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 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178
//! A multi-producer, multi-consumer broadcast queue. Each sent value is seen by
//! all consumers.
//!
//! A [`Sender`] is used to broadcast values to **all** connected [`Receiver`]
//! values. [`Sender`] handles are clone-able, allowing concurrent send and
//! receive actions. [`Sender`] and [`Receiver`] are both `Send` and `Sync` as
//! long as `T` is also `Send` or `Sync` respectively.
//!
//! When a value is sent, **all** [`Receiver`] handles are notified and will
//! receive the value. The value is stored once inside the channel and cloned on
//! demand for each receiver. Once all receivers have received a clone of the
//! value, the value is released from the channel.
//!
//! A channel is created by calling [`channel`], specifying the maximum number
//! of messages the channel can retain at any given time.
//!
//! New [`Receiver`] handles are created by calling [`Sender::subscribe`]. The
//! returned [`Receiver`] will receive values sent **after** the call to
//! `subscribe`.
//!
//! ## Lagging
//!
//! As sent messages must be retained until **all** [`Receiver`] handles receive
//! a clone, broadcast channels are susceptible to the "slow receiver" problem.
//! In this case, all but one receiver are able to receive values at the rate
//! they are sent. Because one receiver is stalled, the channel starts to fill
//! up.
//!
//! This broadcast channel implementation handles this case by setting a hard
//! upper bound on the number of values the channel may retain at any given
//! time. This upper bound is passed to the [`channel`] function as an argument.
//!
//! If a value is sent when the channel is at capacity, the oldest value
//! currently held by the channel is released. This frees up space for the new
//! value. Any receiver that has not yet seen the released value will return
//! [`RecvError::Lagged`] the next time [`recv`] is called.
//!
//! Once [`RecvError::Lagged`] is returned, the lagging receiver's position is
//! updated to the oldest value contained by the channel. The next call to
//! [`recv`] will return this value.
//!
//! This behavior enables a receiver to detect when it has lagged so far behind
//! that data has been dropped. The caller may decide how to respond to this:
//! either by aborting its task or by tolerating lost messages and resuming
//! consumption of the channel.
//!
//! ## Closing
//!
//! When **all** [`Sender`] handles have been dropped, no new values may be
//! sent. At this point, the channel is "closed". Once a receiver has received
//! all values retained by the channel, the next call to [`recv`] will return
//! with [`RecvError::Closed`].
//!
//! [`Sender`]: crate::sync::broadcast::Sender
//! [`Sender::subscribe`]: crate::sync::broadcast::Sender::subscribe
//! [`Receiver`]: crate::sync::broadcast::Receiver
//! [`channel`]: crate::sync::broadcast::channel
//! [`RecvError::Lagged`]: crate::sync::broadcast::error::RecvError::Lagged
//! [`RecvError::Closed`]: crate::sync::broadcast::error::RecvError::Closed
//! [`recv`]: crate::sync::broadcast::Receiver::recv
//!
//! # Examples
//!
//! Basic usage
//!
//! ```
//! use tokio::sync::broadcast;
//!
//! #[tokio::main]
//! async fn main() {
//! let (tx, mut rx1) = broadcast::channel(16);
//! let mut rx2 = tx.subscribe();
//!
//! tokio::spawn(async move {
//! assert_eq!(rx1.recv().await.unwrap(), 10);
//! assert_eq!(rx1.recv().await.unwrap(), 20);
//! });
//!
//! tokio::spawn(async move {
//! assert_eq!(rx2.recv().await.unwrap(), 10);
//! assert_eq!(rx2.recv().await.unwrap(), 20);
//! });
//!
//! tx.send(10).unwrap();
//! tx.send(20).unwrap();
//! }
//! ```
//!
//! Handling lag
//!
//! ```
//! use tokio::sync::broadcast;
//!
//! #[tokio::main]
//! async fn main() {
//! let (tx, mut rx) = broadcast::channel(2);
//!
//! tx.send(10).unwrap();
//! tx.send(20).unwrap();
//! tx.send(30).unwrap();
//!
//! // The receiver lagged behind
//! assert!(rx.recv().await.is_err());
//!
//! // At this point, we can abort or continue with lost messages
//!
//! assert_eq!(20, rx.recv().await.unwrap());
//! assert_eq!(30, rx.recv().await.unwrap());
//! }
//! ```
use crate::loom::cell::UnsafeCell;
use crate::loom::sync::atomic::AtomicUsize;
use crate::loom::sync::{Arc, Mutex, RwLock, RwLockReadGuard};
use crate::util::linked_list::{self, LinkedList};
use std::fmt;
use std::future::Future;
use std::marker::PhantomPinned;
use std::pin::Pin;
use std::ptr::NonNull;
use std::sync::atomic::Ordering::SeqCst;
use std::task::{Context, Poll, Waker};
use std::usize;
/// Sending-half of the [`broadcast`] channel.
///
/// May be used from many threads. Messages can be sent with
/// [`send`][Sender::send].
///
/// # Examples
///
/// ```
/// use tokio::sync::broadcast;
///
/// #[tokio::main]
/// async fn main() {
/// let (tx, mut rx1) = broadcast::channel(16);
/// let mut rx2 = tx.subscribe();
///
/// tokio::spawn(async move {
/// assert_eq!(rx1.recv().await.unwrap(), 10);
/// assert_eq!(rx1.recv().await.unwrap(), 20);
/// });
///
/// tokio::spawn(async move {
/// assert_eq!(rx2.recv().await.unwrap(), 10);
/// assert_eq!(rx2.recv().await.unwrap(), 20);
/// });
///
/// tx.send(10).unwrap();
/// tx.send(20).unwrap();
/// }
/// ```
///
/// [`broadcast`]: crate::sync::broadcast
pub struct Sender<T> {
shared: Arc<Shared<T>>,
}
/// Receiving-half of the [`broadcast`] channel.
///
/// Must not be used concurrently. Messages may be retrieved using
/// [`recv`][Receiver::recv].
///
/// To turn this receiver into a `Stream`, you can use the [`BroadcastStream`]
/// wrapper.
///
/// [`BroadcastStream`]: https://docs.rs/tokio-stream/0.1/tokio_stream/wrappers/struct.BroadcastStream.html
///
/// # Examples
///
/// ```
/// use tokio::sync::broadcast;
///
/// #[tokio::main]
/// async fn main() {
/// let (tx, mut rx1) = broadcast::channel(16);
/// let mut rx2 = tx.subscribe();
///
/// tokio::spawn(async move {
/// assert_eq!(rx1.recv().await.unwrap(), 10);
/// assert_eq!(rx1.recv().await.unwrap(), 20);
/// });
///
/// tokio::spawn(async move {
/// assert_eq!(rx2.recv().await.unwrap(), 10);
/// assert_eq!(rx2.recv().await.unwrap(), 20);
/// });
///
/// tx.send(10).unwrap();
/// tx.send(20).unwrap();
/// }
/// ```
///
/// [`broadcast`]: crate::sync::broadcast
pub struct Receiver<T> {
/// State shared with all receivers and senders.
shared: Arc<Shared<T>>,
/// Next position to read from
next: u64,
}
pub mod error {
//! Broadcast error types
use std::fmt;
/// Error returned by from the [`send`] function on a [`Sender`].
///
/// A **send** operation can only fail if there are no active receivers,
/// implying that the message could never be received. The error contains the
/// message being sent as a payload so it can be recovered.
///
/// [`send`]: crate::sync::broadcast::Sender::send
/// [`Sender`]: crate::sync::broadcast::Sender
#[derive(Debug)]
pub struct SendError<T>(pub T);
impl<T> fmt::Display for SendError<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "channel closed")
}
}
impl<T: fmt::Debug> std::error::Error for SendError<T> {}
/// An error returned from the [`recv`] function on a [`Receiver`].
///
/// [`recv`]: crate::sync::broadcast::Receiver::recv
/// [`Receiver`]: crate::sync::broadcast::Receiver
#[derive(Debug, PartialEq, Clone)]
pub enum RecvError {
/// There are no more active senders implying no further messages will ever
/// be sent.
Closed,
/// The receiver lagged too far behind. Attempting to receive again will
/// return the oldest message still retained by the channel.
///
/// Includes the number of skipped messages.
Lagged(u64),
}
impl fmt::Display for RecvError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self {
RecvError::Closed => write!(f, "channel closed"),
RecvError::Lagged(amt) => write!(f, "channel lagged by {}", amt),
}
}
}
impl std::error::Error for RecvError {}
/// An error returned from the [`try_recv`] function on a [`Receiver`].
///
/// [`try_recv`]: crate::sync::broadcast::Receiver::try_recv
/// [`Receiver`]: crate::sync::broadcast::Receiver
#[derive(Debug, PartialEq, Clone)]
pub enum TryRecvError {
/// The channel is currently empty. There are still active
/// [`Sender`] handles, so data may yet become available.
///
/// [`Sender`]: crate::sync::broadcast::Sender
Empty,
/// There are no more active senders implying no further messages will ever
/// be sent.
Closed,
/// The receiver lagged too far behind and has been forcibly disconnected.
/// Attempting to receive again will return the oldest message still
/// retained by the channel.
///
/// Includes the number of skipped messages.
Lagged(u64),
}
impl fmt::Display for TryRecvError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self {
TryRecvError::Empty => write!(f, "channel empty"),
TryRecvError::Closed => write!(f, "channel closed"),
TryRecvError::Lagged(amt) => write!(f, "channel lagged by {}", amt),
}
}
}
impl std::error::Error for TryRecvError {}
}
use self::error::*;
/// Data shared between senders and receivers.
struct Shared<T> {
/// slots in the channel.
buffer: Box<[RwLock<Slot<T>>]>,
/// Mask a position -> index.
mask: usize,
/// Tail of the queue. Includes the rx wait list.
tail: Mutex<Tail>,
/// Number of outstanding Sender handles.
num_tx: AtomicUsize,
}
/// Next position to write a value.
struct Tail {
/// Next position to write to.
pos: u64,
/// Number of active receivers.
rx_cnt: usize,
/// True if the channel is closed.
closed: bool,
/// Receivers waiting for a value.
waiters: LinkedList<Waiter, <Waiter as linked_list::Link>::Target>,
}
/// Slot in the buffer.
struct Slot<T> {
/// Remaining number of receivers that are expected to see this value.
///
/// When this goes to zero, the value is released.
///
/// An atomic is used as it is mutated concurrently with the slot read lock
/// acquired.
rem: AtomicUsize,
/// Uniquely identifies the `send` stored in the slot.
pos: u64,
/// True signals the channel is closed.
closed: bool,
/// The value being broadcast.
///
/// The value is set by `send` when the write lock is held. When a reader
/// drops, `rem` is decremented. When it hits zero, the value is dropped.
val: UnsafeCell<Option<T>>,
}
/// An entry in the wait queue.
struct Waiter {
/// True if queued.
queued: bool,
/// Task waiting on the broadcast channel.
waker: Option<Waker>,
/// Intrusive linked-list pointers.
pointers: linked_list::Pointers<Waiter>,
/// Should not be `Unpin`.
_p: PhantomPinned,
}
struct RecvGuard<'a, T> {
slot: RwLockReadGuard<'a, Slot<T>>,
}
/// Receive a value future.
struct Recv<'a, T> {
/// Receiver being waited on.
receiver: &'a mut Receiver<T>,
/// Entry in the waiter `LinkedList`.
waiter: UnsafeCell<Waiter>,
}
unsafe impl<'a, T: Send> Send for Recv<'a, T> {}
unsafe impl<'a, T: Send> Sync for Recv<'a, T> {}
/// Max number of receivers. Reserve space to lock.
const MAX_RECEIVERS: usize = usize::MAX >> 2;
/// Create a bounded, multi-producer, multi-consumer channel where each sent
/// value is broadcasted to all active receivers.
///
/// All data sent on [`Sender`] will become available on every active
/// [`Receiver`] in the same order as it was sent.
///
/// The `Sender` can be cloned to `send` to the same channel from multiple
/// points in the process or it can be used concurrently from an `Arc`. New
/// `Receiver` handles are created by calling [`Sender::subscribe`].
///
/// If all [`Receiver`] handles are dropped, the `send` method will return a
/// [`SendError`]. Similarly, if all [`Sender`] handles are dropped, the [`recv`]
/// method will return a [`RecvError`].
///
/// [`Sender`]: crate::sync::broadcast::Sender
/// [`Sender::subscribe`]: crate::sync::broadcast::Sender::subscribe
/// [`Receiver`]: crate::sync::broadcast::Receiver
/// [`recv`]: crate::sync::broadcast::Receiver::recv
/// [`SendError`]: crate::sync::broadcast::error::SendError
/// [`RecvError`]: crate::sync::broadcast::error::RecvError
///
/// # Examples
///
/// ```
/// use tokio::sync::broadcast;
///
/// #[tokio::main]
/// async fn main() {
/// let (tx, mut rx1) = broadcast::channel(16);
/// let mut rx2 = tx.subscribe();
///
/// tokio::spawn(async move {
/// assert_eq!(rx1.recv().await.unwrap(), 10);
/// assert_eq!(rx1.recv().await.unwrap(), 20);
/// });
///
/// tokio::spawn(async move {
/// assert_eq!(rx2.recv().await.unwrap(), 10);
/// assert_eq!(rx2.recv().await.unwrap(), 20);
/// });
///
/// tx.send(10).unwrap();
/// tx.send(20).unwrap();
/// }
/// ```
///
/// # Panics
///
/// This will panic if `capacity` is equal to `0` or larger
/// than `usize::MAX / 2`.
pub fn channel<T: Clone>(mut capacity: usize) -> (Sender<T>, Receiver<T>) {
assert!(capacity > 0, "capacity is empty");
assert!(capacity <= usize::MAX >> 1, "requested capacity too large");
// Round to a power of two
capacity = capacity.next_power_of_two();
let mut buffer = Vec::with_capacity(capacity);
for i in 0..capacity {
buffer.push(RwLock::new(Slot {
rem: AtomicUsize::new(0),
pos: (i as u64).wrapping_sub(capacity as u64),
closed: false,
val: UnsafeCell::new(None),
}));
}
let shared = Arc::new(Shared {
buffer: buffer.into_boxed_slice(),
mask: capacity - 1,
tail: Mutex::new(Tail {
pos: 0,
rx_cnt: 1,
closed: false,
waiters: LinkedList::new(),
}),
num_tx: AtomicUsize::new(1),
});
let rx = Receiver {
shared: shared.clone(),
next: 0,
};
let tx = Sender { shared };
(tx, rx)
}
unsafe impl<T: Send> Send for Sender<T> {}
unsafe impl<T: Send> Sync for Sender<T> {}
unsafe impl<T: Send> Send for Receiver<T> {}
unsafe impl<T: Send> Sync for Receiver<T> {}
impl<T> Sender<T> {
/// Attempts to send a value to all active [`Receiver`] handles, returning
/// it back if it could not be sent.
///
/// A successful send occurs when there is at least one active [`Receiver`]
/// handle. An unsuccessful send would be one where all associated
/// [`Receiver`] handles have already been dropped.
///
/// # Return
///
/// On success, the number of subscribed [`Receiver`] handles is returned.
/// This does not mean that this number of receivers will see the message as
/// a receiver may drop before receiving the message.
///
/// # Note
///
/// A return value of `Ok` **does not** mean that the sent value will be
/// observed by all or any of the active [`Receiver`] handles. [`Receiver`]
/// handles may be dropped before receiving the sent message.
///
/// A return value of `Err` **does not** mean that future calls to `send`
/// will fail. New [`Receiver`] handles may be created by calling
/// [`subscribe`].
///
/// [`Receiver`]: crate::sync::broadcast::Receiver
/// [`subscribe`]: crate::sync::broadcast::Sender::subscribe
///
/// # Examples
///
/// ```
/// use tokio::sync::broadcast;
///
/// #[tokio::main]
/// async fn main() {
/// let (tx, mut rx1) = broadcast::channel(16);
/// let mut rx2 = tx.subscribe();
///
/// tokio::spawn(async move {
/// assert_eq!(rx1.recv().await.unwrap(), 10);
/// assert_eq!(rx1.recv().await.unwrap(), 20);
/// });
///
/// tokio::spawn(async move {
/// assert_eq!(rx2.recv().await.unwrap(), 10);
/// assert_eq!(rx2.recv().await.unwrap(), 20);
/// });
///
/// tx.send(10).unwrap();
/// tx.send(20).unwrap();
/// }
/// ```
pub fn send(&self, value: T) -> Result<usize, SendError<T>> {
self.send2(Some(value))
.map_err(|SendError(maybe_v)| SendError(maybe_v.unwrap()))
}
/// Creates a new [`Receiver`] handle that will receive values sent **after**
/// this call to `subscribe`.
///
/// # Examples
///
/// ```
/// use tokio::sync::broadcast;
///
/// #[tokio::main]
/// async fn main() {
/// let (tx, _rx) = broadcast::channel(16);
///
/// // Will not be seen
/// tx.send(10).unwrap();
///
/// let mut rx = tx.subscribe();
///
/// tx.send(20).unwrap();
///
/// let value = rx.recv().await.unwrap();
/// assert_eq!(20, value);
/// }
/// ```
pub fn subscribe(&self) -> Receiver<T> {
let shared = self.shared.clone();
new_receiver(shared)
}
/// Returns the number of active receivers
///
/// An active receiver is a [`Receiver`] handle returned from [`channel`] or
/// [`subscribe`]. These are the handles that will receive values sent on
/// this [`Sender`].
///
/// # Note
///
/// It is not guaranteed that a sent message will reach this number of
/// receivers. Active receivers may never call [`recv`] again before
/// dropping.
///
/// [`recv`]: crate::sync::broadcast::Receiver::recv
/// [`Receiver`]: crate::sync::broadcast::Receiver
/// [`Sender`]: crate::sync::broadcast::Sender
/// [`subscribe`]: crate::sync::broadcast::Sender::subscribe
/// [`channel`]: crate::sync::broadcast::channel
///
/// # Examples
///
/// ```
/// use tokio::sync::broadcast;
///
/// #[tokio::main]
/// async fn main() {
/// let (tx, _rx1) = broadcast::channel(16);
///
/// assert_eq!(1, tx.receiver_count());
///
/// let mut _rx2 = tx.subscribe();
///
/// assert_eq!(2, tx.receiver_count());
///
/// tx.send(10).unwrap();
/// }
/// ```
pub fn receiver_count(&self) -> usize {
let tail = self.shared.tail.lock();
tail.rx_cnt
}
fn send2(&self, value: Option<T>) -> Result<usize, SendError<Option<T>>> {
let mut tail = self.shared.tail.lock();
if tail.rx_cnt == 0 {
return Err(SendError(value));
}
// Position to write into
let pos = tail.pos;
let rem = tail.rx_cnt;
let idx = (pos & self.shared.mask as u64) as usize;
// Update the tail position
tail.pos = tail.pos.wrapping_add(1);
// Get the slot
let mut slot = self.shared.buffer[idx].write().unwrap();
// Track the position
slot.pos = pos;
// Set remaining receivers
slot.rem.with_mut(|v| *v = rem);
// Set the closed bit if the value is `None`; otherwise write the value
if value.is_none() {
tail.closed = true;
slot.closed = true;
} else {
slot.val.with_mut(|ptr| unsafe { *ptr = value });
}
// Release the slot lock before notifying the receivers.
drop(slot);
tail.notify_rx();
// Release the mutex. This must happen after the slot lock is released,
// otherwise the writer lock bit could be cleared while another thread
// is in the critical section.
drop(tail);
Ok(rem)
}
}
/// Create a new `Receiver` which reads starting from the tail.
fn new_receiver<T>(shared: Arc<Shared<T>>) -> Receiver<T> {
let mut tail = shared.tail.lock();
if tail.rx_cnt == MAX_RECEIVERS {
panic!("max receivers");
}
tail.rx_cnt = tail.rx_cnt.checked_add(1).expect("overflow");
let next = tail.pos;
drop(tail);
Receiver { shared, next }
}
impl Tail {
fn notify_rx(&mut self) {
while let Some(mut waiter) = self.waiters.pop_back() {
// Safety: `waiters` lock is still held.
let waiter = unsafe { waiter.as_mut() };
assert!(waiter.queued);
waiter.queued = false;
let waker = waiter.waker.take().unwrap();
waker.wake();
}
}
}
impl<T> Clone for Sender<T> {
fn clone(&self) -> Sender<T> {
let shared = self.shared.clone();
shared.num_tx.fetch_add(1, SeqCst);
Sender { shared }
}
}
impl<T> Drop for Sender<T> {
fn drop(&mut self) {
if 1 == self.shared.num_tx.fetch_sub(1, SeqCst) {
let _ = self.send2(None);
}
}
}
impl<T> Receiver<T> {
/// Returns the number of messages that were sent into the channel and that
/// this [`Receiver`] has yet to receive.
///
/// If the returned value from `len` is larger than the next largest power of 2
/// of the capacity of the channel any call to [`recv`] will return an
/// `Err(RecvError::Lagged)` and any call to [`try_recv`] will return an
/// `Err(TryRecvError::Lagged)`, e.g. if the capacity of the channel is 10,
/// [`recv`] will start to return `Err(RecvError::Lagged)` once `len` returns
/// values larger than 16.
///
/// [`Receiver`]: crate::sync::broadcast::Receiver
/// [`recv`]: crate::sync::broadcast::Receiver::recv
/// [`try_recv`]: crate::sync::broadcast::Receiver::try_recv
///
/// # Examples
///
/// ```
/// use tokio::sync::broadcast;
///
/// #[tokio::main]
/// async fn main() {
/// let (tx, mut rx1) = broadcast::channel(16);
///
/// tx.send(10).unwrap();
/// tx.send(20).unwrap();
///
/// assert_eq!(rx1.len(), 2);
/// assert_eq!(rx1.recv().await.unwrap(), 10);
/// assert_eq!(rx1.len(), 1);
/// assert_eq!(rx1.recv().await.unwrap(), 20);
/// assert_eq!(rx1.len(), 0);
/// }
/// ```
pub fn len(&self) -> usize {
let next_send_pos = self.shared.tail.lock().pos;
(next_send_pos - self.next) as usize
}
/// Returns true if there aren't any messages in the channel that the [`Receiver`]
/// has yet to receive.
///
/// [`Receiver]: create::sync::broadcast::Receiver
///
/// # Examples
///
/// ```
/// use tokio::sync::broadcast;
///
/// #[tokio::main]
/// async fn main() {
/// let (tx, mut rx1) = broadcast::channel(16);
///
/// assert!(rx1.is_empty());
///
/// tx.send(10).unwrap();
/// tx.send(20).unwrap();
///
/// assert!(!rx1.is_empty());
/// assert_eq!(rx1.recv().await.unwrap(), 10);
/// assert_eq!(rx1.recv().await.unwrap(), 20);
/// assert!(rx1.is_empty());
/// }
/// ```
pub fn is_empty(&self) -> bool {
self.len() == 0
}
/// Locks the next value if there is one.
fn recv_ref(
&mut self,
waiter: Option<(&UnsafeCell<Waiter>, &Waker)>,
) -> Result<RecvGuard<'_, T>, TryRecvError> {
let idx = (self.next & self.shared.mask as u64) as usize;
// The slot holding the next value to read
let mut slot = self.shared.buffer[idx].read().unwrap();
if slot.pos != self.next {
let next_pos = slot.pos.wrapping_add(self.shared.buffer.len() as u64);
// The receiver has read all current values in the channel and there
// is no waiter to register
if waiter.is_none() && next_pos == self.next {
return Err(TryRecvError::Empty);
}
// Release the `slot` lock before attempting to acquire the `tail`
// lock. This is required because `send2` acquires the tail lock
// first followed by the slot lock. Acquiring the locks in reverse
// order here would result in a potential deadlock: `recv_ref`
// acquires the `slot` lock and attempts to acquire the `tail` lock
// while `send2` acquired the `tail` lock and attempts to acquire
// the slot lock.
drop(slot);
let mut tail = self.shared.tail.lock();
// Acquire slot lock again
slot = self.shared.buffer[idx].read().unwrap();
// Make sure the position did not change. This could happen in the
// unlikely event that the buffer is wrapped between dropping the
// read lock and acquiring the tail lock.
if slot.pos != self.next {
let next_pos = slot.pos.wrapping_add(self.shared.buffer.len() as u64);
if next_pos == self.next {
// Store the waker
if let Some((waiter, waker)) = waiter {
// Safety: called while locked.
unsafe {
// Only queue if not already queued
waiter.with_mut(|ptr| {
// If there is no waker **or** if the currently
// stored waker references a **different** task,
// track the tasks' waker to be notified on
// receipt of a new value.
match (*ptr).waker {
Some(ref w) if w.will_wake(waker) => {}
_ => {
(*ptr).waker = Some(waker.clone());
}
}
if !(*ptr).queued {
(*ptr).queued = true;
tail.waiters.push_front(NonNull::new_unchecked(&mut *ptr));
}
});
}
}
return Err(TryRecvError::Empty);
}
// At this point, the receiver has lagged behind the sender by
// more than the channel capacity. The receiver will attempt to
// catch up by skipping dropped messages and setting the
// internal cursor to the **oldest** message stored by the
// channel.
//
// However, finding the oldest position is a bit more
// complicated than `tail-position - buffer-size`. When
// the channel is closed, the tail position is incremented to
// signal a new `None` message, but `None` is not stored in the
// channel itself (see issue #2425 for why).
//
// To account for this, if the channel is closed, the tail
// position is decremented by `buffer-size + 1`.
let mut adjust = 0;
if tail.closed {
adjust = 1
}
let next = tail
.pos
.wrapping_sub(self.shared.buffer.len() as u64 + adjust);
let missed = next.wrapping_sub(self.next);
drop(tail);
// The receiver is slow but no values have been missed
if missed == 0 {
self.next = self.next.wrapping_add(1);
return Ok(RecvGuard { slot });
}
self.next = next;
return Err(TryRecvError::Lagged(missed));
}
}
self.next = self.next.wrapping_add(1);
if slot.closed {
return Err(TryRecvError::Closed);
}
Ok(RecvGuard { slot })
}
}
impl<T: Clone> Receiver<T> {
/// Re-subscribes to the channel starting from the current tail element.
///
/// This [`Receiver`] handle will receive a clone of all values sent
/// **after** it has resubscribed. This will not include elements that are
/// in the queue of the current receiver. Consider the following example.
///
/// # Examples
///
/// ```
/// use tokio::sync::broadcast;
///
/// #[tokio::main]
/// async fn main() {
/// let (tx, mut rx) = broadcast::channel(2);
///
/// tx.send(1).unwrap();
/// let mut rx2 = rx.resubscribe();
/// tx.send(2).unwrap();
///
/// assert_eq!(rx2.recv().await.unwrap(), 2);
/// assert_eq!(rx.recv().await.unwrap(), 1);
/// }
/// ```
pub fn resubscribe(&self) -> Self {
let shared = self.shared.clone();
new_receiver(shared)
}
/// Receives the next value for this receiver.
///
/// Each [`Receiver`] handle will receive a clone of all values sent
/// **after** it has subscribed.
///
/// `Err(RecvError::Closed)` is returned when all `Sender` halves have
/// dropped, indicating that no further values can be sent on the channel.
///
/// If the [`Receiver`] handle falls behind, once the channel is full, newly
/// sent values will overwrite old values. At this point, a call to [`recv`]
/// will return with `Err(RecvError::Lagged)` and the [`Receiver`]'s
/// internal cursor is updated to point to the oldest value still held by
/// the channel. A subsequent call to [`recv`] will return this value
/// **unless** it has been since overwritten.
///
/// # Cancel safety
///
/// This method is cancel safe. If `recv` is used as the event in a
/// [`tokio::select!`](crate::select) statement and some other branch
/// completes first, it is guaranteed that no messages were received on this
/// channel.
///
/// [`Receiver`]: crate::sync::broadcast::Receiver
/// [`recv`]: crate::sync::broadcast::Receiver::recv
///
/// # Examples
///
/// ```
/// use tokio::sync::broadcast;
///
/// #[tokio::main]
/// async fn main() {
/// let (tx, mut rx1) = broadcast::channel(16);
/// let mut rx2 = tx.subscribe();
///
/// tokio::spawn(async move {
/// assert_eq!(rx1.recv().await.unwrap(), 10);
/// assert_eq!(rx1.recv().await.unwrap(), 20);
/// });
///
/// tokio::spawn(async move {
/// assert_eq!(rx2.recv().await.unwrap(), 10);
/// assert_eq!(rx2.recv().await.unwrap(), 20);
/// });
///
/// tx.send(10).unwrap();
/// tx.send(20).unwrap();
/// }
/// ```
///
/// Handling lag
///
/// ```
/// use tokio::sync::broadcast;
///
/// #[tokio::main]
/// async fn main() {
/// let (tx, mut rx) = broadcast::channel(2);
///
/// tx.send(10).unwrap();
/// tx.send(20).unwrap();
/// tx.send(30).unwrap();
///
/// // The receiver lagged behind
/// assert!(rx.recv().await.is_err());
///
/// // At this point, we can abort or continue with lost messages
///
/// assert_eq!(20, rx.recv().await.unwrap());
/// assert_eq!(30, rx.recv().await.unwrap());
/// }
/// ```
pub async fn recv(&mut self) -> Result<T, RecvError> {
let fut = Recv::new(self);
fut.await
}
/// Attempts to return a pending value on this receiver without awaiting.
///
/// This is useful for a flavor of "optimistic check" before deciding to
/// await on a receiver.
///
/// Compared with [`recv`], this function has three failure cases instead of two
/// (one for closed, one for an empty buffer, one for a lagging receiver).
///
/// `Err(TryRecvError::Closed)` is returned when all `Sender` halves have
/// dropped, indicating that no further values can be sent on the channel.
///
/// If the [`Receiver`] handle falls behind, once the channel is full, newly
/// sent values will overwrite old values. At this point, a call to [`recv`]
/// will return with `Err(TryRecvError::Lagged)` and the [`Receiver`]'s
/// internal cursor is updated to point to the oldest value still held by
/// the channel. A subsequent call to [`try_recv`] will return this value
/// **unless** it has been since overwritten. If there are no values to
/// receive, `Err(TryRecvError::Empty)` is returned.
///
/// [`recv`]: crate::sync::broadcast::Receiver::recv
/// [`try_recv`]: crate::sync::broadcast::Receiver::try_recv
/// [`Receiver`]: crate::sync::broadcast::Receiver
///
/// # Examples
///
/// ```
/// use tokio::sync::broadcast;
///
/// #[tokio::main]
/// async fn main() {
/// let (tx, mut rx) = broadcast::channel(16);
///
/// assert!(rx.try_recv().is_err());
///
/// tx.send(10).unwrap();
///
/// let value = rx.try_recv().unwrap();
/// assert_eq!(10, value);
/// }
/// ```
pub fn try_recv(&mut self) -> Result<T, TryRecvError> {
let guard = self.recv_ref(None)?;
guard.clone_value().ok_or(TryRecvError::Closed)
}
}
impl<T> Drop for Receiver<T> {
fn drop(&mut self) {
let mut tail = self.shared.tail.lock();
tail.rx_cnt -= 1;
let until = tail.pos;
drop(tail);
while self.next < until {
match self.recv_ref(None) {
Ok(_) => {}
// The channel is closed
Err(TryRecvError::Closed) => break,
// Ignore lagging, we will catch up
Err(TryRecvError::Lagged(..)) => {}
// Can't be empty
Err(TryRecvError::Empty) => panic!("unexpected empty broadcast channel"),
}
}
}
}
impl<'a, T> Recv<'a, T> {
fn new(receiver: &'a mut Receiver<T>) -> Recv<'a, T> {
Recv {
receiver,
waiter: UnsafeCell::new(Waiter {
queued: false,
waker: None,
pointers: linked_list::Pointers::new(),
_p: PhantomPinned,
}),
}
}
/// A custom `project` implementation is used in place of `pin-project-lite`
/// as a custom drop implementation is needed.
fn project(self: Pin<&mut Self>) -> (&mut Receiver<T>, &UnsafeCell<Waiter>) {
unsafe {
// Safety: Receiver is Unpin
is_unpin::<&mut Receiver<T>>();
let me = self.get_unchecked_mut();
(me.receiver, &me.waiter)
}
}
}
impl<'a, T> Future for Recv<'a, T>
where
T: Clone,
{
type Output = Result<T, RecvError>;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<T, RecvError>> {
let (receiver, waiter) = self.project();
let guard = match receiver.recv_ref(Some((waiter, cx.waker()))) {
Ok(value) => value,
Err(TryRecvError::Empty) => return Poll::Pending,
Err(TryRecvError::Lagged(n)) => return Poll::Ready(Err(RecvError::Lagged(n))),
Err(TryRecvError::Closed) => return Poll::Ready(Err(RecvError::Closed)),
};
Poll::Ready(guard.clone_value().ok_or(RecvError::Closed))
}
}
impl<'a, T> Drop for Recv<'a, T> {
fn drop(&mut self) {
// Acquire the tail lock. This is required for safety before accessing
// the waiter node.
let mut tail = self.receiver.shared.tail.lock();
// safety: tail lock is held
let queued = self.waiter.with(|ptr| unsafe { (*ptr).queued });
if queued {
// Remove the node
//
// safety: tail lock is held and the wait node is verified to be in
// the list.
unsafe {
self.waiter.with_mut(|ptr| {
tail.waiters.remove((&mut *ptr).into());
});
}
}
}
}
/// # Safety
///
/// `Waiter` is forced to be !Unpin.
unsafe impl linked_list::Link for Waiter {
type Handle = NonNull<Waiter>;
type Target = Waiter;
fn as_raw(handle: &NonNull<Waiter>) -> NonNull<Waiter> {
*handle
}
unsafe fn from_raw(ptr: NonNull<Waiter>) -> NonNull<Waiter> {
ptr
}
unsafe fn pointers(mut target: NonNull<Waiter>) -> NonNull<linked_list::Pointers<Waiter>> {
NonNull::from(&mut target.as_mut().pointers)
}
}
impl<T> fmt::Debug for Sender<T> {
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(fmt, "broadcast::Sender")
}
}
impl<T> fmt::Debug for Receiver<T> {
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(fmt, "broadcast::Receiver")
}
}
impl<'a, T> RecvGuard<'a, T> {
fn clone_value(&self) -> Option<T>
where
T: Clone,
{
self.slot.val.with(|ptr| unsafe { (*ptr).clone() })
}
}
impl<'a, T> Drop for RecvGuard<'a, T> {
fn drop(&mut self) {
// Decrement the remaining counter
if 1 == self.slot.rem.fetch_sub(1, SeqCst) {
// Safety: Last receiver, drop the value
self.slot.val.with_mut(|ptr| unsafe { *ptr = None });
}
}
}
fn is_unpin<T: Unpin>() {}