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
//! A concurrent multi-producer multi-consumer queue.
//!
//! There are two kinds of queues:
//!
//! 1. [Bounded] queue with limited capacity.
//! 2. [Unbounded] queue with unlimited capacity.
//!
//! Queues also have the capability to get [closed] at any point. When closed, no more items can be
//! pushed into the queue, although the remaining items can still be popped.
//!
//! These features make it easy to build channels similar to [`std::sync::mpsc`] on top of this
//! crate.
//!
//! # Examples
//!
//! ```
//! use concurrent_queue::ConcurrentQueue;
//!
//! let q = ConcurrentQueue::unbounded();
//! q.push(1).unwrap();
//! q.push(2).unwrap();
//!
//! assert_eq!(q.pop(), Ok(1));
//! assert_eq!(q.pop(), Ok(2));
//! ```
//!
//! # Features
//!
//! `concurrent-queue` uses an `std` default feature. With this feature enabled, this crate will
//! use [`std::thread::yield_now`] to avoid busy waiting in tight loops. However, with this
//! feature disabled, [`core::hint::spin_loop`] will be used instead. Disabling `std` will allow
//! this crate to be used on `no_std` platforms at the potential expense of more busy waiting.
//!
//! There is also a `portable-atomic` feature, which uses a polyfill from the
//! [`portable-atomic`] crate to provide atomic operations on platforms that do not support them.
//! See the [`README`] for the [`portable-atomic`] crate for more information on how to use it.
//! Note that even with this feature enabled, `concurrent-queue` still requires a global allocator
//! to be available. See the documentation for the [`std::alloc::GlobalAlloc`] trait for more
//! information.
//!
//! [Bounded]: `ConcurrentQueue::bounded()`
//! [Unbounded]: `ConcurrentQueue::unbounded()`
//! [closed]: `ConcurrentQueue::close()`
//! [`portable-atomic`]: https://crates.io/crates/portable-atomic
//! [`README`]: https://github.com/taiki-e/portable-atomic/blob/main/README.md#optional-cfg

#![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]
#![no_std]
#![doc(
    html_favicon_url = "https://raw.githubusercontent.com/smol-rs/smol/master/assets/images/logo_fullsize_transparent.png"
)]
#![doc(
    html_logo_url = "https://raw.githubusercontent.com/smol-rs/smol/master/assets/images/logo_fullsize_transparent.png"
)]

extern crate alloc;
#[cfg(feature = "std")]
extern crate std;

use core::fmt;
use core::panic::{RefUnwindSafe, UnwindSafe};
use sync::atomic::{self, Ordering};

#[cfg(feature = "std")]
use std::error;

use crate::bounded::Bounded;
use crate::single::Single;
use crate::sync::busy_wait;
use crate::unbounded::Unbounded;

mod bounded;
mod single;
mod unbounded;

mod sync;

/// A concurrent queue.
///
/// # Examples
///
/// ```
/// use concurrent_queue::{ConcurrentQueue, PopError, PushError};
///
/// let q = ConcurrentQueue::bounded(2);
///
/// assert_eq!(q.push('a'), Ok(()));
/// assert_eq!(q.push('b'), Ok(()));
/// assert_eq!(q.push('c'), Err(PushError::Full('c')));
///
/// assert_eq!(q.pop(), Ok('a'));
/// assert_eq!(q.pop(), Ok('b'));
/// assert_eq!(q.pop(), Err(PopError::Empty));
/// ```
pub struct ConcurrentQueue<T>(Inner<T>);

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

impl<T> UnwindSafe for ConcurrentQueue<T> {}
impl<T> RefUnwindSafe for ConcurrentQueue<T> {}

#[allow(clippy::large_enum_variant)]
enum Inner<T> {
    Single(Single<T>),
    Bounded(Bounded<T>),
    Unbounded(Unbounded<T>),
}

impl<T> ConcurrentQueue<T> {
    /// Creates a new bounded queue.
    ///
    /// The queue allocates enough space for `cap` items.
    ///
    /// # Panics
    ///
    /// If the capacity is zero, this constructor will panic.
    ///
    /// # Examples
    ///
    /// ```
    /// use concurrent_queue::ConcurrentQueue;
    ///
    /// let q = ConcurrentQueue::<i32>::bounded(100);
    /// ```
    pub fn bounded(cap: usize) -> ConcurrentQueue<T> {
        if cap == 1 {
            ConcurrentQueue(Inner::Single(Single::new()))
        } else {
            ConcurrentQueue(Inner::Bounded(Bounded::new(cap)))
        }
    }

    /// Creates a new unbounded queue.
    ///
    /// # Examples
    ///
    /// ```
    /// use concurrent_queue::ConcurrentQueue;
    ///
    /// let q = ConcurrentQueue::<i32>::unbounded();
    /// ```
    pub fn unbounded() -> ConcurrentQueue<T> {
        ConcurrentQueue(Inner::Unbounded(Unbounded::new()))
    }

    /// Attempts to push an item into the queue.
    ///
    /// If the queue is full or closed, the item is returned back as an error.
    ///
    /// # Examples
    ///
    /// ```
    /// use concurrent_queue::{ConcurrentQueue, PushError};
    ///
    /// let q = ConcurrentQueue::bounded(1);
    ///
    /// // Push succeeds because there is space in the queue.
    /// assert_eq!(q.push(10), Ok(()));
    ///
    /// // Push errors because the queue is now full.
    /// assert_eq!(q.push(20), Err(PushError::Full(20)));
    ///
    /// // Close the queue, which will prevent further pushes.
    /// q.close();
    ///
    /// // Pushing now errors indicating the queue is closed.
    /// assert_eq!(q.push(20), Err(PushError::Closed(20)));
    ///
    /// // Pop the single item in the queue.
    /// assert_eq!(q.pop(), Ok(10));
    ///
    /// // Even though there is space, no more items can be pushed.
    /// assert_eq!(q.push(20), Err(PushError::Closed(20)));
    /// ```
    pub fn push(&self, value: T) -> Result<(), PushError<T>> {
        match &self.0 {
            Inner::Single(q) => q.push(value),
            Inner::Bounded(q) => q.push(value),
            Inner::Unbounded(q) => q.push(value),
        }
    }

    /// Attempts to pop an item from the queue.
    ///
    /// If the queue is empty, an error is returned.
    ///
    /// # Examples
    ///
    /// ```
    /// use concurrent_queue::{ConcurrentQueue, PopError};
    ///
    /// let q = ConcurrentQueue::bounded(1);
    ///
    /// // Pop errors when the queue is empty.
    /// assert_eq!(q.pop(), Err(PopError::Empty));
    ///
    /// // Push one item and close the queue.
    /// assert_eq!(q.push(10), Ok(()));
    /// q.close();
    ///
    /// // Remaining items can be popped.
    /// assert_eq!(q.pop(), Ok(10));
    ///
    /// // Again, pop errors when the queue is empty,
    /// // but now also indicates that the queue is closed.
    /// assert_eq!(q.pop(), Err(PopError::Closed));
    /// ```
    pub fn pop(&self) -> Result<T, PopError> {
        match &self.0 {
            Inner::Single(q) => q.pop(),
            Inner::Bounded(q) => q.pop(),
            Inner::Unbounded(q) => q.pop(),
        }
    }

    /// Get an iterator over the items in the queue.
    ///
    /// The iterator will continue until the queue is empty or closed. It will never block;
    /// if the queue is empty, the iterator will return `None`. If new items are pushed into
    /// the queue, the iterator may return `Some` in the future after returning `None`.
    ///
    /// # Examples
    ///
    /// ```
    /// use concurrent_queue::ConcurrentQueue;
    ///
    /// let q = ConcurrentQueue::bounded(5);
    /// q.push(1).unwrap();
    /// q.push(2).unwrap();
    /// q.push(3).unwrap();
    ///
    /// let mut iter = q.try_iter();
    /// assert_eq!(iter.by_ref().sum::<i32>(), 6);
    /// assert_eq!(iter.next(), None);
    ///
    /// // Pushing more items will make them available to the iterator.
    /// q.push(4).unwrap();
    /// assert_eq!(iter.next(), Some(4));
    /// assert_eq!(iter.next(), None);
    /// ```
    pub fn try_iter(&self) -> TryIter<'_, T> {
        TryIter { queue: self }
    }

    /// Returns `true` if the queue is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use concurrent_queue::ConcurrentQueue;
    ///
    /// let q = ConcurrentQueue::<i32>::unbounded();
    ///
    /// assert!(q.is_empty());
    /// q.push(1).unwrap();
    /// assert!(!q.is_empty());
    /// ```
    pub fn is_empty(&self) -> bool {
        match &self.0 {
            Inner::Single(q) => q.is_empty(),
            Inner::Bounded(q) => q.is_empty(),
            Inner::Unbounded(q) => q.is_empty(),
        }
    }

    /// Returns `true` if the queue is full.
    ///
    /// An unbounded queue is never full.
    ///
    /// # Examples
    ///
    /// ```
    /// use concurrent_queue::ConcurrentQueue;
    ///
    /// let q = ConcurrentQueue::bounded(1);
    ///
    /// assert!(!q.is_full());
    /// q.push(1).unwrap();
    /// assert!(q.is_full());
    /// ```
    pub fn is_full(&self) -> bool {
        match &self.0 {
            Inner::Single(q) => q.is_full(),
            Inner::Bounded(q) => q.is_full(),
            Inner::Unbounded(q) => q.is_full(),
        }
    }

    /// Returns the number of items in the queue.
    ///
    /// # Examples
    ///
    /// ```
    /// use concurrent_queue::ConcurrentQueue;
    ///
    /// let q = ConcurrentQueue::unbounded();
    /// assert_eq!(q.len(), 0);
    ///
    /// assert_eq!(q.push(10), Ok(()));
    /// assert_eq!(q.len(), 1);
    ///
    /// assert_eq!(q.push(20), Ok(()));
    /// assert_eq!(q.len(), 2);
    /// ```
    pub fn len(&self) -> usize {
        match &self.0 {
            Inner::Single(q) => q.len(),
            Inner::Bounded(q) => q.len(),
            Inner::Unbounded(q) => q.len(),
        }
    }

    /// Returns the capacity of the queue.
    ///
    /// Unbounded queues have infinite capacity, represented as [`None`].
    ///
    /// # Examples
    ///
    /// ```
    /// use concurrent_queue::ConcurrentQueue;
    ///
    /// let q = ConcurrentQueue::<i32>::bounded(7);
    /// assert_eq!(q.capacity(), Some(7));
    ///
    /// let q = ConcurrentQueue::<i32>::unbounded();
    /// assert_eq!(q.capacity(), None);
    /// ```
    pub fn capacity(&self) -> Option<usize> {
        match &self.0 {
            Inner::Single(_) => Some(1),
            Inner::Bounded(q) => Some(q.capacity()),
            Inner::Unbounded(_) => None,
        }
    }

    /// Closes the queue.
    ///
    /// Returns `true` if this call closed the queue, or `false` if it was already closed.
    ///
    /// When a queue is closed, no more items can be pushed but the remaining items can still be
    /// popped.
    ///
    /// # Examples
    ///
    /// ```
    /// use concurrent_queue::{ConcurrentQueue, PopError, PushError};
    ///
    /// let q = ConcurrentQueue::unbounded();
    /// assert_eq!(q.push(10), Ok(()));
    ///
    /// assert!(q.close());  // `true` because this call closes the queue.
    /// assert!(!q.close()); // `false` because the queue is already closed.
    ///
    /// // Cannot push any more items when closed.
    /// assert_eq!(q.push(20), Err(PushError::Closed(20)));
    ///
    /// // Remaining items can still be popped.
    /// assert_eq!(q.pop(), Ok(10));
    ///
    /// // When no more items are present, the error is `Closed`.
    /// assert_eq!(q.pop(), Err(PopError::Closed));
    /// ```
    pub fn close(&self) -> bool {
        match &self.0 {
            Inner::Single(q) => q.close(),
            Inner::Bounded(q) => q.close(),
            Inner::Unbounded(q) => q.close(),
        }
    }

    /// Returns `true` if the queue is closed.
    ///
    /// # Examples
    ///
    /// ```
    /// use concurrent_queue::ConcurrentQueue;
    ///
    /// let q = ConcurrentQueue::<i32>::unbounded();
    ///
    /// assert!(!q.is_closed());
    /// q.close();
    /// assert!(q.is_closed());
    /// ```
    pub fn is_closed(&self) -> bool {
        match &self.0 {
            Inner::Single(q) => q.is_closed(),
            Inner::Bounded(q) => q.is_closed(),
            Inner::Unbounded(q) => q.is_closed(),
        }
    }
}

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

/// An iterator that pops items from a [`ConcurrentQueue`].
///
/// This iterator will never block; it will return `None` once the queue has
/// been exhausted. Calling `next` after `None` may yield `Some(item)` if more items
/// are pushed to the queue.
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[derive(Clone)]
pub struct TryIter<'a, T> {
    queue: &'a ConcurrentQueue<T>,
}

impl<T> fmt::Debug for TryIter<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("Iter").field(&self.queue).finish()
    }
}

impl<T> Iterator for TryIter<'_, T> {
    type Item = T;

    fn next(&mut self) -> Option<Self::Item> {
        self.queue.pop().ok()
    }
}

/// Error which occurs when popping from an empty queue.
#[derive(Clone, Copy, Eq, PartialEq)]
pub enum PopError {
    /// The queue is empty but not closed.
    Empty,

    /// The queue is empty and closed.
    Closed,
}

impl PopError {
    /// Returns `true` if the queue is empty but not closed.
    pub fn is_empty(&self) -> bool {
        match self {
            PopError::Empty => true,
            PopError::Closed => false,
        }
    }

    /// Returns `true` if the queue is empty and closed.
    pub fn is_closed(&self) -> bool {
        match self {
            PopError::Empty => false,
            PopError::Closed => true,
        }
    }
}

#[cfg(feature = "std")]
impl error::Error for PopError {}

impl fmt::Debug for PopError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            PopError::Empty => write!(f, "Empty"),
            PopError::Closed => write!(f, "Closed"),
        }
    }
}

impl fmt::Display for PopError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            PopError::Empty => write!(f, "Empty"),
            PopError::Closed => write!(f, "Closed"),
        }
    }
}

/// Error which occurs when pushing into a full or closed queue.
#[derive(Clone, Copy, Eq, PartialEq)]
pub enum PushError<T> {
    /// The queue is full but not closed.
    Full(T),

    /// The queue is closed.
    Closed(T),
}

impl<T> PushError<T> {
    /// Unwraps the item that couldn't be pushed.
    pub fn into_inner(self) -> T {
        match self {
            PushError::Full(t) => t,
            PushError::Closed(t) => t,
        }
    }

    /// Returns `true` if the queue is full but not closed.
    pub fn is_full(&self) -> bool {
        match self {
            PushError::Full(_) => true,
            PushError::Closed(_) => false,
        }
    }

    /// Returns `true` if the queue is closed.
    pub fn is_closed(&self) -> bool {
        match self {
            PushError::Full(_) => false,
            PushError::Closed(_) => true,
        }
    }
}

#[cfg(feature = "std")]
impl<T: fmt::Debug> error::Error for PushError<T> {}

impl<T: fmt::Debug> fmt::Debug for PushError<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            PushError::Full(t) => f.debug_tuple("Full").field(t).finish(),
            PushError::Closed(t) => f.debug_tuple("Closed").field(t).finish(),
        }
    }
}

impl<T> fmt::Display for PushError<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            PushError::Full(_) => write!(f, "Full"),
            PushError::Closed(_) => write!(f, "Closed"),
        }
    }
}

/// Equivalent to `atomic::fence(Ordering::SeqCst)`, but in some cases faster.
#[inline]
fn full_fence() {
    #[cfg(all(any(target_arch = "x86", target_arch = "x86_64"), not(miri), not(loom)))]
    {
        use core::{arch::asm, cell::UnsafeCell};
        // HACK(stjepang): On x86 architectures there are two different ways of executing
        // a `SeqCst` fence.
        //
        // 1. `atomic::fence(SeqCst)`, which compiles into a `mfence` instruction.
        // 2. A `lock <op>` instruction.
        //
        // Both instructions have the effect of a full barrier, but empirical benchmarks have shown
        // that the second one is sometimes a bit faster.
        let a = UnsafeCell::new(0_usize);
        // It is common to use `lock or` here, but when using a local variable, `lock not`, which
        // does not change the flag, should be slightly more efficient.
        // Refs: https://www.felixcloutier.com/x86/not
        unsafe {
            #[cfg(target_pointer_width = "64")]
            asm!("lock not qword ptr [{0}]", in(reg) a.get(), options(nostack, preserves_flags));
            #[cfg(target_pointer_width = "32")]
            asm!("lock not dword ptr [{0:e}]", in(reg) a.get(), options(nostack, preserves_flags));
        }
        return;
    }
    #[allow(unreachable_code)]
    {
        atomic::fence(Ordering::SeqCst);
    }
}