use crate::channel::{
decode_state, encode_state, queue::Queue, Priority, SendError, SendErrorKind, TryRecvError,
TrySendError, INIT_STATE, MAX_BUFFER, MAX_CAPACITY, OPEN_MASK,
};
use crate::lock::Mutex;
use futures::{
future::poll_fn,
stream::{FusedStream, Stream},
task::{AtomicWaker, Waker},
};
use std::{
pin::Pin,
sync::{
atomic::{
AtomicBool, AtomicUsize,
Ordering::{Relaxed, SeqCst},
},
Arc,
},
task::{Context, Poll},
};
/// 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(),
quick_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: AtomicBool::new(false),
};
let rx = Receiver { inner: Some(inner) };
(Sender(Some(tx)), rx)
}
#[derive(Debug)]
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 messages to the receiver
quick_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,
}
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 BoundedInner<T> {}
unsafe impl<T: Send> Sync for BoundedInner<T> {}
// 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();
}
}
}
#[derive(Debug)]
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: AtomicBool,
}
impl<T> BoundedSenderInner<T> {
/// Attempts to send a message on this `Sender`, returning the message
/// if there was an error.
fn try_send(&self, msg: T, priority: Priority) -> 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, priority)
}
// Do the send without failing.
// Can be called only by bounded sender.
#[allow(clippy::debug_assert_with_mut_call)]
fn do_send_b(&self, msg: T, priority: Priority) -> 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
}
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, priority);
Ok(())
}
// Push message to the queue and signal to the receiver
fn queue_push_and_signal(&self, msg: T, priority: Priority) {
// Push the message onto the message queue
match priority {
Priority::High => self.inner.quick_message_queue.push(msg),
Priority::Normal => 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(&self) {
{
let mut sender = self.sender_task.lock();
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.store(state.is_open, Relaxed);
}
/// 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(&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 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(&self, cx: Option<&mut Context<'_>>) -> Poll<()> {
// First check the `maybe_parked` variable. This avoids acquiring the
// lock in most cases
if self.maybe_parked.load(Relaxed) {
// Get a lock on the task handle
let mut task = self.sender_task.lock();
if !task.is_parked {
self.maybe_parked.store(false, Relaxed);
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> Clone for BoundedSenderInner<T> {
fn clone(&self) -> BoundedSenderInner<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;
match self
.inner
.num_senders
.compare_exchange(curr, next, SeqCst, SeqCst)
{
Ok(actual) => {
// The ABA problem doesn't matter here. We only care that the
// number of senders never exceeds the maximum.
if actual == curr {
return BoundedSenderInner {
inner: self.inner.clone(),
sender_task: Arc::new(Mutex::new(SenderTask::new())),
maybe_parked: AtomicBool::new(false),
};
}
}
Err(actual) => curr = actual,
}
}
}
}
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();
}
}
}
/// The transmission end of a bounded mpsc channel.
///
/// This value is created by the [`channel`](channel) function.
#[derive(Debug)]
pub struct Sender<T>(Option<BoundedSenderInner<T>>);
impl<T> Sender<T> {
/// Attempts to send a message on this `Sender`, returning the message
/// if there was an error.
pub fn try_send(&self, msg: T) -> Result<(), TrySendError<T>> {
if let Some(inner) = &self.0 {
inner.try_send(msg, Priority::Normal)
} else {
Err(TrySendError {
err: SendError {
kind: SendErrorKind::Disconnected,
},
val: msg,
})
}
}
/// Attempts to send a message on this `Sender`, returning the message
/// if there was an error.
pub fn try_quick_send(&self, msg: T) -> Result<(), TrySendError<T>> {
if let Some(inner) = &self.0 {
inner.try_send(msg, Priority::High)
} else {
Err(TrySendError {
err: SendError {
kind: SendErrorKind::Disconnected,
},
val: msg,
})
}
}
// Send a message on the channel with async fn, doesn't use Sink trait
pub async fn async_send(&self, msg: T) -> Result<(), SendError> {
let mut msg = Some(msg);
poll_fn(|cx| {
let item = msg.take().unwrap();
match self.poll_ready(cx)? {
Poll::Ready(()) => Poll::Ready(self.start_send(item)),
Poll::Pending => {
msg = Some(item);
Poll::Pending
}
}
})
.await
}
// Send a message on the channel with async fn, doesn't use Sink trait
pub async fn async_quick_send(&self, msg: T) -> Result<(), SendError> {
let mut msg = Some(msg);
poll_fn(|cx| {
let item = msg.take().unwrap();
match self.poll_ready(cx)? {
Poll::Ready(()) => Poll::Ready(self.start_quick_send(item)),
Poll::Pending => {
msg = Some(item);
Poll::Pending
}
}
})
.await
}
/// 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(&self, msg: T) -> Result<(), SendError> {
self.try_send(msg).map_err(|e| e.err)
}
/// 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_quick_send(&self, msg: T) -> Result<(), SendError> {
self.try_quick_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(&self, cx: &mut Context<'_>) -> Poll<Result<(), SendError>> {
let inner = self.0.as_ref().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,
}
}
/// 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) -> Sender<T> {
Sender(self.0.clone())
}
}
/// 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<BoundedInner<T>>>,
}
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().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<(Priority, T)>, TryRecvError> {
match self.next_message() {
Poll::Ready(msg) => Ok(msg),
Poll::Pending => Err(TryRecvError { _priv: () }),
}
}
fn next_message(&mut self) -> Poll<Option<(Priority, T)>> {
let inner = self
.inner
.as_mut()
.expect("Receiver::next_message called after `None`");
// Pop off a message
match unsafe { inner.quick_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((Priority::High, msg)))
}
None => {
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((Priority::Normal, 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().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 = (Priority, T);
fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
// 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() {
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.
#[cfg(any(
target_arch = "x86",
target_arch = "x86_64",
target_arch = "aarch64",
target_arch = "arm"
))]
std::hint::spin_loop();
#[cfg(not(any(
target_arch = "x86",
target_arch = "x86_64",
target_arch = "aarch64",
target_arch = "arm"
)))]
std::thread::yield_now();
}
}
}
}
}
}