1//! Unbounded channel implemented as a linked list.
2
3use std::alloc::{alloc_zeroed, handle_alloc_error, Layout};
4use std::boxed::Box;
5use std::cell::UnsafeCell;
6use std::marker::PhantomData;
7use std::mem::MaybeUninit;
8use std::ptr;
9use std::sync::atomic::{self, AtomicPtr, AtomicUsize, Ordering};
10use std::time::Instant;
11
12use crossbeam_utils::{Backoff, CachePadded};
13
14use crate::context::Context;
15use crate::err::{RecvTimeoutError, SendTimeoutError, TryRecvError, TrySendError};
16use crate::select::{Operation, SelectHandle, Selected, Token};
17use crate::waker::SyncWaker;
18
19// TODO(stjepang): Once we bump the minimum required Rust version to 1.28 or newer, re-apply the
20// following changes by @kleimkuhler:
21//
22// 1. https://github.com/crossbeam-rs/crossbeam-channel/pull/100
23// 2. https://github.com/crossbeam-rs/crossbeam-channel/pull/101
24
25// Bits indicating the state of a slot:
26// * If a message has been written into the slot, `WRITE` is set.
27// * If a message has been read from the slot, `READ` is set.
28// * If the block is being destroyed, `DESTROY` is set.
29const WRITE: usize = 1;
30const READ: usize = 2;
31const DESTROY: usize = 4;
32
33// Each block covers one "lap" of indices.
34const LAP: usize = 32;
35// The maximum number of messages a block can hold.
36const BLOCK_CAP: usize = LAP - 1;
37// How many lower bits are reserved for metadata.
38const SHIFT: usize = 1;
39// Has two different purposes:
40// * If set in head, indicates that the block is not the last one.
41// * If set in tail, indicates that the channel is disconnected.
42const MARK_BIT: usize = 1;
43
44/// A slot in a block.
45struct Slot<T> {
46 /// The message.
47 msg: UnsafeCell<MaybeUninit<T>>,
48
49 /// The state of the slot.
50 state: AtomicUsize,
51}
52
53impl<T> Slot<T> {
54 /// Waits until a message is written into the slot.
55 fn wait_write(&self) {
56 let backoff: Backoff = Backoff::new();
57 while self.state.load(order:Ordering::Acquire) & WRITE == 0 {
58 backoff.snooze();
59 }
60 }
61}
62
63/// A block in a linked list.
64///
65/// Each block in the list can hold up to `BLOCK_CAP` messages.
66struct Block<T> {
67 /// The next block in the linked list.
68 next: AtomicPtr<Block<T>>,
69
70 /// Slots for messages.
71 slots: [Slot<T>; BLOCK_CAP],
72}
73
74impl<T> Block<T> {
75 const LAYOUT: Layout = {
76 let layout = Layout::new::<Self>();
77 assert!(
78 layout.size() != 0,
79 "Block should never be zero-sized, as it has an AtomicPtr field"
80 );
81 layout
82 };
83
84 /// Creates an empty block.
85 fn new() -> Box<Self> {
86 // SAFETY: layout is not zero-sized
87 let ptr = unsafe { alloc_zeroed(Self::LAYOUT) };
88 // Handle allocation failure
89 if ptr.is_null() {
90 handle_alloc_error(Self::LAYOUT)
91 }
92 // SAFETY: This is safe because:
93 // [1] `Block::next` (AtomicPtr) may be safely zero initialized.
94 // [2] `Block::slots` (Array) may be safely zero initialized because of [3, 4].
95 // [3] `Slot::msg` (UnsafeCell) may be safely zero initialized because it
96 // holds a MaybeUninit.
97 // [4] `Slot::state` (AtomicUsize) may be safely zero initialized.
98 // TODO: unsafe { Box::new_zeroed().assume_init() }
99 unsafe { Box::from_raw(ptr.cast()) }
100 }
101
102 /// Waits until the next pointer is set.
103 fn wait_next(&self) -> *mut Block<T> {
104 let backoff = Backoff::new();
105 loop {
106 let next = self.next.load(Ordering::Acquire);
107 if !next.is_null() {
108 return next;
109 }
110 backoff.snooze();
111 }
112 }
113
114 /// Sets the `DESTROY` bit in slots starting from `start` and destroys the block.
115 unsafe fn destroy(this: *mut Block<T>, start: usize) {
116 // It is not necessary to set the `DESTROY` bit in the last slot because that slot has
117 // begun destruction of the block.
118 for i in start..BLOCK_CAP - 1 {
119 let slot = (*this).slots.get_unchecked(i);
120
121 // Mark the `DESTROY` bit if a thread is still using the slot.
122 if slot.state.load(Ordering::Acquire) & READ == 0
123 && slot.state.fetch_or(DESTROY, Ordering::AcqRel) & READ == 0
124 {
125 // If a thread is still using the slot, it will continue destruction of the block.
126 return;
127 }
128 }
129
130 // No thread is using the block, now it is safe to destroy it.
131 drop(Box::from_raw(this));
132 }
133}
134
135/// A position in a channel.
136#[derive(Debug)]
137struct Position<T> {
138 /// The index in the channel.
139 index: AtomicUsize,
140
141 /// The block in the linked list.
142 block: AtomicPtr<Block<T>>,
143}
144
145/// The token type for the list flavor.
146#[derive(Debug)]
147pub(crate) struct ListToken {
148 /// The block of slots.
149 block: *const u8,
150
151 /// The offset into the block.
152 offset: usize,
153}
154
155impl Default for ListToken {
156 #[inline]
157 fn default() -> Self {
158 ListToken {
159 block: ptr::null(),
160 offset: 0,
161 }
162 }
163}
164
165/// Unbounded channel implemented as a linked list.
166///
167/// Each message sent into the channel is assigned a sequence number, i.e. an index. Indices are
168/// represented as numbers of type `usize` and wrap on overflow.
169///
170/// Consecutive messages are grouped into blocks in order to put less pressure on the allocator and
171/// improve cache efficiency.
172pub(crate) struct Channel<T> {
173 /// The head of the channel.
174 head: CachePadded<Position<T>>,
175
176 /// The tail of the channel.
177 tail: CachePadded<Position<T>>,
178
179 /// Receivers waiting while the channel is empty and not disconnected.
180 receivers: SyncWaker,
181
182 /// Indicates that dropping a `Channel<T>` may drop messages of type `T`.
183 _marker: PhantomData<T>,
184}
185
186impl<T> Channel<T> {
187 /// Creates a new unbounded channel.
188 pub(crate) fn new() -> Self {
189 Channel {
190 head: CachePadded::new(Position {
191 block: AtomicPtr::new(ptr::null_mut()),
192 index: AtomicUsize::new(0),
193 }),
194 tail: CachePadded::new(Position {
195 block: AtomicPtr::new(ptr::null_mut()),
196 index: AtomicUsize::new(0),
197 }),
198 receivers: SyncWaker::new(),
199 _marker: PhantomData,
200 }
201 }
202
203 /// Returns a receiver handle to the channel.
204 pub(crate) fn receiver(&self) -> Receiver<'_, T> {
205 Receiver(self)
206 }
207
208 /// Returns a sender handle to the channel.
209 pub(crate) fn sender(&self) -> Sender<'_, T> {
210 Sender(self)
211 }
212
213 /// Attempts to reserve a slot for sending a message.
214 fn start_send(&self, token: &mut Token) -> bool {
215 let backoff = Backoff::new();
216 let mut tail = self.tail.index.load(Ordering::Acquire);
217 let mut block = self.tail.block.load(Ordering::Acquire);
218 let mut next_block = None;
219
220 loop {
221 // Check if the channel is disconnected.
222 if tail & MARK_BIT != 0 {
223 token.list.block = ptr::null();
224 return true;
225 }
226
227 // Calculate the offset of the index into the block.
228 let offset = (tail >> SHIFT) % LAP;
229
230 // If we reached the end of the block, wait until the next one is installed.
231 if offset == BLOCK_CAP {
232 backoff.snooze();
233 tail = self.tail.index.load(Ordering::Acquire);
234 block = self.tail.block.load(Ordering::Acquire);
235 continue;
236 }
237
238 // If we're going to have to install the next block, allocate it in advance in order to
239 // make the wait for other threads as short as possible.
240 if offset + 1 == BLOCK_CAP && next_block.is_none() {
241 next_block = Some(Block::<T>::new());
242 }
243
244 // If this is the first message to be sent into the channel, we need to allocate the
245 // first block and install it.
246 if block.is_null() {
247 let new = Box::into_raw(Block::<T>::new());
248
249 if self
250 .tail
251 .block
252 .compare_exchange(block, new, Ordering::Release, Ordering::Relaxed)
253 .is_ok()
254 {
255 self.head.block.store(new, Ordering::Release);
256 block = new;
257 } else {
258 next_block = unsafe { Some(Box::from_raw(new)) };
259 tail = self.tail.index.load(Ordering::Acquire);
260 block = self.tail.block.load(Ordering::Acquire);
261 continue;
262 }
263 }
264
265 let new_tail = tail + (1 << SHIFT);
266
267 // Try advancing the tail forward.
268 match self.tail.index.compare_exchange_weak(
269 tail,
270 new_tail,
271 Ordering::SeqCst,
272 Ordering::Acquire,
273 ) {
274 Ok(_) => unsafe {
275 // If we've reached the end of the block, install the next one.
276 if offset + 1 == BLOCK_CAP {
277 let next_block = Box::into_raw(next_block.unwrap());
278 self.tail.block.store(next_block, Ordering::Release);
279 self.tail.index.fetch_add(1 << SHIFT, Ordering::Release);
280 (*block).next.store(next_block, Ordering::Release);
281 }
282
283 token.list.block = block as *const u8;
284 token.list.offset = offset;
285 return true;
286 },
287 Err(t) => {
288 tail = t;
289 block = self.tail.block.load(Ordering::Acquire);
290 backoff.spin();
291 }
292 }
293 }
294 }
295
296 /// Writes a message into the channel.
297 pub(crate) unsafe fn write(&self, token: &mut Token, msg: T) -> Result<(), T> {
298 // If there is no slot, the channel is disconnected.
299 if token.list.block.is_null() {
300 return Err(msg);
301 }
302
303 // Write the message into the slot.
304 let block = token.list.block.cast::<Block<T>>();
305 let offset = token.list.offset;
306 let slot = (*block).slots.get_unchecked(offset);
307 slot.msg.get().write(MaybeUninit::new(msg));
308 slot.state.fetch_or(WRITE, Ordering::Release);
309
310 // Wake a sleeping receiver.
311 self.receivers.notify();
312 Ok(())
313 }
314
315 /// Attempts to reserve a slot for receiving a message.
316 fn start_recv(&self, token: &mut Token) -> bool {
317 let backoff = Backoff::new();
318 let mut head = self.head.index.load(Ordering::Acquire);
319 let mut block = self.head.block.load(Ordering::Acquire);
320
321 loop {
322 // Calculate the offset of the index into the block.
323 let offset = (head >> SHIFT) % LAP;
324
325 // If we reached the end of the block, wait until the next one is installed.
326 if offset == BLOCK_CAP {
327 backoff.snooze();
328 head = self.head.index.load(Ordering::Acquire);
329 block = self.head.block.load(Ordering::Acquire);
330 continue;
331 }
332
333 let mut new_head = head + (1 << SHIFT);
334
335 if new_head & MARK_BIT == 0 {
336 atomic::fence(Ordering::SeqCst);
337 let tail = self.tail.index.load(Ordering::Relaxed);
338
339 // If the tail equals the head, that means the channel is empty.
340 if head >> SHIFT == tail >> SHIFT {
341 // If the channel is disconnected...
342 if tail & MARK_BIT != 0 {
343 // ...then receive an error.
344 token.list.block = ptr::null();
345 return true;
346 } else {
347 // Otherwise, the receive operation is not ready.
348 return false;
349 }
350 }
351
352 // If head and tail are not in the same block, set `MARK_BIT` in head.
353 if (head >> SHIFT) / LAP != (tail >> SHIFT) / LAP {
354 new_head |= MARK_BIT;
355 }
356 }
357
358 // The block can be null here only if the first message is being sent into the channel.
359 // In that case, just wait until it gets initialized.
360 if block.is_null() {
361 backoff.snooze();
362 head = self.head.index.load(Ordering::Acquire);
363 block = self.head.block.load(Ordering::Acquire);
364 continue;
365 }
366
367 // Try moving the head index forward.
368 match self.head.index.compare_exchange_weak(
369 head,
370 new_head,
371 Ordering::SeqCst,
372 Ordering::Acquire,
373 ) {
374 Ok(_) => unsafe {
375 // If we've reached the end of the block, move to the next one.
376 if offset + 1 == BLOCK_CAP {
377 let next = (*block).wait_next();
378 let mut next_index = (new_head & !MARK_BIT).wrapping_add(1 << SHIFT);
379 if !(*next).next.load(Ordering::Relaxed).is_null() {
380 next_index |= MARK_BIT;
381 }
382
383 self.head.block.store(next, Ordering::Release);
384 self.head.index.store(next_index, Ordering::Release);
385 }
386
387 token.list.block = block as *const u8;
388 token.list.offset = offset;
389 return true;
390 },
391 Err(h) => {
392 head = h;
393 block = self.head.block.load(Ordering::Acquire);
394 backoff.spin();
395 }
396 }
397 }
398 }
399
400 /// Reads a message from the channel.
401 pub(crate) unsafe fn read(&self, token: &mut Token) -> Result<T, ()> {
402 if token.list.block.is_null() {
403 // The channel is disconnected.
404 return Err(());
405 }
406
407 // Read the message.
408 let block = token.list.block as *mut Block<T>;
409 let offset = token.list.offset;
410 let slot = (*block).slots.get_unchecked(offset);
411 slot.wait_write();
412 let msg = slot.msg.get().read().assume_init();
413
414 // Destroy the block if we've reached the end, or if another thread wanted to destroy but
415 // couldn't because we were busy reading from the slot.
416 if offset + 1 == BLOCK_CAP {
417 Block::destroy(block, 0);
418 } else if slot.state.fetch_or(READ, Ordering::AcqRel) & DESTROY != 0 {
419 Block::destroy(block, offset + 1);
420 }
421
422 Ok(msg)
423 }
424
425 /// Attempts to send a message into the channel.
426 pub(crate) fn try_send(&self, msg: T) -> Result<(), TrySendError<T>> {
427 self.send(msg, None).map_err(|err| match err {
428 SendTimeoutError::Disconnected(msg) => TrySendError::Disconnected(msg),
429 SendTimeoutError::Timeout(_) => unreachable!(),
430 })
431 }
432
433 /// Sends a message into the channel.
434 pub(crate) fn send(
435 &self,
436 msg: T,
437 _deadline: Option<Instant>,
438 ) -> Result<(), SendTimeoutError<T>> {
439 let token = &mut Token::default();
440 assert!(self.start_send(token));
441 unsafe {
442 self.write(token, msg)
443 .map_err(SendTimeoutError::Disconnected)
444 }
445 }
446
447 /// Attempts to receive a message without blocking.
448 pub(crate) fn try_recv(&self) -> Result<T, TryRecvError> {
449 let token = &mut Token::default();
450
451 if self.start_recv(token) {
452 unsafe { self.read(token).map_err(|_| TryRecvError::Disconnected) }
453 } else {
454 Err(TryRecvError::Empty)
455 }
456 }
457
458 /// Receives a message from the channel.
459 pub(crate) fn recv(&self, deadline: Option<Instant>) -> Result<T, RecvTimeoutError> {
460 let token = &mut Token::default();
461 loop {
462 // Try receiving a message several times.
463 let backoff = Backoff::new();
464 loop {
465 if self.start_recv(token) {
466 unsafe {
467 return self.read(token).map_err(|_| RecvTimeoutError::Disconnected);
468 }
469 }
470
471 if backoff.is_completed() {
472 break;
473 } else {
474 backoff.snooze();
475 }
476 }
477
478 if let Some(d) = deadline {
479 if Instant::now() >= d {
480 return Err(RecvTimeoutError::Timeout);
481 }
482 }
483
484 // Prepare for blocking until a sender wakes us up.
485 Context::with(|cx| {
486 let oper = Operation::hook(token);
487 self.receivers.register(oper, cx);
488
489 // Has the channel become ready just now?
490 if !self.is_empty() || self.is_disconnected() {
491 let _ = cx.try_select(Selected::Aborted);
492 }
493
494 // Block the current thread.
495 let sel = cx.wait_until(deadline);
496
497 match sel {
498 Selected::Waiting => unreachable!(),
499 Selected::Aborted | Selected::Disconnected => {
500 self.receivers.unregister(oper).unwrap();
501 // If the channel was disconnected, we still have to check for remaining
502 // messages.
503 }
504 Selected::Operation(_) => {}
505 }
506 });
507 }
508 }
509
510 /// Returns the current number of messages inside the channel.
511 pub(crate) fn len(&self) -> usize {
512 loop {
513 // Load the tail index, then load the head index.
514 let mut tail = self.tail.index.load(Ordering::SeqCst);
515 let mut head = self.head.index.load(Ordering::SeqCst);
516
517 // If the tail index didn't change, we've got consistent indices to work with.
518 if self.tail.index.load(Ordering::SeqCst) == tail {
519 // Erase the lower bits.
520 tail &= !((1 << SHIFT) - 1);
521 head &= !((1 << SHIFT) - 1);
522
523 // Fix up indices if they fall onto block ends.
524 if (tail >> SHIFT) & (LAP - 1) == LAP - 1 {
525 tail = tail.wrapping_add(1 << SHIFT);
526 }
527 if (head >> SHIFT) & (LAP - 1) == LAP - 1 {
528 head = head.wrapping_add(1 << SHIFT);
529 }
530
531 // Rotate indices so that head falls into the first block.
532 let lap = (head >> SHIFT) / LAP;
533 tail = tail.wrapping_sub((lap * LAP) << SHIFT);
534 head = head.wrapping_sub((lap * LAP) << SHIFT);
535
536 // Remove the lower bits.
537 tail >>= SHIFT;
538 head >>= SHIFT;
539
540 // Return the difference minus the number of blocks between tail and head.
541 return tail - head - tail / LAP;
542 }
543 }
544 }
545
546 /// Returns the capacity of the channel.
547 pub(crate) fn capacity(&self) -> Option<usize> {
548 None
549 }
550
551 /// Disconnects senders and wakes up all blocked receivers.
552 ///
553 /// Returns `true` if this call disconnected the channel.
554 pub(crate) fn disconnect_senders(&self) -> bool {
555 let tail = self.tail.index.fetch_or(MARK_BIT, Ordering::SeqCst);
556
557 if tail & MARK_BIT == 0 {
558 self.receivers.disconnect();
559 true
560 } else {
561 false
562 }
563 }
564
565 /// Disconnects receivers.
566 ///
567 /// Returns `true` if this call disconnected the channel.
568 pub(crate) fn disconnect_receivers(&self) -> bool {
569 let tail = self.tail.index.fetch_or(MARK_BIT, Ordering::SeqCst);
570
571 if tail & MARK_BIT == 0 {
572 // If receivers are dropped first, discard all messages to free
573 // memory eagerly.
574 self.discard_all_messages();
575 true
576 } else {
577 false
578 }
579 }
580
581 /// Discards all messages.
582 ///
583 /// This method should only be called when all receivers are dropped.
584 fn discard_all_messages(&self) {
585 let backoff = Backoff::new();
586 let mut tail = self.tail.index.load(Ordering::Acquire);
587 loop {
588 let offset = (tail >> SHIFT) % LAP;
589 if offset != BLOCK_CAP {
590 break;
591 }
592
593 // New updates to tail will be rejected by MARK_BIT and aborted unless it's
594 // at boundary. We need to wait for the updates take affect otherwise there
595 // can be memory leaks.
596 backoff.snooze();
597 tail = self.tail.index.load(Ordering::Acquire);
598 }
599
600 let mut head = self.head.index.load(Ordering::Acquire);
601 // The channel may be uninitialized, so we have to swap to avoid overwriting any sender's attempts
602 // to initialize the first block before noticing that the receivers disconnected. Late allocations
603 // will be deallocated by the sender in Drop
604 let mut block = self.head.block.swap(ptr::null_mut(), Ordering::AcqRel);
605
606 // If we're going to be dropping messages we need to synchronize with initialization
607 if head >> SHIFT != tail >> SHIFT {
608 // The block can be null here only if a sender is in the process of initializing the
609 // channel while another sender managed to send a message by inserting it into the
610 // semi-initialized channel and advanced the tail.
611 // In that case, just wait until it gets initialized.
612 while block.is_null() {
613 backoff.snooze();
614 block = self.head.block.swap(ptr::null_mut(), Ordering::AcqRel);
615 }
616 }
617
618 unsafe {
619 // Drop all messages between head and tail and deallocate the heap-allocated blocks.
620 while head >> SHIFT != tail >> SHIFT {
621 let offset = (head >> SHIFT) % LAP;
622
623 if offset < BLOCK_CAP {
624 // Drop the message in the slot.
625 let slot = (*block).slots.get_unchecked(offset);
626 slot.wait_write();
627 (*slot.msg.get()).assume_init_drop();
628 } else {
629 (*block).wait_next();
630 // Deallocate the block and move to the next one.
631 let next = (*block).next.load(Ordering::Acquire);
632 drop(Box::from_raw(block));
633 block = next;
634 }
635
636 head = head.wrapping_add(1 << SHIFT);
637 }
638
639 // Deallocate the last remaining block.
640 if !block.is_null() {
641 drop(Box::from_raw(block));
642 }
643 }
644 head &= !MARK_BIT;
645 self.head.index.store(head, Ordering::Release);
646 }
647
648 /// Returns `true` if the channel is disconnected.
649 pub(crate) fn is_disconnected(&self) -> bool {
650 self.tail.index.load(Ordering::SeqCst) & MARK_BIT != 0
651 }
652
653 /// Returns `true` if the channel is empty.
654 pub(crate) fn is_empty(&self) -> bool {
655 let head = self.head.index.load(Ordering::SeqCst);
656 let tail = self.tail.index.load(Ordering::SeqCst);
657 head >> SHIFT == tail >> SHIFT
658 }
659
660 /// Returns `true` if the channel is full.
661 pub(crate) fn is_full(&self) -> bool {
662 false
663 }
664}
665
666impl<T> Drop for Channel<T> {
667 fn drop(&mut self) {
668 let mut head = *self.head.index.get_mut();
669 let mut tail = *self.tail.index.get_mut();
670 let mut block = *self.head.block.get_mut();
671
672 // Erase the lower bits.
673 head &= !((1 << SHIFT) - 1);
674 tail &= !((1 << SHIFT) - 1);
675
676 unsafe {
677 // Drop all messages between head and tail and deallocate the heap-allocated blocks.
678 while head != tail {
679 let offset = (head >> SHIFT) % LAP;
680
681 if offset < BLOCK_CAP {
682 // Drop the message in the slot.
683 let slot = (*block).slots.get_unchecked(offset);
684 (*slot.msg.get()).assume_init_drop();
685 } else {
686 // Deallocate the block and move to the next one.
687 let next = *(*block).next.get_mut();
688 drop(Box::from_raw(block));
689 block = next;
690 }
691
692 head = head.wrapping_add(1 << SHIFT);
693 }
694
695 // Deallocate the last remaining block.
696 if !block.is_null() {
697 drop(Box::from_raw(block));
698 }
699 }
700 }
701}
702
703/// Receiver handle to a channel.
704pub(crate) struct Receiver<'a, T>(&'a Channel<T>);
705
706/// Sender handle to a channel.
707pub(crate) struct Sender<'a, T>(&'a Channel<T>);
708
709impl<T> SelectHandle for Receiver<'_, T> {
710 fn try_select(&self, token: &mut Token) -> bool {
711 self.0.start_recv(token)
712 }
713
714 fn deadline(&self) -> Option<Instant> {
715 None
716 }
717
718 fn register(&self, oper: Operation, cx: &Context) -> bool {
719 self.0.receivers.register(oper, cx);
720 self.is_ready()
721 }
722
723 fn unregister(&self, oper: Operation) {
724 self.0.receivers.unregister(oper);
725 }
726
727 fn accept(&self, token: &mut Token, _cx: &Context) -> bool {
728 self.try_select(token)
729 }
730
731 fn is_ready(&self) -> bool {
732 !self.0.is_empty() || self.0.is_disconnected()
733 }
734
735 fn watch(&self, oper: Operation, cx: &Context) -> bool {
736 self.0.receivers.watch(oper, cx);
737 self.is_ready()
738 }
739
740 fn unwatch(&self, oper: Operation) {
741 self.0.receivers.unwatch(oper);
742 }
743}
744
745impl<T> SelectHandle for Sender<'_, T> {
746 fn try_select(&self, token: &mut Token) -> bool {
747 self.0.start_send(token)
748 }
749
750 fn deadline(&self) -> Option<Instant> {
751 None
752 }
753
754 fn register(&self, _oper: Operation, _cx: &Context) -> bool {
755 self.is_ready()
756 }
757
758 fn unregister(&self, _oper: Operation) {}
759
760 fn accept(&self, token: &mut Token, _cx: &Context) -> bool {
761 self.try_select(token)
762 }
763
764 fn is_ready(&self) -> bool {
765 true
766 }
767
768 fn watch(&self, _oper: Operation, _cx: &Context) -> bool {
769 self.is_ready()
770 }
771
772 fn unwatch(&self, _oper: Operation) {}
773}
774