futures_channel/mpsc/mod.rs
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 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 1240 1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252 1253 1254 1255 1256 1257 1258 1259 1260 1261 1262 1263 1264 1265 1266 1267 1268 1269 1270 1271 1272 1273 1274 1275 1276 1277 1278 1279 1280 1281 1282 1283 1284 1285 1286 1287 1288 1289 1290 1291 1292 1293 1294 1295 1296 1297 1298 1299 1300 1301 1302 1303 1304 1305 1306 1307 1308 1309 1310 1311 1312 1313 1314 1315 1316 1317 1318 1319 1320 1321 1322 1323 1324 1325 1326 1327 1328 1329 1330 1331 1332 1333 1334 1335 1336 1337 1338 1339 1340 1341 1342 1343 1344 1345 1346 1347 1348 1349 1350 1351 1352 1353 1354 1355 1356 1357 1358 1359
//! 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::__internal::AtomicWaker;
use futures_core::task::{Context, Poll, Waker};
use std::fmt;
use std::pin::Pin;
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering::SeqCst;
use std::sync::{Arc, Mutex};
use std::thread;
use crate::mpsc::queue::Queue;
mod queue;
#[cfg(feature = "sink")]
mod sink_impl;
struct UnboundedSenderInner<T> {
// Channel state shared between the sender and receiver.
inner: Arc<UnboundedInner<T>>,
}
struct BoundedSenderInner<T> {
// Channel state shared between the sender and receiver.
inner: Arc<BoundedInner<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 UnboundedSenderInner<T> {}
impl<T> Unpin for BoundedSenderInner<T> {}
/// The transmission end of a bounded mpsc channel.
///
/// This value is created by the [`channel`](channel) function.
pub struct Sender<T>(Option<BoundedSenderInner<T>>);
/// The transmission end of an unbounded mpsc channel.
///
/// This value is created by the [`unbounded`](unbounded) function.
pub struct UnboundedSender<T>(Option<UnboundedSenderInner<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.
pub struct Receiver<T> {
inner: Option<Arc<BoundedInner<T>>>,
}
/// The receiving end of an unbounded mpsc channel.
///
/// This value is created by the [`unbounded`](unbounded) function.
pub struct UnboundedReceiver<T> {
inner: Option<Arc<UnboundedInner<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 {
_priv: (),
}
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 std::error::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: core::any::Any> std::error::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 std::error::Error for TryRecvError {}
struct UnboundedInner<T> {
// 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>,
// Number of senders in existence
num_senders: AtomicUsize,
// Handle to the receiver's task.
recv_task: AtomicWaker,
}
struct BoundedInner<T> {
// Max buffer size of the channel. If `None` then the channel is unbounded.
buffer: 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(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
struct SenderTask {
task: Option<Waker>,
is_parked: bool,
}
impl SenderTask {
fn new() -> Self {
Self { 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 inner = Arc::new(BoundedInner {
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 = BoundedSenderInner {
inner: inner.clone(),
sender_task: Arc::new(Mutex::new(SenderTask::new())),
maybe_parked: false,
};
let rx = Receiver { inner: Some(inner) };
(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 inner = Arc::new(UnboundedInner {
state: AtomicUsize::new(INIT_STATE),
message_queue: Queue::new(),
num_senders: AtomicUsize::new(1),
recv_task: AtomicWaker::new(),
});
let tx = UnboundedSenderInner { inner: inner.clone() };
let rx = UnboundedReceiver { inner: Some(inner) };
(UnboundedSender(Some(tx)), rx)
}
/*
*
* ===== impl Sender =====
*
*/
impl<T> UnboundedSenderInner<T> {
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,
}
}
}
/// 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 the sender send to this receiver.
fn is_connected_to(&self, inner: &Arc<UnboundedInner<T>>) -> bool {
Arc::ptr_eq(&self.inner, inner)
}
/// Returns pointer to the Arc containing sender
///
/// The returned pointer is not referenced and should be only used for hashing!
fn ptr(&self) -> *const UnboundedInner<T> {
&*self.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();
}
}
impl<T> BoundedSenderInner<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 calling 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
}
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(())
}
// 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 the sender send to this receiver.
fn is_connected_to(&self, receiver: &Arc<BoundedInner<T>>) -> bool {
Arc::ptr_eq(&self.inner, receiver)
}
/// Returns pointer to the Arc containing sender
///
/// The returned pointer is not referenced and should be only used for hashing!
fn ptr(&self) -> *const BoundedInner<T> {
&*self.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(BoundedSenderInner::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,
}
}
/// Returns whether the sender send to this receiver.
pub fn is_connected_to(&self, receiver: &Receiver<T>) -> bool {
match (&self.0, &receiver.inner) {
(Some(inner), Some(receiver)) => inner.is_connected_to(receiver),
_ => false,
}
}
/// Hashes the receiver into the provided hasher
pub fn hash_receiver<H>(&self, hasher: &mut H)
where
H: std::hash::Hasher,
{
use std::hash::Hash;
let ptr = self.0.as_ref().map(|inner| inner.ptr());
ptr.hash(hasher);
}
}
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(UnboundedSenderInner::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,
}
}
/// Returns whether the sender send to this receiver.
pub fn is_connected_to(&self, receiver: &UnboundedReceiver<T>) -> bool {
match (&self.0, &receiver.inner) {
(Some(inner), Some(receiver)) => inner.is_connected_to(receiver),
_ => false,
}
}
/// Hashes the receiver into the provided hasher
pub fn hash_receiver<H>(&self, hasher: &mut H)
where
H: std::hash::Hasher,
{
use std::hash::Hash;
let ptr = self.0.as_ref().map(|inner| inner.ptr());
ptr.hash(hasher);
}
}
impl<T> Clone for Sender<T> {
fn clone(&self) -> Self {
Self(self.0.clone())
}
}
impl<T> Clone for UnboundedSender<T> {
fn clone(&self) -> Self {
Self(self.0.clone())
}
}
impl<T> Clone for UnboundedSenderInner<T> {
fn clone(&self) -> Self {
// 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 == MAX_BUFFER {
panic!("cannot clone `Sender` -- too many outstanding senders");
}
debug_assert!(curr < MAX_BUFFER);
let next = curr + 1;
match self.inner.num_senders.compare_exchange(curr, next, SeqCst, SeqCst) {
Ok(_) => {
// The ABA problem doesn't matter here. We only care that the
// number of senders never exceeds the maximum.
return Self { inner: self.inner.clone() };
}
Err(actual) => curr = actual,
}
}
}
}
impl<T> Clone for BoundedSenderInner<T> {
fn clone(&self) -> Self {
// 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;
match self.inner.num_senders.compare_exchange(curr, next, SeqCst, SeqCst) {
Ok(_) => {
// The ABA problem doesn't matter here. We only care that the
// number of senders never exceeds the maximum.
return Self {
inner: self.inner.clone(),
sender_task: Arc::new(Mutex::new(SenderTask::new())),
maybe_parked: false,
};
}
Err(actual) => curr = actual,
}
}
}
}
impl<T> Drop for UnboundedSenderInner<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<T> Drop for BoundedSenderInner<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<T> fmt::Debug for Sender<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Sender").field("closed", &self.is_closed()).finish()
}
}
impl<T> fmt::Debug for UnboundedSender<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("UnboundedSender").field("closed", &self.is_closed()).finish()
}
}
/*
*
* ===== 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 returns:
/// * `Ok(Some(t))` when message is fetched
/// * `Ok(None)` when channel is closed and no messages left in the queue
/// * `Err(e)` when there are no messages available, but channel is not yet closed
pub fn try_next(&mut self) -> Result<Option<T>, TryRecvError> {
match self.next_message() {
Poll::Ready(msg) => Ok(msg),
Poll::Pending => Err(TryRecvError { _priv: () }),
}
}
fn next_message(&mut self) -> Poll<Option<T>> {
let inner = match self.inner.as_mut() {
None => return Poll::Ready(None),
Some(inner) => inner,
};
// 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_closed() {
// If closed flag is set AND there are no pending messages
// it means end of stream
self.inner = None;
Poll::Ready(None)
} else {
// 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
}
}
}
}
// 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()
}
}
}
fn size_hint(&self) -> (usize, Option<usize>) {
if let Some(inner) = &self.inner {
decode_state(inner.state.load(SeqCst)).size_hint()
} else {
(0, Some(0))
}
}
}
impl<T> Drop for Receiver<T> {
fn drop(&mut self) {
// Drain the channel of all pending messages
self.close();
if self.inner.is_some() {
loop {
match self.next_message() {
Poll::Ready(Some(_)) => {}
Poll::Ready(None) => break,
Poll::Pending => {
let state = decode_state(self.inner.as_ref().unwrap().state.load(SeqCst));
// If the channel is closed, then there is no need to park.
if state.is_closed() {
break;
}
// TODO: Spinning isn't ideal, it might be worth
// investigating using a condvar or some other strategy
// here. That said, if this case is hit, then another thread
// is about to push the value into the queue and this isn't
// the only spinlock in the impl right now.
thread::yield_now();
}
}
}
}
}
}
impl<T> fmt::Debug for Receiver<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let closed = if let Some(ref inner) = self.inner {
decode_state(inner.state.load(SeqCst)).is_closed()
} else {
false
};
f.debug_struct("Receiver").field("closed", &closed).finish()
}
}
impl<T> UnboundedReceiver<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();
}
}
/// 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 returns:
/// * `Ok(Some(t))` when message is fetched
/// * `Ok(None)` when channel is closed and no messages left in the queue
/// * `Err(e)` when there are no messages available, but channel is not yet closed
pub fn try_next(&mut self) -> Result<Option<T>, TryRecvError> {
match self.next_message() {
Poll::Ready(msg) => Ok(msg),
Poll::Pending => Err(TryRecvError { _priv: () }),
}
}
fn next_message(&mut self) -> Poll<Option<T>> {
let inner = match self.inner.as_mut() {
None => return Poll::Ready(None),
Some(inner) => inner,
};
// Pop off a message
match unsafe { inner.message_queue.pop_spin() } {
Some(msg) => {
// Decrement number of messages
self.dec_num_messages();
Poll::Ready(Some(msg))
}
None => {
let state = decode_state(inner.state.load(SeqCst));
if state.is_closed() {
// If closed flag is set AND there are no pending messages
// it means end of stream
self.inner = None;
Poll::Ready(None)
} else {
// 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
}
}
}
}
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);
}
}
}
impl<T> FusedStream for UnboundedReceiver<T> {
fn is_terminated(&self) -> bool {
self.inner.is_none()
}
}
impl<T> Stream for UnboundedReceiver<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()
}
}
}
fn size_hint(&self) -> (usize, Option<usize>) {
if let Some(inner) = &self.inner {
decode_state(inner.state.load(SeqCst)).size_hint()
} else {
(0, Some(0))
}
}
}
impl<T> Drop for UnboundedReceiver<T> {
fn drop(&mut self) {
// Drain the channel of all pending messages
self.close();
if self.inner.is_some() {
loop {
match self.next_message() {
Poll::Ready(Some(_)) => {}
Poll::Ready(None) => break,
Poll::Pending => {
let state = decode_state(self.inner.as_ref().unwrap().state.load(SeqCst));
// If the channel is closed, then there is no need to park.
if state.is_closed() {
break;
}
// TODO: Spinning isn't ideal, it might be worth
// investigating using a condvar or some other strategy
// here. That said, if this case is hit, then another thread
// is about to push the value into the queue and this isn't
// the only spinlock in the impl right now.
thread::yield_now();
}
}
}
}
}
}
impl<T> fmt::Debug for UnboundedReceiver<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let closed = if let Some(ref inner) = self.inner {
decode_state(inner.state.load(SeqCst)).is_closed()
} else {
false
};
f.debug_struct("Receiver").field("closed", &closed).finish()
}
}
/*
*
* ===== impl Inner =====
*
*/
impl<T> UnboundedInner<T> {
// 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);
}
}
impl<T> BoundedInner<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 {
MAX_CAPACITY - self.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 UnboundedInner<T> {}
unsafe impl<T: Send> Sync for UnboundedInner<T> {}
unsafe impl<T: Send> Send for BoundedInner<T> {}
unsafe impl<T: Send> Sync for BoundedInner<T> {}
impl State {
fn is_closed(&self) -> bool {
!self.is_open && self.num_messages == 0
}
fn size_hint(&self) -> (usize, Option<usize>) {
if self.is_open {
(self.num_messages, None)
} else {
(self.num_messages, Some(self.num_messages))
}
}
}
/*
*
* ===== 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
}