task) = shared.queue.pop_front() {
                    self.metrics.dec_queue_depth();
                    drop(shared);

                    task.shutdown_or_run_if_mandatory();

                    shared = self.shared.lock();
                }

                // Work was produced, and we "took" it (by decrementing num_notify).
                // This means that num_idle was decremented once for our wakeup.
                // But, since we are exiting, we need to "undo" that, as we'll stay idle.
                self.metrics.inc_num_idle_threads();
                // NOTE: Technically we should also do num_notify++ and notify again,
                // but since we're shutting down anyway, that won't be necessary.
                break;
            }
        }

        // Thread exit
        self.metrics.dec_num_threads();

        // num_idle should now be tracked exactly, panic
        // with a descriptive message if it is not the
        // case.
        let prev_idle = self.metrics.dec_num_idle_threads();
        assert!(
            prev_idle >= self.metrics.num_idle_threads(),
            "num_idle_threads underflowed on thread exit"
        );

        if shared.shutdown && self.metrics.num_threads() == 0 {
            self.condvar.notify_one();
        }

        drop(shared);

        if let Some(f) = &self.before_stop {
            f();
        }

        if let Some(handle) = join_on_thread {
            let _ = handle.join();
        }
    }
}

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