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
 679
 680
 681
 682
 683
 684
 685
 686
 687
 688
 689
 690
 691
 692
 693
 694
 695
 696
 697
 698
 699
 700
 701
 702
 703
 704
 705
 706
 707
 708
 709
 710
 711
 712
 713
 714
 715
 716
 717
 718
 719
 720
 721
 722
 723
 724
 725
 726
 727
 728
 729
 730
 731
 732
 733
 734
 735
 736
 737
 738
 739
 740
 741
 742
 743
 744
 745
 746
 747
 748
 749
 750
 751
 752
 753
 754
 755
 756
 757
 758
 759
 760
 761
 762
 763
 764
 765
 766
 767
 768
 769
 770
 771
 772
 773
 774
 775
 776
 777
 778
 779
 780
 781
 782
 783
 784
 785
 786
 787
 788
 789
 790
 791
 792
 793
 794
 795
 796
 797
 798
 799
 800
 801
 802
 803
 804
 805
 806
 807
 808
 809
 810
 811
 812
 813
 814
 815
 816
 817
 818
 819
 820
 821
 822
 823
 824
 825
 826
 827
 828
 829
 830
 831
 832
 833
 834
 835
 836
 837
 838
 839
 840
 841
 842
 843
 844
 845
 846
 847
 848
 849
 850
 851
 852
 853
 854
 855
 856
 857
 858
 859
 860
 861
 862
 863
 864
 865
 866
 867
 868
 869
 870
 871
 872
 873
 874
 875
 876
 877
 878
 879
 880
 881
 882
 883
 884
 885
 886
 887
 888
 889
 890
 891
 892
 893
 894
 895
 896
 897
 898
 899
 900
 901
 902
 903
 904
 905
 906
 907
 908
 909
 910
 911
 912
 913
 914
 915
 916
 917
 918
 919
 920
 921
 922
 923
 924
 925
 926
 927
 928
 929
 930
 931
 932
 933
 934
 935
 936
 937
 938
 939
 940
 941
 942
 943
 944
 945
 946
 947
 948
 949
 950
 951
 952
 953
 954
 955
 956
 957
 958
 959
 960
 961
 962
 963
 964
 965
 966
 967
 968
 969
 970
 971
 972
 973
 974
 975
 976
 977
 978
 979
 980
 981
 982
 983
 984
 985
 986
 987
 988
 989
 990
 991
 992
 993
 994
 995
 996
 997
 998
 999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
//! A multi-producer, single-consumer queue for sending values across
//! asynchronous tasks.
//!
//! Similarly to the `std`, channel creation provides [`Receiver`] and
//! [`Sender`] handles. [`Receiver`] implements [`Stream`] and allows a task to
//! read values out of the channel. If there is no message to read from the
//! channel, the current task will be notified when a new value is sent.
//! [`Sender`] implements the `Sink` trait and allows a task to send messages into
//! the channel. If the channel is at capacity, the send will be rejected and
//! the task will be notified when additional capacity is available. In other
//! words, the channel provides backpressure.
//!
//! Unbounded channels are also available using the `unbounded` constructor.
//!
//! # Disconnection
//!
//! When all [`Sender`] handles have been dropped, it is no longer
//! possible to send values into the channel. This is considered the termination
//! event of the stream. As such, [`Receiver::poll_next`]
//! will return `Ok(Ready(None))`.
//!
//! If the [`Receiver`] handle is dropped, then messages can no longer
//! be read out of the channel. In this case, all further attempts to send will
//! result in an error.
//!
//! # Clean Shutdown
//!
//! If the [`Receiver`] is simply dropped, then it is possible for
//! there to be messages still in the channel that will not be processed. As
//! such, it is usually desirable to perform a "clean" shutdown. To do this, the
//! receiver will first call `close`, which will prevent any further messages to
//! be sent into the channel. Then, the receiver consumes the channel to
//! completion, at which point the receiver can be dropped.
//!
//! [`Sender`]: struct.Sender.html
//! [`Receiver`]: struct.Receiver.html
//! [`Stream`]: ../../futures_core/stream/trait.Stream.html
//! [`Receiver::poll_next`]:
//!     ../../futures_core/stream/trait.Stream.html#tymethod.poll_next

// At the core, the channel uses an atomic FIFO queue for message passing. This
// queue is used as the primary coordination primitive. In order to enforce
// capacity limits and handle back pressure, a secondary FIFO queue is used to
// send parked task handles.
//
// The general idea is that the channel is created with a `buffer` size of `n`.
// The channel capacity is `n + num-senders`. Each sender gets one "guaranteed"
// slot to hold a message. This allows `Sender` to know for a fact that a send
// will succeed *before* starting to do the actual work of sending the value.
// Since most of this work is lock-free, once the work starts, it is impossible
// to safely revert.
//
// If the sender is unable to process a send operation, then the current
// task is parked and the handle is sent on the parked task queue.
//
// Note that the implementation guarantees that the channel capacity will never
// exceed the configured limit, however there is no *strict* guarantee that the
// receiver will wake up a parked task *immediately* when a slot becomes
// available. However, it will almost always unpark a task when a slot becomes
// available and it is *guaranteed* that a sender will be unparked when the
// message that caused the sender to become parked is read out of the channel.
//
// The steps for sending a message are roughly:
//
// 1) Increment the channel message count
// 2) If the channel is at capacity, push the task handle onto the wait queue
// 3) Push the message onto the message queue.
//
// The steps for receiving a message are roughly:
//
// 1) Pop a message from the message queue
// 2) Pop a task handle from the wait queue
// 3) Decrement the channel message count.
//
// It's important for the order of operations on lock-free structures to happen
// in reverse order between the sender and receiver. This makes the message
// queue the primary coordination structure and establishes the necessary
// happens-before semantics required for the acquire / release semantics used
// by the queue structure.

use futures_core::stream::{FusedStream, Stream};
use futures_core::task::{Context, Poll, Waker};
use futures_core::task::__internal::AtomicWaker;
use std::any::Any;
use std::error::Error;
use std::fmt;
use std::pin::Pin;
use std::sync::{Arc, Mutex};
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering::SeqCst;

use crate::mpsc::queue::Queue;

mod queue;
#[cfg(feature = "sink")]
mod sink_impl;

#[derive(Debug)]
struct SenderInner<T> {
    // Channel state shared between the sender and receiver.
    inner: Arc<Inner<T>>,

    // Handle to the task that is blocked on this sender. This handle is sent
    // to the receiver half in order to be notified when the sender becomes
    // unblocked.
    sender_task: Arc<Mutex<SenderTask>>,

    // True if the sender might be blocked. This is an optimization to avoid
    // having to lock the mutex most of the time.
    maybe_parked: bool,
}

// We never project Pin<&mut SenderInner> to `Pin<&mut T>`
impl<T> Unpin for SenderInner<T> {}

/// The transmission end of a bounded mpsc channel.
///
/// This value is created by the [`channel`](channel) function.
#[derive(Debug)]
pub struct Sender<T>(Option<SenderInner<T>>);

/// The transmission end of an unbounded mpsc channel.
///
/// This value is created by the [`unbounded`](unbounded) function.
#[derive(Debug)]
pub struct UnboundedSender<T>(Option<SenderInner<T>>);

trait AssertKinds: Send + Sync + Clone {}
impl AssertKinds for UnboundedSender<u32> {}

/// The receiving end of a bounded mpsc channel.
///
/// This value is created by the [`channel`](channel) function.
#[derive(Debug)]
pub struct Receiver<T> {
    inner: Option<Arc<Inner<T>>>,
}

/// The receiving end of an unbounded mpsc channel.
///
/// This value is created by the [`unbounded`](unbounded) function.
#[derive(Debug)]
pub struct UnboundedReceiver<T>(Receiver<T>);

// `Pin<&mut UnboundedReceiver<T>>` is never projected to `Pin<&mut T>`
impl<T> Unpin for UnboundedReceiver<T> {}

/// The error type for [`Sender`s](Sender) used as `Sink`s.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct SendError {
    kind: SendErrorKind,
}

/// The error type returned from [`try_send`](Sender::try_send).
#[derive(Clone, PartialEq, Eq)]
pub struct TrySendError<T> {
    err: SendError,
    val: T,
}

#[derive(Clone, Debug, PartialEq, Eq)]
enum SendErrorKind {
    Full,
    Disconnected,
}

/// The error type returned from [`try_next`](Receiver::try_next).
pub struct TryRecvError {
    _inner: (),
}

impl fmt::Display for SendError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        if self.is_full() {
            write!(f, "send failed because channel is full")
        } else {
            write!(f, "send failed because receiver is gone")
        }
    }
}

impl Error for SendError {}

impl SendError {
    /// Returns true if this error is a result of the channel being full.
    pub fn is_full(&self) -> bool {
        match self.kind {
            SendErrorKind::Full => true,
            _ => false,
        }
    }

    /// Returns true if this error is a result of the receiver being dropped.
    pub fn is_disconnected(&self) -> bool {
        match self.kind {
            SendErrorKind::Disconnected => true,
            _ => false,
        }
    }
}

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

impl<T> fmt::Display for TrySendError<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        if self.is_full() {
            write!(f, "send failed because channel is full")
        } else {
            write!(f, "send failed because receiver is gone")
        }
    }
}

impl<T: Any> Error for TrySendError<T> {}

impl<T> TrySendError<T> {
    /// Returns true if this error is a result of the channel being full.
    pub fn is_full(&self) -> bool {
        self.err.is_full()
    }

    /// Returns true if this error is a result of the receiver being dropped.
    pub fn is_disconnected(&self) -> bool {
        self.err.is_disconnected()
    }

    /// Returns the message that was attempted to be sent but failed.
    pub fn into_inner(self) -> T {
        self.val
    }

    /// Drops the message and converts into a `SendError`.
    pub fn into_send_error(self) -> SendError {
        self.err
    }
}

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

impl fmt::Display for TryRecvError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "receiver channel is empty")
    }
}

impl Error for TryRecvError {}

#[derive(Debug)]
struct Inner<T> {
    // Max buffer size of the channel. If `None` then the channel is unbounded.
    buffer: Option<usize>,

    // Internal channel state. Consists of the number of messages stored in the
    // channel as well as a flag signalling that the channel is closed.
    state: AtomicUsize,

    // Atomic, FIFO queue used to send messages to the receiver
    message_queue: Queue<T>,

    // Atomic, FIFO queue used to send parked task handles to the receiver.
    parked_queue: Queue<Arc<Mutex<SenderTask>>>,

    // Number of senders in existence
    num_senders: AtomicUsize,

    // Handle to the receiver's task.
    recv_task: AtomicWaker,
}

// Struct representation of `Inner::state`.
#[derive(Debug, Clone, Copy)]
struct State {
    // `true` when the channel is open
    is_open: bool,

    // Number of messages in the channel
    num_messages: usize,
}

// The `is_open` flag is stored in the left-most bit of `Inner::state`
const OPEN_MASK: usize = usize::max_value() - (usize::max_value() >> 1);

// When a new channel is created, it is created in the open state with no
// pending messages.
const INIT_STATE: usize = OPEN_MASK;

// The maximum number of messages that a channel can track is `usize::max_value() >> 1`
const MAX_CAPACITY: usize = !(OPEN_MASK);

// The maximum requested buffer size must be less than the maximum capacity of
// a channel. This is because each sender gets a guaranteed slot.
const MAX_BUFFER: usize = MAX_CAPACITY >> 1;

// Sent to the consumer to wake up blocked producers
#[derive(Debug)]
struct SenderTask {
    task: Option<Waker>,
    is_parked: bool,
}

impl SenderTask {
    fn new() -> Self {
        SenderTask {
            task: None,
            is_parked: false,
        }
    }

    fn notify(&mut self) {
        self.is_parked = false;

        if let Some(task) = self.task.take() {
            task.wake();
        }
    }
}

/// Creates a bounded mpsc channel for communicating between asynchronous tasks.
///
/// Being bounded, this channel provides backpressure to ensure that the sender
/// outpaces the receiver by only a limited amount. The channel's capacity is
/// equal to `buffer + num-senders`. In other words, each sender gets a
/// guaranteed slot in the channel capacity, and on top of that there are
/// `buffer` "first come, first serve" slots available to all senders.
///
/// The [`Receiver`](Receiver) returned implements the
/// [`Stream`](futures_core::stream::Stream) trait, while [`Sender`](Sender) implements
/// `Sink`.
pub fn channel<T>(buffer: usize) -> (Sender<T>, Receiver<T>) {
    // Check that the requested buffer size does not exceed the maximum buffer
    // size permitted by the system.
    assert!(buffer < MAX_BUFFER, "requested buffer size too large");
    let (tx, rx) = channel2(Some(buffer));
    (Sender(Some(tx)), rx)
}

/// Creates an unbounded mpsc channel for communicating between asynchronous
/// tasks.
///
/// A `send` on this channel will always succeed as long as the receive half has
/// not been closed. If the receiver falls behind, messages will be arbitrarily
/// buffered.
///
/// **Note** that the amount of available system memory is an implicit bound to
/// the channel. Using an `unbounded` channel has the ability of causing the
/// process to run out of memory. In this case, the process will be aborted.
pub fn unbounded<T>() -> (UnboundedSender<T>, UnboundedReceiver<T>) {
    let (tx, rx) = channel2(None);
    (UnboundedSender(Some(tx)), UnboundedReceiver(rx))
}

fn channel2<T>(buffer: Option<usize>) -> (SenderInner<T>, Receiver<T>) {
    let inner = Arc::new(Inner {
        buffer,
        state: AtomicUsize::new(INIT_STATE),
        message_queue: Queue::new(),
        parked_queue: Queue::new(),
        num_senders: AtomicUsize::new(1),
        recv_task: AtomicWaker::new(),
    });

    let tx = SenderInner {
        inner: inner.clone(),
        sender_task: Arc::new(Mutex::new(SenderTask::new())),
        maybe_parked: false,
    };

    let rx = Receiver {
        inner: Some(inner),
    };

    (tx, rx)
}

/*
 *
 * ===== impl Sender =====
 *
 */

impl<T> SenderInner<T> {
    /// Attempts to send a message on this `Sender`, returning the message
    /// if there was an error.
    fn try_send(&mut self, msg: T) -> Result<(), TrySendError<T>> {
        // If the sender is currently blocked, reject the message
        if !self.poll_unparked(None).is_ready() {
            return Err(TrySendError {
                err: SendError {
                    kind: SendErrorKind::Full,
                },
                val: msg,
            });
        }

        // The channel has capacity to accept the message, so send it
        self.do_send_b(msg)
    }

    // Do the send without failing.
    // Can be called only by bounded sender.
    fn do_send_b(&mut self, msg: T)
        -> Result<(), TrySendError<T>>
    {
        // Anyone callig do_send *should* make sure there is room first,
        // but assert here for tests as a sanity check.
        debug_assert!(self.poll_unparked(None).is_ready());

        // First, increment the number of messages contained by the channel.
        // This operation will also atomically determine if the sender task
        // should be parked.
        //
        // None is returned in the case that the channel has been closed by the
        // receiver. This happens when `Receiver::close` is called or the
        // receiver is dropped.
        let park_self = match self.inc_num_messages() {
            Some(num_messages) => {
                // Block if the current number of pending messages has exceeded
                // the configured buffer size
                num_messages > self.inner.buffer.unwrap()
            }
            None => return Err(TrySendError {
                err: SendError {
                    kind: SendErrorKind::Disconnected,
                },
                val: msg,
            }),
        };

        // If the channel has reached capacity, then the sender task needs to
        // be parked. This will send the task handle on the parked task queue.
        //
        // However, when `do_send` is called while dropping the `Sender`,
        // `task::current()` can't be called safely. In this case, in order to
        // maintain internal consistency, a blank message is pushed onto the
        // parked task queue.
        if park_self {
            self.park();
        }

        self.queue_push_and_signal(msg);

        Ok(())
    }

    fn poll_ready_nb(&self) -> Poll<Result<(), SendError>> {
        let state = decode_state(self.inner.state.load(SeqCst));
        if state.is_open {
            Poll::Ready(Ok(()))
        } else {
            Poll::Ready(Err(SendError {
                kind: SendErrorKind::Disconnected,
            }))
        }
    }


    // Push message to the queue and signal to the receiver
    fn queue_push_and_signal(&self, msg: T) {
        // Push the message onto the message queue
        self.inner.message_queue.push(msg);

        // Signal to the receiver that a message has been enqueued. If the
        // receiver is parked, this will unpark the task.
        self.inner.recv_task.wake();
    }

    // Increment the number of queued messages. Returns the resulting number.
    fn inc_num_messages(&self) -> Option<usize> {
        let mut curr = self.inner.state.load(SeqCst);

        loop {
            let mut state = decode_state(curr);

            // The receiver end closed the channel.
            if !state.is_open {
                return None;
            }

            // This probably is never hit? Odds are the process will run out of
            // memory first. It may be worth to return something else in this
            // case?
            assert!(state.num_messages < MAX_CAPACITY, "buffer space \
                    exhausted; sending this messages would overflow the state");

            state.num_messages += 1;

            let next = encode_state(&state);
            match self.inner.state.compare_exchange(curr, next, SeqCst, SeqCst) {
                Ok(_) => {
                    return Some(state.num_messages)
                }
                Err(actual) => curr = actual,
            }
        }
    }

    fn park(&mut self) {
        {
            let mut sender = self.sender_task.lock().unwrap();
            sender.task = None;
            sender.is_parked = true;
        }

        // Send handle over queue
        let t = self.sender_task.clone();
        self.inner.parked_queue.push(t);

        // Check to make sure we weren't closed after we sent our task on the
        // queue
        let state = decode_state(self.inner.state.load(SeqCst));
        self.maybe_parked = state.is_open;
    }

    /// Polls the channel to determine if there is guaranteed capacity to send
    /// at least one item without waiting.
    ///
    /// # Return value
    ///
    /// This method returns:
    ///
    /// - `Poll::Ready(Ok(_))` if there is sufficient capacity;
    /// - `Poll::Pending` if the channel may not have
    ///   capacity, in which case the current task is queued to be notified once
    ///   capacity is available;
    /// - `Poll::Ready(Err(SendError))` if the receiver has been dropped.
    fn poll_ready(
        &mut self,
        cx: &mut Context<'_>,
    ) -> Poll<Result<(), SendError>> {
        let state = decode_state(self.inner.state.load(SeqCst));
        if !state.is_open {
            return Poll::Ready(Err(SendError {
                kind: SendErrorKind::Disconnected,
            }));
        }

        self.poll_unparked(Some(cx)).map(Ok)
    }

    /// Returns whether the senders send to the same receiver.
    fn same_receiver(&self, other: &Self) -> bool {
        Arc::ptr_eq(&self.inner, &other.inner)
    }

    /// Returns whether this channel is closed without needing a context.
    fn is_closed(&self) -> bool {
        !decode_state(self.inner.state.load(SeqCst)).is_open
    }

    /// Closes this channel from the sender side, preventing any new messages.
    fn close_channel(&self) {
        // There's no need to park this sender, its dropping,
        // and we don't want to check for capacity, so skip
        // that stuff from `do_send`.

        self.inner.set_closed();
        self.inner.recv_task.wake();
    }

    fn poll_unparked(&mut self, cx: Option<&mut Context<'_>>) -> Poll<()> {
        // First check the `maybe_parked` variable. This avoids acquiring the
        // lock in most cases
        if self.maybe_parked {
            // Get a lock on the task handle
            let mut task = self.sender_task.lock().unwrap();

            if !task.is_parked {
                self.maybe_parked = false;
                return Poll::Ready(())
            }

            // At this point, an unpark request is pending, so there will be an
            // unpark sometime in the future. We just need to make sure that
            // the correct task will be notified.
            //
            // Update the task in case the `Sender` has been moved to another
            // task
            task.task = cx.map(|cx| cx.waker().clone());

            Poll::Pending
        } else {
            Poll::Ready(())
        }
    }
}

impl<T> Sender<T> {
    /// Attempts to send a message on this `Sender`, returning the message
    /// if there was an error.
    pub fn try_send(&mut self, msg: T) -> Result<(), TrySendError<T>> {
        if let Some(inner) = &mut self.0 {
            inner.try_send(msg)
        } else {
            Err(TrySendError {
                err: SendError {
                    kind: SendErrorKind::Disconnected,
                },
                val: msg,
            })
        }
    }

    /// Send a message on the channel.
    ///
    /// This function should only be called after
    /// [`poll_ready`](Sender::poll_ready) has reported that the channel is
    /// ready to receive a message.
    pub fn start_send(&mut self, msg: T) -> Result<(), SendError> {
        self.try_send(msg)
            .map_err(|e| e.err)
    }

    /// Polls the channel to determine if there is guaranteed capacity to send
    /// at least one item without waiting.
    ///
    /// # Return value
    ///
    /// This method returns:
    ///
    /// - `Poll::Ready(Ok(_))` if there is sufficient capacity;
    /// - `Poll::Pending` if the channel may not have
    ///   capacity, in which case the current task is queued to be notified once
    ///   capacity is available;
    /// - `Poll::Ready(Err(SendError))` if the receiver has been dropped.
    pub fn poll_ready(
        &mut self,
        cx: &mut Context<'_>,
    ) -> Poll<Result<(), SendError>> {
        let inner = self.0.as_mut().ok_or(SendError {
            kind: SendErrorKind::Disconnected,
        })?;
        inner.poll_ready(cx)
    }

    /// Returns whether this channel is closed without needing a context.
    pub fn is_closed(&self) -> bool {
        self.0.as_ref().map(SenderInner::is_closed).unwrap_or(true)
    }

    /// Closes this channel from the sender side, preventing any new messages.
    pub fn close_channel(&mut self) {
        if let Some(inner) = &mut self.0 {
            inner.close_channel();
        }
    }

    /// Disconnects this sender from the channel, closing it if there are no more senders left.
    pub fn disconnect(&mut self) {
        self.0 = None;
    }

    /// Returns whether the senders send to the same receiver.
    pub fn same_receiver(&self, other: &Self) -> bool {
        match (&self.0, &other.0) {
            (Some(inner), Some(other)) => inner.same_receiver(other),
            _ => false,
        }
    }
}

impl<T> UnboundedSender<T> {
    /// Check if the channel is ready to receive a message.
    pub fn poll_ready(
        &self,
        _: &mut Context<'_>,
    ) -> Poll<Result<(), SendError>> {
        let inner = self.0.as_ref().ok_or(SendError {
            kind: SendErrorKind::Disconnected,
        })?;
        inner.poll_ready_nb()
    }

    /// Returns whether this channel is closed without needing a context.
    pub fn is_closed(&self) -> bool {
        self.0.as_ref().map(SenderInner::is_closed).unwrap_or(true)
    }

    /// Closes this channel from the sender side, preventing any new messages.
    pub fn close_channel(&self) {
        if let Some(inner) = &self.0 {
            inner.close_channel();
        }
    }

    /// Disconnects this sender from the channel, closing it if there are no more senders left.
    pub fn disconnect(&mut self) {
        self.0 = None;
    }

    // Do the send without parking current task.
    fn do_send_nb(&self, msg: T) -> Result<(), TrySendError<T>> {
        if let Some(inner) = &self.0 {
            if inner.inc_num_messages().is_some() {
                inner.queue_push_and_signal(msg);
                return Ok(());
            }
        }

        Err(TrySendError {
            err: SendError {
                kind: SendErrorKind::Disconnected,
            },
            val: msg,
        })
    }

    /// Send a message on the channel.
    ///
    /// This method should only be called after `poll_ready` has been used to
    /// verify that the channel is ready to receive a message.
    pub fn start_send(&mut self, msg: T) -> Result<(), SendError> {
        self.do_send_nb(msg)
            .map_err(|e| e.err)
    }

    /// Sends a message along this channel.
    ///
    /// This is an unbounded sender, so this function differs from `Sink::send`
    /// by ensuring the return type reflects that the channel is always ready to
    /// receive messages.
    pub fn unbounded_send(&self, msg: T) -> Result<(), TrySendError<T>> {
        self.do_send_nb(msg)
    }

    /// Returns whether the senders send to the same receiver.
    pub fn same_receiver(&self, other: &Self) -> bool {
        match (&self.0, &other.0) {
            (Some(inner), Some(other)) => inner.same_receiver(other),
            _ => false,
        }
    }
}

impl<T> Clone for Sender<T> {
    fn clone(&self) -> Sender<T> {
        Sender(self.0.clone())
    }
}

impl<T> Clone for UnboundedSender<T> {
    fn clone(&self) -> UnboundedSender<T> {
        UnboundedSender(self.0.clone())
    }
}

impl<T> Clone for SenderInner<T> {
    fn clone(&self) -> SenderInner<T> {
        // Since this atomic op isn't actually guarding any memory and we don't
        // care about any orderings besides the ordering on the single atomic
        // variable, a relaxed ordering is acceptable.
        let mut curr = self.inner.num_senders.load(SeqCst);

        loop {
            // If the maximum number of senders has been reached, then fail
            if curr == self.inner.max_senders() {
                panic!("cannot clone `Sender` -- too many outstanding senders");
            }

            debug_assert!(curr < self.inner.max_senders());

            let next = curr + 1;
            let actual = self.inner.num_senders.compare_and_swap(curr, next, SeqCst);

            // The ABA problem doesn't matter here. We only care that the
            // number of senders never exceeds the maximum.
            if actual == curr {
                return SenderInner {
                    inner: self.inner.clone(),
                    sender_task: Arc::new(Mutex::new(SenderTask::new())),
                    maybe_parked: false,
                };
            }

            curr = actual;
        }
    }
}

impl<T> Drop for SenderInner<T> {
    fn drop(&mut self) {
        // Ordering between variables don't matter here
        let prev = self.inner.num_senders.fetch_sub(1, SeqCst);

        if prev == 1 {
            self.close_channel();
        }
    }
}

/*
 *
 * ===== impl Receiver =====
 *
 */

impl<T> Receiver<T> {
    /// Closes the receiving half of a channel, without dropping it.
    ///
    /// This prevents any further messages from being sent on the channel while
    /// still enabling the receiver to drain messages that are buffered.
    pub fn close(&mut self) {
        if let Some(inner) = &mut self.inner {
            inner.set_closed();

            // Wake up any threads waiting as they'll see that we've closed the
            // channel and will continue on their merry way.
            while let Some(task) = unsafe { inner.parked_queue.pop_spin() } {
                task.lock().unwrap().notify();
            }
        }
    }

    /// Tries to receive the next message without notifying a context if empty.
    ///
    /// It is not recommended to call this function from inside of a future,
    /// only when you've otherwise arranged to be notified when the channel is
    /// no longer empty.
    ///
    /// This function will panic if called after `try_next` or `poll_next` has
    /// returned None.
    pub fn try_next(&mut self) -> Result<Option<T>, TryRecvError> {
        match self.next_message() {
            Poll::Ready(msg) => {
                Ok(msg)
            },
            Poll::Pending => Err(TryRecvError { _inner: () }),
        }
    }

    fn next_message(&mut self) -> Poll<Option<T>> {
        let inner = self.inner.as_mut().expect("Receiver::next_message called after `None`");
        // Pop off a message
        match unsafe { inner.message_queue.pop_spin() } {
            Some(msg) => {
                // If there are any parked task handles in the parked queue,
                // pop one and unpark it.
                self.unpark_one();

                // Decrement number of messages
                self.dec_num_messages();

                Poll::Ready(Some(msg))
            }
            None => {
                let state = decode_state(inner.state.load(SeqCst));
                if state.is_open || state.num_messages != 0 {
                    // If queue is open, we need to return Pending
                    // to be woken up when new messages arrive.
                    // If queue is closed but num_messages is non-zero,
                    // it means that senders updated the state,
                    // but didn't put message to queue yet,
                    // so we need to park until sender unparks the task
                    // after queueing the message.
                    Poll::Pending
                } else {
                    // If closed flag is set AND there are no pending messages
                    // it means end of stream
                    self.inner = None;
                    Poll::Ready(None)
                }
            }
        }
    }

    // Unpark a single task handle if there is one pending in the parked queue
    fn unpark_one(&mut self) {
        if let Some(inner) = &mut self.inner {
            if let Some(task) = unsafe { inner.parked_queue.pop_spin() } {
                task.lock().unwrap().notify();
            }
        }
    }

    fn dec_num_messages(&self) {
        if let Some(inner) = &self.inner {
            // OPEN_MASK is highest bit, so it's unaffected by subtraction
            // unless there's underflow, and we know there's no underflow
            // because number of messages at this point is always > 0.
            inner.state.fetch_sub(1, SeqCst);
        }
    }
}

// The receiver does not ever take a Pin to the inner T
impl<T> Unpin for Receiver<T> {}

impl<T> FusedStream for Receiver<T> {
    fn is_terminated(&self) -> bool {
        self.inner.is_none()
    }
}

impl<T> Stream for Receiver<T> {
    type Item = T;

    fn poll_next(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Option<T>> {
            // Try to read a message off of the message queue.
        match self.next_message() {
            Poll::Ready(msg) => {
                if msg.is_none() {
                    self.inner = None;
                }
                Poll::Ready(msg)
            },
            Poll::Pending => {
                // There are no messages to read, in this case, park.
                self.inner.as_ref().unwrap().recv_task.register(cx.waker());
                // Check queue again after parking to prevent race condition:
                // a message could be added to the queue after previous `next_message`
                // before `register` call.
                self.next_message()
            }
        }
    }
}

impl<T> Drop for Receiver<T> {
    fn drop(&mut self) {
        // Drain the channel of all pending messages
        self.close();
        if self.inner.is_some() {
            while let Poll::Ready(Some(..)) = self.next_message() {
                // ...
            }
        }
    }
}

impl<T> UnboundedReceiver<T> {
    /// Closes the receiving half of the channel, without dropping it.
    ///
    /// This prevents any further messages from being sent on the channel while
    /// still enabling the receiver to drain messages that are buffered.
    pub fn close(&mut self) {
        self.0.close();
    }

    /// Tries to receive the next message without notifying a context if empty.
    ///
    /// It is not recommended to call this function from inside of a future,
    /// only when you've otherwise arranged to be notified when the channel is
    /// no longer empty.
    ///
    /// This function will panic if called after `try_next` or `poll_next` has
    /// returned None.
    pub fn try_next(&mut self) -> Result<Option<T>, TryRecvError> {
        self.0.try_next()
    }
}

impl<T> FusedStream for UnboundedReceiver<T> {
    fn is_terminated(&self) -> bool {
        self.0.is_terminated()
    }
}

impl<T> Stream for UnboundedReceiver<T> {
    type Item = T;

    fn poll_next(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Option<T>> {
        Pin::new(&mut self.0).poll_next(cx)
    }
}

/*
 *
 * ===== impl Inner =====
 *
 */

impl<T> Inner<T> {
    // The return value is such that the total number of messages that can be
    // enqueued into the channel will never exceed MAX_CAPACITY
    fn max_senders(&self) -> usize {
        match self.buffer {
            Some(buffer) => MAX_CAPACITY - buffer,
            None => MAX_BUFFER,
        }
    }

    // Clear `open` flag in the state, keep `num_messages` intact.
    fn set_closed(&self) {
        let curr = self.state.load(SeqCst);
        if !decode_state(curr).is_open {
            return;
        }

        self.state.fetch_and(!OPEN_MASK, SeqCst);
    }
}

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

/*
 *
 * ===== Helpers =====
 *
 */

fn decode_state(num: usize) -> State {
    State {
        is_open: num & OPEN_MASK == OPEN_MASK,
        num_messages: num & MAX_CAPACITY,
    }
}

fn encode_state(state: &State) -> usize {
    let mut num = state.num_messages;

    if state.is_open {
        num |= OPEN_MASK;
    }

    num
}