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//! Multi-producer, single-consumer FIFO queue communication primitives.
use crate::runtime::execution::ExecutionState;
use crate::runtime::task::clock::VectorClock;
use crate::runtime::task::{TaskId, DEFAULT_INLINE_TASKS};
use crate::runtime::thread;
use smallvec::SmallVec;
use std::cell::RefCell;
use std::fmt::Debug;
use std::rc::Rc;
use std::result::Result;
pub use std::sync::mpsc::{RecvError, RecvTimeoutError, SendError, TryRecvError, TrySendError};
use std::sync::Arc;
use std::time::Duration;
use tracing::trace;
const MAX_INLINE_MESSAGES: usize = 32;
/// Create an unbounded channel
pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
let channel = Arc::new(Channel::new(None));
let sender = Sender {
inner: Arc::clone(&channel),
};
let receiver = Receiver {
inner: Arc::clone(&channel),
};
(sender, receiver)
}
/// Create a bounded channel
pub fn sync_channel<T>(bound: usize) -> (SyncSender<T>, Receiver<T>) {
let channel = Arc::new(Channel::new(Some(bound)));
let sender = SyncSender {
inner: Arc::clone(&channel),
};
let receiver = Receiver {
inner: Arc::clone(&channel),
};
(sender, receiver)
}
#[derive(Debug)]
struct Channel<T> {
bound: Option<usize>, // None for an unbounded channel, Some(k) for a bounded channel of size k
state: Rc<RefCell<ChannelState<T>>>,
}
// For tracking causality on channels, we timestamp each message with the clock of the sender.
// When the receiver gets the message, it updates its clock with the the associated timestamp.
// For unbounded channels, that's all the work we need to do.
//
// For bounded and rendezvous channels, things get a bit more interesting.
// Consider a bounded channel of depth K. As soon as the sender successfully sends its K+1'th
// message, it knows that the receiver has received at least 1 message. At this point, the
// first receive event causally precedes the (K+1)'th send. By the rule for vector clocks,
// (clock of the first receive) < (clock of the K+1'th send)
// In order to ensure this ordering, we add a return queue of depth K to bounded channels.
// Initially, this queue contains K empty vector clocks. On each receive, we push the
// receiver's clock at the time of the receive to the end of this queue. Whenever the sender
// successfully sends a message, it pops the clock at the front of the queue, and updates its
// own clock with this value. Thus, on the (K+1)'th send, the sender's clock will be updated
// with the clock at the first receive, as needed.
//
// The story is similar for rendezvous channels, except we have to handle things a bit more
// specially because K=0.
struct TimestampedValue<T> {
value: T,
clock: VectorClock,
}
impl<T> TimestampedValue<T> {
fn new(value: T, clock: VectorClock) -> Self {
Self { value, clock }
}
}
// Note: The channels in std::sync::mpsc only support a single Receiver (which cannot be
// cloned). The state below admits a more general use case, where multiple Senders
// and Receivers can share a single channel.
struct ChannelState<T> {
messages: SmallVec<[TimestampedValue<T>; MAX_INLINE_MESSAGES]>, // messages in the channel
receiver_clock: Option<SmallVec<[VectorClock; MAX_INLINE_MESSAGES]>>, // receiver vector clocks for bounded case
known_senders: usize, // number of senders referencing this channel
known_receivers: usize, // number or receivers referencing this channel
waiting_senders: SmallVec<[TaskId; DEFAULT_INLINE_TASKS]>, // list of currently blocked senders
waiting_receivers: SmallVec<[TaskId; DEFAULT_INLINE_TASKS]>, // list of currently blocked receivers
}
impl<T> Debug for ChannelState<T> {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
write!(f, "Channel {{ ")?;
write!(f, "num_messages: {} ", self.messages.len())?;
write!(
f,
"known_senders {} known_receivers {} ",
self.known_senders, self.known_receivers
)?;
write!(f, "waiting_senders: [{:?}] ", self.waiting_senders)?;
write!(f, "waiting_receivers: [{:?}] ", self.waiting_receivers)?;
write!(f, "}}")
}
}
impl<T> Channel<T> {
fn new(bound: Option<usize>) -> Self {
let receiver_clock = if let Some(bound) = bound {
let mut s = SmallVec::with_capacity(bound);
for _ in 0..bound {
s.push(VectorClock::new());
}
Some(s)
} else {
None
};
Self {
bound,
state: Rc::new(RefCell::new(ChannelState {
messages: SmallVec::new(),
receiver_clock,
known_senders: 1,
known_receivers: 1,
waiting_senders: SmallVec::new(),
waiting_receivers: SmallVec::new(),
})),
}
}
fn try_send(&self, message: T) -> Result<(), TrySendError<T>> {
self.send_internal(message, false)
}
fn send(&self, message: T) -> Result<(), SendError<T>> {
self.send_internal(message, true).map_err(|e| match e {
TrySendError::Full(_) => unreachable!(),
TrySendError::Disconnected(m) => SendError(m),
})
}
fn send_internal(&self, message: T, can_block: bool) -> Result<(), TrySendError<T>> {
let me = ExecutionState::me();
let mut state = self.state.borrow_mut();
trace!(
state = ?state,
"sender {:?} starting send on channel {:p}",
me,
self,
);
if state.known_receivers == 0 {
// No receivers are left, so the channel is disconnected. Stop and return failure.
return Err(TrySendError::Disconnected(message));
}
let (is_rendezvous, is_full) = if let Some(bound) = self.bound {
// For a rendezvous channel (bound = 0), "is_full" holds when there is a message in the channel.
// For a non-rendezvous channel (bound > 0), "is_full" holds when the capacity is reached.
// We cover both these cases at once using max(bound, 1) below.
(bound == 0, state.messages.len() >= std::cmp::max(bound, 1))
} else {
(false, false)
};
// The sender should block in any of the following situations:
// the channel is full (as defined above)
// there are already waiting senders
// this is a rendezvous channel and there are no waiting receivers
let sender_should_block =
is_full || !state.waiting_senders.is_empty() || (is_rendezvous && state.waiting_receivers.is_empty());
if sender_should_block {
if !can_block {
return Err(TrySendError::Full(message));
}
state.waiting_senders.push(me);
trace!(
state = ?state,
"blocking sender {:?} on channel {:p}",
me,
self,
);
ExecutionState::with(|s| s.current_mut().block(false));
drop(state);
thread::switch();
state = self.state.borrow_mut();
trace!(
state = ?state,
"unblocked sender {:?} on channel {:p}",
me,
self,
);
// Check again that we still have a receiver; if not, return with error.
// We repeat this check because the receivers may have disconnected while the sender was blocked.
if state.known_receivers == 0 {
state.waiting_senders.retain(|t| *t != me);
// No receivers are left, so the channel is disconnected. Stop and return failure.
return Err(TrySendError::Disconnected(message));
}
let head = state.waiting_senders.remove(0);
assert_eq!(head, me);
}
ExecutionState::with(|s| {
let clock = s.increment_clock();
state.messages.push(TimestampedValue::new(message, clock.clone()));
});
// The sender has just added a message to the channel, so unblock the first waiting receiver if any
if let Some(&tid) = state.waiting_receivers.first() {
ExecutionState::with(|s| {
s.get_mut(tid).unblock();
// When a sender successfully sends on a rendezvous channel, it knows that the receiver will perform
// the matching receive, so we need to update the sender's clock with the receiver's.
if is_rendezvous {
let recv_clock = s.get_clock(tid).clone();
s.update_clock(&recv_clock);
}
});
}
// Check and unblock the next the waiting sender, if eligible
if let Some(&tid) = state.waiting_senders.first() {
let bound = self.bound.expect("can't have waiting senders on an unbounded channel");
if state.messages.len() < bound {
ExecutionState::with(|s| s.get_mut(tid).unblock());
}
}
if !is_rendezvous {
if let Some(receiver_clock) = &mut state.receiver_clock {
let recv_clock = receiver_clock.remove(0);
ExecutionState::with(|s| s.update_clock(&recv_clock));
}
}
Ok(())
}
fn recv(&self) -> Result<T, RecvError> {
self.recv_internal(true).map_err(|e| match e {
TryRecvError::Disconnected => RecvError,
TryRecvError::Empty => unreachable!(),
})
}
fn try_recv(&self) -> Result<T, TryRecvError> {
self.recv_internal(false)
}
fn recv_internal(&self, can_block: bool) -> Result<T, TryRecvError> {
let me = ExecutionState::me();
let mut state = self.state.borrow_mut();
trace!(
state = ?state,
"starting recv on channel {:p}",
self,
);
// Check if there are any senders left; if not, and the channel is empty, fail with error
// (If there are no senders, but the channel is nonempty, the receiver can successfully consume that message.)
if state.messages.is_empty() && state.known_senders == 0 {
return Err(TryRecvError::Disconnected);
}
let is_rendezvous = self.bound == Some(0);
// If this is a rendezvous channel, and the channel is empty, and there are waiting senders,
// notify the first waiting sender
if is_rendezvous && state.messages.is_empty() {
if let Some(&tid) = state.waiting_senders.first() {
// Note: another receiver may have unblocked the sender already
ExecutionState::with(|s| s.get_mut(tid).unblock());
} else if !can_block {
// Nobody to rendezvous with
return Err(TryRecvError::Empty);
}
}
// Handle the try_recv case, accounting for the number of msgs available and already waiting receivers.
if !is_rendezvous && !can_block && state.waiting_receivers.len() >= state.messages.len() {
return Err(TryRecvError::Empty);
}
// Pre-increment the receiver's clock before continuing
//
// Note: The reason for pre-incrementing the receiver's clock is to deal properly with rendezvous channels.
// Here's the scenario we have to handle:
// 1. the receiver arrives at a rendezvous channel and blocks
// 2. the sender arrives, sees the receiver is waiting and does not block
// 3. the sender drops the message in the channel and updates its clock with the receiver's clock and continues
// 4. later, the receiver unblocks and picks up the message and updates its clock with the sender's
// Without the pre-increment, in step 3, the sender would update its clock with the receiver's clock before
// it is incremented. (The increment records the fact that the receiver arrived at the synchronization point.)
ExecutionState::with(|s| {
let _ = s.increment_clock();
});
// The receiver should block in any of the following situations:
// the channel is empty
// there are waiting receivers
let should_block = state.messages.is_empty() || !state.waiting_receivers.is_empty();
if should_block {
state.waiting_receivers.push(me);
trace!(
state = ?state,
"blocking receiver {:?} on channel {:p}",
me,
self,
);
ExecutionState::with(|s| s.current_mut().block(false));
drop(state);
thread::switch();
state = self.state.borrow_mut();
trace!(
state = ?state,
"unblocked receiver {:?} on channel {:p}",
me,
self,
);
// Check again if there are any senders left; if not, and the channel is empty, fail with error
// (If there are no senders, but the channel is nonempty, the receiver can successfully consume that message.)
// We repeat this check because the senders may have disconnected while the receiver was blocked.
if state.messages.is_empty() && state.known_senders == 0 {
state.waiting_receivers.retain(|t| *t != me);
return Err(TryRecvError::Disconnected);
}
let head = state.waiting_receivers.remove(0);
assert_eq!(head, me);
}
let item = state.messages.remove(0);
// The receiver has just removed an element from the channel. Check if any waiting senders
// need to be notified.
if let Some(&tid) = state.waiting_senders.first() {
let bound = self.bound.expect("can't have waiting senders on an unbounded channel");
// Unblock the first waiting sender provided one of the following conditions hold:
// - this is a non-rendezvous bounded channel (bound > 0)
// - this is a rendezvous channel and we have additional waiting receivers
if bound > 0 || !state.waiting_receivers.is_empty() {
ExecutionState::with(|s| s.get_mut(tid).unblock());
}
}
// Check and unblock the next the waiting receiver, if eligible
// Note: this is a no-op for mpsc channels, since there can only be one receiver
if let Some(&tid) = state.waiting_receivers.first() {
if !state.messages.is_empty() {
ExecutionState::with(|s| s.get_mut(tid).unblock());
}
}
// Update receiver clock from the clock attached to the message received
let TimestampedValue { value, clock } = item;
ExecutionState::with(|s| {
// Since we already incremented the receiver's clock above, just update it here
s.get_clock_mut(me).update(&clock);
// If this is a (non-rendezvous) bounded channel, propagate causality backwards to sender
if let Some(receiver_clock) = &mut state.receiver_clock {
let bound = self.bound.expect("unexpected internal error"); // must be defined for bounded channels
if bound > 0 {
// non-rendezvous
assert!(receiver_clock.len() < bound);
receiver_clock.push(s.get_clock(me).clone());
}
}
});
Ok(value)
}
}
// Safety: A Channel is never actually passed across true threads, only across continuations. The
// Rc<RefCell<_>> type therefore can't be preempted mid-bookkeeping-operation.
// TODO We use this workaround in several places in Shuttle. Maybe there's a cleaner solution.
unsafe impl<T: Send> Send for Channel<T> {}
unsafe impl<T: Send> Sync for Channel<T> {}
/// The receiving half of Rust's [`channel`] (or [`sync_channel`]) type.
/// This half can only be owned by one thread.
#[derive(Debug)]
pub struct Receiver<T> {
inner: Arc<Channel<T>>,
}
impl<T> Receiver<T> {
/// Attempts to wait for a value on this receiver, returning an error if the
/// corresponding channel has hung up.
pub fn recv(&self) -> Result<T, RecvError> {
self.inner.recv()
}
/// Attempts to wait for a value on this receiver, returning an error if the
/// corresponding channel has hung up.
pub fn try_recv(&self) -> Result<T, TryRecvError> {
self.inner.try_recv()
}
/// Attempts to wait for a value on this receiver, returning an error if the
/// corresponding channel has hung up, or if it waits more than timeout.
pub fn recv_timeout(&self, _timeout: Duration) -> Result<T, RecvTimeoutError> {
// TODO support the timeout case -- this method never times out
self.inner.recv().map_err(|_| RecvTimeoutError::Disconnected)
}
/// Returns an iterator that will block waiting for messages, but never
/// [`panic!`]. It will return [`None`] when the channel has hung up.
pub fn iter(&self) -> Iter<'_, T> {
Iter { rx: self }
}
/// Returns an iterator that will attempt to yield all pending values.
/// It will return `None` if there are no more pending values or if the
/// channel has hung up. The iterator will never [`panic!`] or block the
/// user by waiting for values.
pub fn try_iter(&self) -> TryIter<'_, T> {
TryIter { rx: self }
}
}
impl<T> Drop for Receiver<T> {
fn drop(&mut self) {
if ExecutionState::should_stop() {
return;
}
let mut state = self.inner.state.borrow_mut();
assert!(state.known_receivers > 0);
state.known_receivers -= 1;
if state.known_receivers == 0 {
// Last receiver was dropped; wake up all senders
for &tid in state.waiting_senders.iter() {
ExecutionState::with(|s| s.get_mut(tid).unblock());
}
}
}
}
/// An iterator over messages on a [`Receiver`], created by [`iter`].
///
/// This iterator will block whenever [`next`] is called,
/// waiting for a new message, and [`None`] will be returned
/// when the corresponding channel has hung up.
///
/// [`iter`]: Receiver::iter
/// [`next`]: Iterator::next
#[derive(Debug)]
pub struct Iter<'a, T: 'a> {
rx: &'a Receiver<T>,
}
/// An iterator that attempts to yield all pending values for a [`Receiver`],
/// created by [`try_iter`].
///
/// [`None`] will be returned when there are no pending values remaining or
/// if the corresponding channel has hung up.
///
/// This iterator will never block the caller in order to wait for data to
/// become available. Instead, it will return [`None`].
///
/// [`try_iter`]: Receiver::try_iter
#[derive(Debug)]
pub struct TryIter<'a, T: 'a> {
rx: &'a Receiver<T>,
}
/// An owning iterator over messages on a [`Receiver`],
/// created by [`into_iter`].
///
/// This iterator will block whenever [`next`]
/// is called, waiting for a new message, and [`None`] will be
/// returned if the corresponding channel has hung up.
///
/// [`into_iter`]: Receiver::into_iter
/// [`next`]: Iterator::next
#[derive(Debug)]
pub struct IntoIter<T> {
rx: Receiver<T>,
}
impl<'a, T> Iterator for Iter<'a, T> {
type Item = T;
fn next(&mut self) -> Option<T> {
self.rx.recv().ok()
}
}
impl<'a, T> Iterator for TryIter<'a, T> {
type Item = T;
fn next(&mut self) -> Option<T> {
self.rx.try_recv().ok()
}
}
impl<'a, T> IntoIterator for &'a Receiver<T> {
type Item = T;
type IntoIter = Iter<'a, T>;
fn into_iter(self) -> Iter<'a, T> {
self.iter()
}
}
impl<T> Iterator for IntoIter<T> {
type Item = T;
fn next(&mut self) -> Option<T> {
self.rx.recv().ok()
}
}
impl<T> IntoIterator for Receiver<T> {
type Item = T;
type IntoIter = IntoIter<T>;
fn into_iter(self) -> IntoIter<T> {
IntoIter { rx: self }
}
}
/// The sending-half of Rust's asynchronous [`channel`] type. This half can only be
/// owned by one thread, but it can be cloned to send to other threads.
#[derive(Debug)]
pub struct Sender<T> {
inner: Arc<Channel<T>>,
}
impl<T> Sender<T> {
/// Attempts to send a value on this channel, returning it back if it could
/// not be sent.
pub fn send(&self, t: T) -> Result<(), SendError<T>> {
self.inner.send(t)
}
}
impl<T> Clone for Sender<T> {
fn clone(&self) -> Self {
let mut state = self.inner.state.borrow_mut();
state.known_senders += 1;
drop(state);
Self {
inner: self.inner.clone(),
}
}
}
impl<T> Drop for Sender<T> {
fn drop(&mut self) {
if ExecutionState::should_stop() {
return;
}
let mut state = self.inner.state.borrow_mut();
assert!(state.known_senders > 0);
state.known_senders -= 1;
if state.known_senders == 0 {
// Last sender was dropped; wake up all receivers
for &tid in state.waiting_receivers.iter() {
ExecutionState::with(|s| s.get_mut(tid).unblock());
}
}
}
}
/// The sending-half of Rust's synchronous [`sync_channel`] type.
///
/// Messages can be sent through this channel with [`SyncSender::send`] or \[`try_send`\] (TODO)
///
/// [`SyncSender::send`] will block if there is no space in the internal buffer.
#[derive(Debug)]
pub struct SyncSender<T> {
inner: Arc<Channel<T>>,
}
impl<T> SyncSender<T> {
/// Sends a value on this synchronous channel.
///
/// This function will *block* until space in the internal buffer becomes
/// available or a receiver is available to hand off the message to.
pub fn send(&self, t: T) -> Result<(), SendError<T>> {
self.inner.send(t)
}
/// Attempts to send a value on this channel without blocking.
///
/// This method differs from [`send`] by returning immediately if the
/// channel's buffer is full or no receiver is waiting to acquire some
/// data. Compared with [`send`], this function has two failure cases
/// instead of one (one for disconnection, one for a full buffer).
///
/// [`send`]: Self::send
pub fn try_send(&self, t: T) -> Result<(), TrySendError<T>> {
self.inner.try_send(t)
}
}
impl<T> Clone for SyncSender<T> {
fn clone(&self) -> Self {
let mut state = self.inner.state.borrow_mut();
state.known_senders += 1;
drop(state);
Self {
inner: self.inner.clone(),
}
}
}
impl<T> Drop for SyncSender<T> {
fn drop(&mut self) {
if ExecutionState::should_stop() {
return;
}
let mut state = self.inner.state.borrow_mut();
assert!(state.known_senders > 0);
state.known_senders -= 1;
if state.known_senders == 0 {
// Last sender was dropped; wake up any receivers
for &tid in state.waiting_receivers.iter() {
ExecutionState::with(|s| s.get_mut(tid).unblock());
}
}
}
}