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
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
//! Timer state structures.
//!
//! This module contains the heart of the intrusive timer implementation, and as
//! such the structures inside are full of tricky concurrency and unsafe code.
//!
//! # Ground rules
//!
//! The heart of the timer implementation here is the [`TimerShared`] structure,
//! shared between the [`TimerEntry`] and the driver. Generally, we permit access
//! to [`TimerShared`] ONLY via either 1) a mutable reference to [`TimerEntry`] or
//! 2) a held driver lock.
//!
//! It follows from this that any changes made while holding BOTH 1 and 2 will
//! be reliably visible, regardless of ordering. This is because of the `acq/rel`
//! fences on the driver lock ensuring ordering with 2, and rust mutable
//! reference rules for 1 (a mutable reference to an object can't be passed
//! between threads without an `acq/rel` barrier, and same-thread we have local
//! happens-before ordering).
//!
//! # State field
//!
//! Each timer has a state field associated with it. This field contains either
//! the current scheduled time, or a special flag value indicating its state.
//! This state can either indicate that the timer is on the 'pending' queue (and
//! thus will be fired with an `Ok(())` result soon) or that it has already been
//! fired/deregistered.
//!
//! This single state field allows for code that is firing the timer to
//! synchronize with any racing `reset` calls reliably.
//!
//! # Cached vs true timeouts
//!
//! To allow for the use case of a timeout that is periodically reset before
//! expiration to be as lightweight as possible, we support optimistically
//! lock-free timer resets, in the case where a timer is rescheduled to a later
//! point than it was originally scheduled for.
//!
//! This is accomplished by lazily rescheduling timers. That is, we update the
//! state field with the true expiration of the timer from the holder of
//! the [`TimerEntry`]. When the driver services timers (ie, whenever it's
//! walking lists of timers), it checks this "true when" value, and reschedules
//! based on it.
//!
//! We do, however, also need to track what the expiration time was when we
//! originally registered the timer; this is used to locate the right linked
//! list when the timer is being cancelled. This is referred to as the "cached
//! when" internally.
//!
//! There is of course a race condition between timer reset and timer
//! expiration. If the driver fails to observe the updated expiration time, it
//! could trigger expiration of the timer too early. However, because
//! [`mark_pending`][mark_pending] performs a compare-and-swap, it will identify this race and
//! refuse to mark the timer as pending.
//!
//! [mark_pending]: TimerHandle::mark_pending

use crate::loom::cell::UnsafeCell;
use crate::loom::sync::atomic::AtomicU64;
use crate::loom::sync::atomic::Ordering;

use crate::runtime::context;
use crate::runtime::scheduler;
use crate::sync::AtomicWaker;
use crate::time::Instant;
use crate::util::linked_list;

use std::cell::UnsafeCell as StdUnsafeCell;
use std::task::{Context, Poll, Waker};
use std::{marker::PhantomPinned, pin::Pin, ptr::NonNull};

type TimerResult = Result<(), crate::time::error::Error>;

const STATE_DEREGISTERED: u64 = u64::MAX;
const STATE_PENDING_FIRE: u64 = STATE_DEREGISTERED - 1;
const STATE_MIN_VALUE: u64 = STATE_PENDING_FIRE;
/// The largest safe integer to use for ticks.
///
/// This value should be updated if any other signal values are added above.
pub(super) const MAX_SAFE_MILLIS_DURATION: u64 = STATE_MIN_VALUE - 1;

/// This structure holds the current shared state of the timer - its scheduled
/// time (if registered), or otherwise the result of the timer completing, as
/// well as the registered waker.
///
/// Generally, the `StateCell` is only permitted to be accessed from two contexts:
/// Either a thread holding the corresponding `&mut TimerEntry`, or a thread
/// holding the timer driver lock. The write actions on the `StateCell` amount to
/// passing "ownership" of the `StateCell` between these contexts; moving a timer
/// from the `TimerEntry` to the driver requires _both_ holding the `&mut
/// TimerEntry` and the driver lock, while moving it back (firing the timer)
/// requires only the driver lock.
pub(super) struct StateCell {
    /// Holds either the scheduled expiration time for this timer, or (if the
    /// timer has been fired and is unregistered), `u64::MAX`.
    state: AtomicU64,
    /// If the timer is fired (an Acquire order read on state shows
    /// `u64::MAX`), holds the result that should be returned from
    /// polling the timer. Otherwise, the contents are unspecified and reading
    /// without holding the driver lock is undefined behavior.
    result: UnsafeCell<TimerResult>,
    /// The currently-registered waker
    waker: AtomicWaker,
}

impl Default for StateCell {
    fn default() -> Self {
        Self::new()
    }
}

impl std::fmt::Debug for StateCell {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "StateCell({:?})", self.read_state())
    }
}

impl StateCell {
    fn new() -> Self {
        Self {
            state: AtomicU64::new(STATE_DEREGISTERED),
            result: UnsafeCell::new(Ok(())),
            waker: AtomicWaker::new(),
        }
    }

    fn is_pending(&self) -> bool {
        self.state.load(Ordering::Relaxed) == STATE_PENDING_FIRE
    }

    /// Returns the current expiration time, or None if not currently scheduled.
    fn when(&self) -> Option<u64> {
        let cur_state = self.state.load(Ordering::Relaxed);

        if cur_state == STATE_DEREGISTERED {
            None
        } else {
            Some(cur_state)
        }
    }

    /// If the timer is completed, returns the result of the timer. Otherwise,
    /// returns None and registers the waker.
    fn poll(&self, waker: &Waker) -> Poll<TimerResult> {
        // We must register first. This ensures that either `fire` will
        // observe the new waker, or we will observe a racing fire to have set
        // the state, or both.
        self.waker.register_by_ref(waker);

        self.read_state()
    }

    fn read_state(&self) -> Poll<TimerResult> {
        let cur_state = self.state.load(Ordering::Acquire);

        if cur_state == STATE_DEREGISTERED {
            // SAFETY: The driver has fired this timer; this involves writing
            // the result, and then writing (with release ordering) the state
            // field.
            Poll::Ready(unsafe { self.result.with(|p| *p) })
        } else {
            Poll::Pending
        }
    }

    /// Marks this timer as being moved to the pending list, if its scheduled
    /// time is not after `not_after`.
    ///
    /// If the timer is scheduled for a time after `not_after`, returns an Err
    /// containing the current scheduled time.
    ///
    /// SAFETY: Must hold the driver lock.
    unsafe fn mark_pending(&self, not_after: u64) -> Result<(), u64> {
        // Quick initial debug check to see if the timer is already fired. Since
        // firing the timer can only happen with the driver lock held, we know
        // we shouldn't be able to "miss" a transition to a fired state, even
        // with relaxed ordering.
        let mut cur_state = self.state.load(Ordering::Relaxed);

        loop {
            // improve the error message for things like
            // https://github.com/tokio-rs/tokio/issues/3675
            assert!(
                cur_state < STATE_MIN_VALUE,
                "mark_pending called when the timer entry is in an invalid state"
            );

            if cur_state > not_after {
                break Err(cur_state);
            }

            match self.state.compare_exchange_weak(
                cur_state,
                STATE_PENDING_FIRE,
                Ordering::AcqRel,
                Ordering::Acquire,
            ) {
                Ok(_) => break Ok(()),
                Err(actual_state) => cur_state = actual_state,
            }
        }
    }

    /// Fires the timer, setting the result to the provided result.
    ///
    /// Returns:
    /// * `Some(waker)` - if fired and a waker needs to be invoked once the
    ///   driver lock is released
    /// * `None` - if fired and a waker does not need to be invoked, or if
    ///   already fired
    ///
    /// SAFETY: The driver lock must be held.
    unsafe fn fire(&self, result: TimerResult) -> Option<Waker> {
        // Quick initial check to see if the timer is already fired. Since
        // firing the timer can only happen with the driver lock held, we know
        // we shouldn't be able to "miss" a transition to a fired state, even
        // with relaxed ordering.
        let cur_state = self.state.load(Ordering::Relaxed);
        if cur_state == STATE_DEREGISTERED {
            return None;
        }

        // SAFETY: We assume the driver lock is held and the timer is not
        // fired, so only the driver is accessing this field.
        //
        // We perform a release-ordered store to state below, to ensure this
        // write is visible before the state update is visible.
        unsafe { self.result.with_mut(|p| *p = result) };

        self.state.store(STATE_DEREGISTERED, Ordering::Release);

        self.waker.take_waker()
    }

    /// Marks the timer as registered (poll will return None) and sets the
    /// expiration time.
    ///
    /// While this function is memory-safe, it should only be called from a
    /// context holding both `&mut TimerEntry` and the driver lock.
    fn set_expiration(&self, timestamp: u64) {
        debug_assert!(timestamp < STATE_MIN_VALUE);

        // We can use relaxed ordering because we hold the driver lock and will
        // fence when we release the lock.
        self.state.store(timestamp, Ordering::Relaxed);
    }

    /// Attempts to adjust the timer to a new timestamp.
    ///
    /// If the timer has already been fired, is pending firing, or the new
    /// timestamp is earlier than the old timestamp, (or occasionally
    /// spuriously) returns Err without changing the timer's state. In this
    /// case, the timer must be deregistered and re-registered.
    fn extend_expiration(&self, new_timestamp: u64) -> Result<(), ()> {
        let mut prior = self.state.load(Ordering::Relaxed);
        loop {
            if new_timestamp < prior || prior >= STATE_MIN_VALUE {
                return Err(());
            }

            match self.state.compare_exchange_weak(
                prior,
                new_timestamp,
                Ordering::AcqRel,
                Ordering::Acquire,
            ) {
                Ok(_) => return Ok(()),
                Err(true_prior) => prior = true_prior,
            }
        }
    }

    /// Returns true if the state of this timer indicates that the timer might
    /// be registered with the driver. This check is performed with relaxed
    /// ordering, but is conservative - if it returns false, the timer is
    /// definitely _not_ registered.
    pub(super) fn might_be_registered(&self) -> bool {
        self.state.load(Ordering::Relaxed) != u64::MAX
    }
}

/// A timer entry.
///
/// This is the handle to a timer that is controlled by the requester of the
/// timer. As this participates in intrusive data structures, it must be pinned
/// before polling.
#[derive(Debug)]
pub(crate) struct TimerEntry {
    /// Arc reference to the runtime handle. We can only free the driver after
    /// deregistering everything from their respective timer wheels.
    driver: scheduler::Handle,
    /// Shared inner structure; this is part of an intrusive linked list, and
    /// therefore other references can exist to it while mutable references to
    /// Entry exist.
    ///
    /// This is manipulated only under the inner mutex. TODO: Can we use loom
    /// cells for this?
    inner: StdUnsafeCell<Option<TimerShared>>,
    /// Deadline for the timer. This is used to register on the first
    /// poll, as we can't register prior to being pinned.
    deadline: Instant,
    /// Whether the deadline has been registered.
    registered: bool,
    /// Ensure the type is !Unpin
    _m: std::marker::PhantomPinned,
}

unsafe impl Send for TimerEntry {}
unsafe impl Sync for TimerEntry {}

/// An `TimerHandle` is the (non-enforced) "unique" pointer from the driver to the
/// timer entry. Generally, at most one `TimerHandle` exists for a timer at a time
/// (enforced by the timer state machine).
///
/// SAFETY: An `TimerHandle` is essentially a raw pointer, and the usual caveats
/// of pointer safety apply. In particular, `TimerHandle` does not itself enforce
/// that the timer does still exist; however, normally an `TimerHandle` is created
/// immediately before registering the timer, and is consumed when firing the
/// timer, to help minimize mistakes. Still, because `TimerHandle` cannot enforce
/// memory safety, all operations are unsafe.
#[derive(Debug)]
pub(crate) struct TimerHandle {
    inner: NonNull<TimerShared>,
}

pub(super) type EntryList = crate::util::linked_list::LinkedList<TimerShared, TimerShared>;

/// The shared state structure of a timer. This structure is shared between the
/// frontend (`Entry`) and driver backend.
///
/// Note that this structure is located inside the `TimerEntry` structure.
pub(crate) struct TimerShared {
    /// The shard id. We should never change it.
    shard_id: u32,
    /// A link within the doubly-linked list of timers on a particular level and
    /// slot. Valid only if state is equal to Registered.
    ///
    /// Only accessed under the entry lock.
    pointers: linked_list::Pointers<TimerShared>,

    /// The expiration time for which this entry is currently registered.
    /// Generally owned by the driver, but is accessed by the entry when not
    /// registered.
    cached_when: AtomicU64,

    /// Current state. This records whether the timer entry is currently under
    /// the ownership of the driver, and if not, its current state (not
    /// complete, fired, error, etc).
    state: StateCell,

    _p: PhantomPinned,
}

unsafe impl Send for TimerShared {}
unsafe impl Sync for TimerShared {}

impl std::fmt::Debug for TimerShared {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("TimerShared")
            .field("cached_when", &self.cached_when.load(Ordering::Relaxed))
            .field("state", &self.state)
            .finish()
    }
}

generate_addr_of_methods! {
    impl<> TimerShared {
        unsafe fn addr_of_pointers(self: NonNull<Self>) -> NonNull<linked_list::Pointers<TimerShared>> {
            &self.pointers
        }
    }
}

impl TimerShared {
    pub(super) fn new(shard_id: u32) -> Self {
        Self {
            shard_id,
            cached_when: AtomicU64::new(0),
            pointers: linked_list::Pointers::new(),
            state: StateCell::default(),
            _p: PhantomPinned,
        }
    }

    /// Gets the cached time-of-expiration value.
    pub(super) fn cached_when(&self) -> u64 {
        // Cached-when is only accessed under the driver lock, so we can use relaxed
        self.cached_when.load(Ordering::Relaxed)
    }

    /// Gets the true time-of-expiration value, and copies it into the cached
    /// time-of-expiration value.
    ///
    /// SAFETY: Must be called with the driver lock held, and when this entry is
    /// not in any timer wheel lists.
    pub(super) unsafe fn sync_when(&self) -> u64 {
        let true_when = self.true_when();

        self.cached_when.store(true_when, Ordering::Relaxed);

        true_when
    }

    /// Sets the cached time-of-expiration value.
    ///
    /// SAFETY: Must be called with the driver lock held, and when this entry is
    /// not in any timer wheel lists.
    unsafe fn set_cached_when(&self, when: u64) {
        self.cached_when.store(when, Ordering::Relaxed);
    }

    /// Returns the true time-of-expiration value, with relaxed memory ordering.
    pub(super) fn true_when(&self) -> u64 {
        self.state.when().expect("Timer already fired")
    }

    /// Sets the true time-of-expiration value, even if it is less than the
    /// current expiration or the timer is deregistered.
    ///
    /// SAFETY: Must only be called with the driver lock held and the entry not
    /// in the timer wheel.
    pub(super) unsafe fn set_expiration(&self, t: u64) {
        self.state.set_expiration(t);
        self.cached_when.store(t, Ordering::Relaxed);
    }

    /// Sets the true time-of-expiration only if it is after the current.
    pub(super) fn extend_expiration(&self, t: u64) -> Result<(), ()> {
        self.state.extend_expiration(t)
    }

    /// Returns a `TimerHandle` for this timer.
    pub(super) fn handle(&self) -> TimerHandle {
        TimerHandle {
            inner: NonNull::from(self),
        }
    }

    /// Returns true if the state of this timer indicates that the timer might
    /// be registered with the driver. This check is performed with relaxed
    /// ordering, but is conservative - if it returns false, the timer is
    /// definitely _not_ registered.
    pub(super) fn might_be_registered(&self) -> bool {
        self.state.might_be_registered()
    }

    /// Gets the shard id.
    pub(super) fn shard_id(&self) -> u32 {
        self.shard_id
    }
}

unsafe impl linked_list::Link for TimerShared {
    type Handle = TimerHandle;

    type Target = TimerShared;

    fn as_raw(handle: &Self::Handle) -> NonNull<Self::Target> {
        handle.inner
    }

    unsafe fn from_raw(ptr: NonNull<Self::Target>) -> Self::Handle {
        TimerHandle { inner: ptr }
    }

    unsafe fn pointers(
        target: NonNull<Self::Target>,
    ) -> NonNull<linked_list::Pointers<Self::Target>> {
        TimerShared::addr_of_pointers(target)
    }
}

// ===== impl Entry =====

impl TimerEntry {
    #[track_caller]
    pub(crate) fn new(handle: scheduler::Handle, deadline: Instant) -> Self {
        // Panic if the time driver is not enabled
        let _ = handle.driver().time();

        Self {
            driver: handle,
            inner: StdUnsafeCell::new(None),
            deadline,
            registered: false,
            _m: std::marker::PhantomPinned,
        }
    }

    fn is_inner_init(&self) -> bool {
        unsafe { &*self.inner.get() }.is_some()
    }

    // This lazy initialization is for performance purposes.
    fn inner(&self) -> &TimerShared {
        let inner = unsafe { &*self.inner.get() };
        if inner.is_none() {
            let shard_size = self.driver.driver().time().inner.get_shard_size();
            let shard_id = generate_shard_id(shard_size);
            unsafe {
                *self.inner.get() = Some(TimerShared::new(shard_id));
            }
        }
        return inner.as_ref().unwrap();
    }

    pub(crate) fn deadline(&self) -> Instant {
        self.deadline
    }

    pub(crate) fn is_elapsed(&self) -> bool {
        self.is_inner_init() && !self.inner().state.might_be_registered() && self.registered
    }

    /// Cancels and deregisters the timer. This operation is irreversible.
    pub(crate) fn cancel(self: Pin<&mut Self>) {
        // Avoid calling the `clear_entry` method, because it has not been initialized yet.
        if !self.is_inner_init() {
            return;
        }
        // We need to perform an acq/rel fence with the driver thread, and the
        // simplest way to do so is to grab the driver lock.
        //
        // Why is this necessary? We're about to release this timer's memory for
        // some other non-timer use. However, we've been doing a bunch of
        // relaxed (or even non-atomic) writes from the driver thread, and we'll
        // be doing more from _this thread_ (as this memory is interpreted as
        // something else).
        //
        // It is critical to ensure that, from the point of view of the driver,
        // those future non-timer writes happen-after the timer is fully fired,
        // and from the purpose of this thread, the driver's writes all
        // happen-before we drop the timer. This in turn requires us to perform
        // an acquire-release barrier in _both_ directions between the driver
        // and dropping thread.
        //
        // The lock acquisition in clear_entry serves this purpose. All of the
        // driver manipulations happen with the lock held, so we can just take
        // the lock and be sure that this drop happens-after everything the
        // driver did so far and happens-before everything the driver does in
        // the future. While we have the lock held, we also go ahead and
        // deregister the entry if necessary.
        unsafe { self.driver().clear_entry(NonNull::from(self.inner())) };
    }

    pub(crate) fn reset(mut self: Pin<&mut Self>, new_time: Instant, reregister: bool) {
        let this = unsafe { self.as_mut().get_unchecked_mut() };
        this.deadline = new_time;
        this.registered = reregister;

        let tick = self.driver().time_source().deadline_to_tick(new_time);

        if self.inner().extend_expiration(tick).is_ok() {
            return;
        }

        if reregister {
            unsafe {
                self.driver()
                    .reregister(&self.driver.driver().io, tick, self.inner().into());
            }
        }
    }

    pub(crate) fn poll_elapsed(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Result<(), super::Error>> {
        assert!(
            !self.driver().is_shutdown(),
            "{}",
            crate::util::error::RUNTIME_SHUTTING_DOWN_ERROR
        );

        if !self.registered {
            let deadline = self.deadline;
            self.as_mut().reset(deadline, true);
        }

        self.inner().state.poll(cx.waker())
    }

    pub(crate) fn driver(&self) -> &super::Handle {
        self.driver.driver().time()
    }

    #[cfg(all(tokio_unstable, feature = "tracing"))]
    pub(crate) fn clock(&self) -> &super::Clock {
        self.driver.driver().clock()
    }
}

impl TimerHandle {
    pub(super) unsafe fn cached_when(&self) -> u64 {
        unsafe { self.inner.as_ref().cached_when() }
    }

    pub(super) unsafe fn sync_when(&self) -> u64 {
        unsafe { self.inner.as_ref().sync_when() }
    }

    pub(super) unsafe fn is_pending(&self) -> bool {
        unsafe { self.inner.as_ref().state.is_pending() }
    }

    /// Forcibly sets the true and cached expiration times to the given tick.
    ///
    /// SAFETY: The caller must ensure that the handle remains valid, the driver
    /// lock is held, and that the timer is not in any wheel linked lists.
    pub(super) unsafe fn set_expiration(&self, tick: u64) {
        self.inner.as_ref().set_expiration(tick);
    }

    /// Attempts to mark this entry as pending. If the expiration time is after
    /// `not_after`, however, returns an Err with the current expiration time.
    ///
    /// If an `Err` is returned, the `cached_when` value will be updated to this
    /// new expiration time.
    ///
    /// SAFETY: The caller must ensure that the handle remains valid, the driver
    /// lock is held, and that the timer is not in any wheel linked lists.
    /// After returning Ok, the entry must be added to the pending list.
    pub(super) unsafe fn mark_pending(&self, not_after: u64) -> Result<(), u64> {
        match self.inner.as_ref().state.mark_pending(not_after) {
            Ok(()) => {
                // mark this as being on the pending queue in cached_when
                self.inner.as_ref().set_cached_when(u64::MAX);
                Ok(())
            }
            Err(tick) => {
                self.inner.as_ref().set_cached_when(tick);
                Err(tick)
            }
        }
    }

    /// Attempts to transition to a terminal state. If the state is already a
    /// terminal state, does nothing.
    ///
    /// Because the entry might be dropped after the state is moved to a
    /// terminal state, this function consumes the handle to ensure we don't
    /// access the entry afterwards.
    ///
    /// Returns the last-registered waker, if any.
    ///
    /// SAFETY: The driver lock must be held while invoking this function, and
    /// the entry must not be in any wheel linked lists.
    pub(super) unsafe fn fire(self, completed_state: TimerResult) -> Option<Waker> {
        self.inner.as_ref().state.fire(completed_state)
    }
}

impl Drop for TimerEntry {
    fn drop(&mut self) {
        unsafe { Pin::new_unchecked(self) }.as_mut().cancel();
    }
}

// Generates a shard id. If current thread is a worker thread, we use its worker index as a shard id.
// Otherwise, we use a random number generator to obtain the shard id.
cfg_rt! {
    fn generate_shard_id(shard_size: u32) -> u32 {
        let id = context::with_scheduler(|ctx| match ctx {
            Some(scheduler::Context::CurrentThread(_ctx)) => 0,
            #[cfg(feature = "rt-multi-thread")]
            Some(scheduler::Context::MultiThread(ctx)) => ctx.get_worker_index() as u32,
            #[cfg(all(tokio_unstable, feature = "rt-multi-thread"))]
            Some(scheduler::Context::MultiThreadAlt(ctx)) => ctx.get_worker_index() as u32,
            None => context::thread_rng_n(shard_size),
        });
        id % shard_size
    }
}

cfg_not_rt! {
    fn generate_shard_id(shard_size: u32) -> u32 {
        context::thread_rng_n(shard_size)
    }
}