//! Bounded channel based on a preallocated array.
//!
//! This flavor has a fixed, positive capacity.
//!
//! The implementation is based on Dmitry Vyukov's bounded MPMC queue.
//!
//! Source:
//! - <
http://www.1024cores.net/home/lock-free-algorithms/queues/bounded-mpmc-queue>
//! - <
https://docs.google.com/document/d/1yIAYmbvL3JxOKOjuCyon7JhW4cSv1wy5hC0ApeGMV9s/pub>
use std::boxed::Box;
use std::cell::UnsafeCell;
use std::mem::{self, MaybeUninit};
use std::ptr;
use std::sync::atomic::{self, AtomicUsize, Ordering};
use std::time::Instant;
use crossbeam_utils::{Backoff, CachePadded};
use crate::context::Context;
use crate::err::{RecvTimeoutError, SendTimeoutError, TryRecvError, TrySendError};
use crate::select::{Operation, SelectHandle, Selected, Token};
use crate::waker::SyncWaker;
/// A slot in a channel.
struct Slot<T> {
/// The current stamp.
stamp: AtomicUsize,
/// The message in this slot.
msg: UnsafeCell<MaybeUninit<T>>,
}
/// The token type for the array flavor.
#[derive(Debug)]
pub(crate) struct ArrayToken {
/// Slot to read from or write to.
slot: *const u8,
/// Stamp to store into the slot after reading or writing.
stamp: usize,
}
impl Default for ArrayToken {
#[inline]
fn default() -> Self {
ArrayToken {
slot: ptr::null(),
stamp: 0,
}
}
}
/// Bounded channel based on a preallocated array.
pub(crate) struct Channel<T> {
/// The head of the channel.
///
/// This value is a "stamp" consisting of an index into the buffer, a mark bit, and a lap, but
/// packed into a single `usize`. The lower bits represent the index, while the upper bits
/// represent the lap. The mark bit in the head is always zero.
///
/// Messages are popped from the head of the channel.
head: CachePadded<AtomicUsize>,
/// The tail of the channel.
///
/// This value is a "stamp" consisting of an index into the buffer, a mark bit, and a lap, but
/// packed into a single `usize`. The lower bits represent the index, while the upper bits
/// represent the lap. The mark bit indicates that the channel is disconnected.
///
/// Messages are pushed into the tail of the channel.
tail: CachePadded<AtomicUsize>,
/// The buffer holding slots.
buffer: Box<[Slot<T>]>,
/// The channel capacity.
cap: usize,
/// A stamp with the value of `{ lap: 1, mark: 0, index: 0 }`.
one_lap: usize,
/// If this bit is set in the tail, that means the channel is disconnected.
mark_bit: usize,
/// Senders waiting while the channel is full.
senders: SyncWaker,
/// Receivers waiting while the channel is empty and not disconnected.
receivers: SyncWaker,
}
impl<T> Channel<T> {
/// Creates a bounded channel of capacity `cap`.
pub(crate) fn with_capacity(cap: usize) -> Self {
assert!(cap > 0, "capacity must be positive");
// Compute constants `mark_bit` and `one_lap`.
let mark_bit = (cap + 1).next_power_of_two();
let one_lap = mark_bit * 2;
// Head is initialized to `{ lap: 0, mark: 0, index: 0 }`.
let head = 0;
// Tail is initialized to `{ lap: 0, mark: 0, index: 0 }`.
let tail = 0;
// Allocate a buffer of `cap` slots initialized
// with stamps.
let buffer: Box<[Slot<T>]> = (0..cap)
.map(|i| {
// Set the stamp to `{ lap: 0, mark: 0, index: i }`.
Slot {
stamp: AtomicUsize::new(i),
msg: UnsafeCell::new(MaybeUninit::uninit()),
}
})
.collect();
Channel {
buffer,
cap,
one_lap,
mark_bit,
head: CachePadded::new(AtomicUsize::new(head)),
tail: CachePadded::new(AtomicUsize::new(tail)),
senders: SyncWaker::new(),
receivers: SyncWaker::new(),
}
}
/// Returns a receiver handle to the channel.
pub(crate) fn receiver(&self) -> Receiver<'_, T> {
Receiver(self)
}
/// Returns a sender handle to the channel.
pub(crate) fn sender(&self) -> Sender<'_, T> {
Sender(self)
}
/// Attempts to reserve a slot for sending a message.
fn start_send(&self, token: &mut Token) -> bool {
let backoff = Backoff::new();
let mut tail = self.tail.load(Ordering::Relaxed);
loop {
// Check if the channel is disconnected.
if tail & self.mark_bit != 0 {
token.array.slot = ptr::null();
token.array.stamp = 0;
return true;
}
// Deconstruct the tail.
let index = tail & (self.mark_bit - 1);
let lap = tail & !(self.one_lap - 1);
// Inspect the corresponding slot.
debug_assert!(index < self.buffer.len());
let slot = unsafe { self.buffer.get_unchecked(index) };
let stamp = slot.stamp.load(Ordering::Acquire);
// If the tail and the stamp match, we may attempt to push.
if tail == stamp {
let new_tail = if index + 1 < self.cap {
// Same lap, incremented index.
// Set to `{ lap: lap, mark: 0, index: index + 1 }`.
tail + 1
} else {
// One lap forward, index wraps around to zero.
// Set to `{ lap: lap.wrapping_add(1), mark: 0, index: 0 }`.
lap.wrapping_add(self.one_lap)
};
// Try moving the tail.
match self.tail.compare_exchange_weak(
tail,
new_tail,
Ordering::SeqCst,
Ordering::Relaxed,
) {
Ok(_) => {
// Prepare the token for the follow-up call to `write`.
token.array.slot = slot as *const Slot<T> as *const u8;
token.array.stamp = tail + 1;
return true;
}
Err(t) => {
tail = t;
backoff.spin();
}
}
} else if stamp.wrapping_add(self.one_lap) == tail + 1 {
atomic::fence(Ordering::SeqCst);
let head = self.head.load(Ordering::Relaxed);
// If the head lags one lap behind the tail as well...
if head.wrapping_add(self.one_lap) == tail {
// ...then the channel is full.
return false;
}
backoff.spin();
tail = self.tail.load(Ordering::Relaxed);
} else {
// Snooze because we need to wait for the stamp to get updated.
backoff.snooze();
tail = self.tail.load(Ordering::Relaxed);
}
}
}
/// Writes a message into the channel.
pub(crate) unsafe fn write(&self, token: &mut Token, msg: T) -> Result<(), T> {
// If there is no slot, the channel is disconnected.
if token.array.slot.is_null() {
return Err(msg);
}
let slot: &Slot<T> = &*token.array.slot.cast::<Slot<T>>();
// Write the message into the slot and update the stamp.
slot.msg.get().write(MaybeUninit::new(msg));
slot.stamp.store(token.array.stamp, Ordering::Release);
// Wake a sleeping receiver.
self.receivers.notify();
Ok(())
}
/// Attempts to reserve a slot for receiving a message.
fn start_recv(&self, token: &mut Token) -> bool {
let backoff = Backoff::new();
let mut head = self.head.load(Ordering::Relaxed);
loop {
// Deconstruct the head.
let index = head & (self.mark_bit - 1);
let lap = head & !(self.one_lap - 1);
// Inspect the corresponding slot.
debug_assert!(index < self.buffer.len());
let slot = unsafe { self.buffer.get_unchecked(index) };
let stamp = slot.stamp.load(Ordering::Acquire);
// If the stamp is ahead of the head by 1, we may attempt to pop.
if head + 1 == stamp {
let new = if index + 1 < self.cap {
// Same lap, incremented index.
// Set to `{ lap: lap, mark: 0, index: index + 1 }`.
head + 1
} else {
// One lap forward, index wraps around to zero.
// Set to `{ lap: lap.wrapping_add(1), mark: 0, index: 0 }`.
lap.wrapping_add(self.one_lap)
};
// Try moving the head.
match self.head.compare_exchange_weak(
head,
new,
Ordering::SeqCst,
Ordering::Relaxed,
) {
Ok(_) => {
// Prepare the token for the follow-up call to `read`.
token.array.slot = slot as *const Slot<T> as *const u8;
token.array.stamp = head.wrapping_add(self.one_lap);
return true;
}
Err(h) => {
head = h;
backoff.spin();
}
}
} else if stamp == head {
atomic::fence(Ordering::SeqCst);
let tail = self.tail.load(Ordering::Relaxed);
// If the tail equals the head, that means the channel is empty.
if (tail & !self.mark_bit) == head {
// If the channel is disconnected...
if tail & self.mark_bit != 0 {
// ...then receive an error.
token.array.slot = ptr::null();
token.array.stamp = 0;
return true;
} else {
// Otherwise, the receive operation is not ready.
return false;
}
}
backoff.spin();
head = self.head.load(Ordering::Relaxed);
} else {
// Snooze because we need to wait for the stamp to get updated.
backoff.snooze();
head = self.head.load(Ordering::Relaxed);
}
}
}
/// Reads a message from the channel.
pub(crate) unsafe fn read(&self, token: &mut Token) -> Result<T, ()> {
if token.array.slot.is_null() {
// The channel is disconnected.
return Err(());
}
let slot: &Slot<T> = &*token.array.slot.cast::<Slot<T>>();
// Read the message from the slot and update the stamp.
let msg = slot.msg.get().read().assume_init();
slot.stamp.store(token.array.stamp, Ordering::Release);
// Wake a sleeping sender.
self.senders.notify();
Ok(msg)
}
/// Attempts to send a message into the channel.
pub(crate) fn try_send(&self, msg: T) -> Result<(), TrySendError<T>> {
let token = &mut Token::default();
if self.start_send(token) {
unsafe { self.write(token, msg).map_err(TrySendError::Disconnected) }
} else {
Err(TrySendError::Full(msg))
}
}
/// Sends a message into the channel.
pub(crate) fn send(
&self,
msg: T,
deadline: Option<Instant>,
) -> Result<(), SendTimeoutError<T>> {
let token = &mut Token::default();
loop {
// Try sending a message several times.
let backoff = Backoff::new();
loop {
if self.start_send(token) {
let res = unsafe { self.write(token, msg) };
return res.map_err(SendTimeoutError::Disconnected);
}
if backoff.is_completed() {
break;
} else {
backoff.snooze();
}
}
if let Some(d) = deadline {
if Instant::now() >= d {
return Err(SendTimeoutError::Timeout(msg));
}
}
Context::with(|cx| {
// Prepare for blocking until a receiver wakes us up.
let oper = Operation::hook(token);
self.senders.register(oper, cx);
// Has the channel become ready just now?
if !self.is_full() || self.is_disconnected() {
let _ = cx.try_select(Selected::Aborted);
}
// Block the current thread.
let sel = cx.wait_until(deadline);
match sel {
Selected::Waiting => unreachable!(),
Selected::Aborted | Selected::Disconnected => {
self.senders.unregister(oper).unwrap();
}
Selected::Operation(_) => {}
}
});
}
}
/// Attempts to receive a message without blocking.
pub(crate) fn try_recv(&self) -> Result<T, TryRecvError> {
let token = &mut Token::default();
if self.start_recv(token) {
unsafe { self.read(token).map_err(|_| TryRecvError::Disconnected) }
} else {
Err(TryRecvError::Empty)
}
}
/// Receives a message from the channel.
pub(crate) fn recv(&self, deadline: Option<Instant>) -> Result<T, RecvTimeoutError> {
let token = &mut Token::default();
loop {
// Try receiving a message several times.
let backoff = Backoff::new();
loop {
if self.start_recv(token) {
let res = unsafe { self.read(token) };
return res.map_err(|_| RecvTimeoutError::Disconnected);
}
if backoff.is_completed() {
break;
} else {
backoff.snooze();
}
}
if let Some(d) = deadline {
if Instant::now() >= d {
return Err(RecvTimeoutError::Timeout);
}
}
Context::with(|cx| {
// Prepare for blocking until a sender wakes us up.
let oper = Operation::hook(token);
self.receivers.register(oper, cx);
// Has the channel become ready just now?
if !self.is_empty() || self.is_disconnected() {
let _ = cx.try_select(Selected::Aborted);
}
// Block the current thread.
let sel = cx.wait_until(deadline);
match sel {
Selected::Waiting => unreachable!(),
Selected::Aborted | Selected::Disconnected => {
self.receivers.unregister(oper).unwrap();
// If the channel was disconnected, we still have to check for remaining
// messages.
}
Selected::Operation(_) => {}
}
});
}
}
/// Returns the current number of messages inside the channel.
pub(crate) fn len(&self) -> usize {
loop {
// Load the tail, then load the head.
let tail = self.tail.load(Ordering::SeqCst);
let head = self.head.load(Ordering::SeqCst);
// If the tail didn't change, we've got consistent values to work with.
if self.tail.load(Ordering::SeqCst) == tail {
let hix = head & (self.mark_bit - 1);
let tix = tail & (self.mark_bit - 1);
return if hix < tix {
tix - hix
} else if hix > tix {
self.cap - hix + tix
} else if (tail & !self.mark_bit) == head {
0
} else {
self.cap
};
}
}
}
/// Returns the capacity of the channel.
pub(crate) fn capacity(&self) -> Option<usize> {
Some(self.cap)
}
/// Disconnects the channel and wakes up all blocked senders and receivers.
///
/// Returns `true` if this call disconnected the channel.
pub(crate) fn disconnect(&self) -> bool {
let tail = self.tail.fetch_or(self.mark_bit, Ordering::SeqCst);
if tail & self.mark_bit == 0 {
self.senders.disconnect();
self.receivers.disconnect();
true
} else {
false
}
}
/// Returns `true` if the channel is disconnected.
pub(crate) fn is_disconnected(&self) -> bool {
self.tail.load(Ordering::SeqCst) & self.mark_bit != 0
}
/// Returns `true` if the channel is empty.
pub(crate) fn is_empty(&self) -> bool {
let head = self.head.load(Ordering::SeqCst);
let tail = self.tail.load(Ordering::SeqCst);
// Is the tail equal to the head?
//
// Note: If the head changes just before we load the tail, that means there was a moment
// when the channel was not empty, so it is safe to just return `false`.
(tail & !self.mark_bit) == head
}
/// Returns `true` if the channel is full.
pub(crate) fn is_full(&self) -> bool {
let tail = self.tail.load(Ordering::SeqCst);
let head = self.head.load(Ordering::SeqCst);
// Is the head lagging one lap behind tail?
//
// Note: If the tail changes just before we load the head, that means there was a moment
// when the channel was not full, so it is safe to just return `false`.
head.wrapping_add(self.one_lap) == tail & !self.mark_bit
}
}
impl<T> Drop for Channel<T> {
fn drop(&mut self) {
if mem::needs_drop::<T>() {
// Get the index of the head.
let head = *self.head.get_mut();
let tail = *self.tail.get_mut();
let hix = head & (self.mark_bit - 1);
let tix = tail & (self.mark_bit - 1);
let len = if hix < tix {
tix - hix
} else if hix > tix {
self.cap - hix + tix
} else if (tail & !self.mark_bit) == head {
0
} else {
self.cap
};
// Loop over all slots that hold a message and drop them.
for i in 0..len {
// Compute the index of the next slot holding a message.
let index = if hix + i < self.cap {
hix + i
} else {
hix + i - self.cap
};
unsafe {
debug_assert!(index < self.buffer.len());
let slot = self.buffer.get_unchecked_mut(index);
(*slot.msg.get()).assume_init_drop();
}
}
}
}
}
/// Receiver handle to a channel.
pub(crate) struct Receiver<'a, T>(&'a Channel<T>);
/// Sender handle to a channel.
pub(crate) struct Sender<'a, T>(&'a Channel<T>);
impl<T> SelectHandle for Receiver<'_, T> {
fn try_select(&self, token: &mut Token) -> bool {
self.0.start_recv(token)
}
fn deadline(&self) -> Option<Instant> {
None
}
fn register(&self, oper: Operation, cx: &Context) -> bool {
self.0.receivers.register(oper, cx);
self.is_ready()
}
fn unregister(&self, oper: Operation) {
self.0.receivers.unregister(oper);
}
fn accept(&self, token: &mut Token, _cx: &Context) -> bool {
self.try_select(token)
}
fn is_ready(&self) -> bool {
!self.0.is_empty() || self.0.is_disconnected()
}
fn watch(&self, oper: Operation, cx: &Context) -> bool {
self.0.receivers.watch(oper, cx);
self.is_ready()
}
fn unwatch(&self, oper: Operation) {
self.0.receivers.unwatch(oper);
}
}
impl<T> SelectHandle for Sender<'_, T> {
fn try_select(&self, token: &mut Token) -> bool {
self.0.start_send(token)
}
fn deadline(&self) -> Option<Instant> {
None
}
fn register(&self, oper: Operation, cx: &Context) -> bool {
self.0.senders.register(oper, cx);
self.is_ready()
}
fn unregister(&self, oper: Operation) {
self.0.senders.unregister(oper);
}
fn accept(&self, token: &mut Token, _cx: &Context) -> bool {
self.try_select(token)
}
fn is_ready(&self) -> bool {
!self.0.is_full() || self.0.is_disconnected()
}
fn watch(&self, oper: Operation, cx: &Context) -> bool {
self.0.senders.watch(oper, cx);
self.is_ready()
}
fn unwatch(&self, oper: Operation) {
self.0.senders.unwatch(oper);
}
}
