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//! This implements a bounded lock-free Queue
//!
//! # Example
//! ```
//! use nolock::queues::spsc::bounded;
//!
//! // Creates a new BoundedQueue with the Capacity for 5 Items
//! let (mut rx, mut tx) = bounded::queue(5);
//!
//! // Enqueues the Value 13 on the Queue
//! tx.try_enqueue(13);
//! // Dequeues 13 from the Queue again
//! assert_eq!(Ok(13), rx.try_dequeue());
//! ```
//!
//! # Reference:
//! * [FastForward for Efficient Pipeline Parallelism - A Cache-Optimized Concurrent Lock-Free Queue](https://www.researchgate.net/publication/213894711_FastForward_for_Efficient_Pipeline_Parallelism_A_Cache-Optimized_Concurrent_Lock-Free_Queue)
use alloc::{sync::Arc, vec::Vec};
use core::{fmt::Debug, sync::atomic};
use crate::queues::{DequeueError, EnqueueError};
#[cfg(feature = "async")]
mod async_queue;
#[cfg(feature = "async")]
pub use async_queue::*;
mod node;
use node::Node;
/// The Sending-Half for the queue
pub struct BoundedSender<T> {
/// Indicates if the Queue has been closed or not
closed: Arc<atomic::AtomicBool>,
/// The Index of the next Node to read in the Buffer
head: usize,
/// The underlying Buffer of Nodes
buffer: Arc<Vec<Node<T>>>,
}
/// The Receiving-Half for the Queue
pub struct BoundedReceiver<T> {
/// Indicates if the Queue has been closed or not
closed: Arc<atomic::AtomicBool>,
/// The Index of the next Node to store Data into
tail: usize,
/// The underlying Buffer of Nodes
buffer: Arc<Vec<Node<T>>>,
}
/// Calculates the Index of the next Element in the Buffer and wraps around
/// if the End of the Buffer has been reached
#[inline(always)]
const fn next_element(current: usize, length: usize) -> usize {
// This code is logically speaking identical to simply returning
// `(current + 1) % length`
// However after performing some benchmarks, this code seems to be
// significantly, in this context, faster than the simple naive variant
let target = current + 1;
if target >= length {
0
} else {
target
}
}
impl<T> BoundedSender<T> {
/// Returns whether or not the Queue has been closed by the Consumer
///
/// # Example
/// ```
/// # use nolock::queues::spsc::bounded;
/// let (rx, tx) = bounded::queue::<usize>(3);
///
/// // Drop the Consumer and therefore also close the Queue
/// drop(rx);
///
/// assert_eq!(true, tx.is_closed());
/// ```
pub fn is_closed(&self) -> bool {
self.closed.load(atomic::Ordering::Acquire)
}
/// Attempts to Enqueue the given piece of Data
///
/// # Example:
/// Enqueue Data when there is still space
/// ```
/// # use nolock::queues::spsc::bounded;
/// // Create a new Queue with the capacity for 16 Elements
/// let (mut rx, mut tx) = bounded::queue::<usize>(16);
///
/// // Enqueue some Data
/// assert_eq!(Ok(()), tx.try_enqueue(13));
///
/// # assert_eq!(Ok(13), rx.try_dequeue());
/// ```
///
/// Enqueue Data when there is no more space
/// ```
/// # use nolock::queues::spsc::bounded;
/// # use nolock::queues::EnqueueError;
/// // Create a new Queue with the capacity for 16 Elements
/// let (mut rx, mut tx) = bounded::queue::<usize>(16);
///
/// // Fill up the Queue
/// for i in 0..16 {
/// assert_eq!(Ok(()), tx.try_enqueue(i));
/// }
///
/// // Attempt to enqueue some Data, but there is no more room
/// assert_eq!(Err((13, EnqueueError::Full)), tx.try_enqueue(13));
///
/// # drop(rx);
/// ```
pub fn try_enqueue(&mut self, data: T) -> Result<(), (T, EnqueueError)> {
if self.is_closed() {
return Err((data, EnqueueError::Closed));
}
// Get a reference to the current Entry where we would enqueue the next
// Element
let buffer_entry = unsafe { self.buffer.get_unchecked(self.head) };
// If the Node is already set, that means we don't have anywhere to
// store the new Element, meaning that the Buffer is full and we should
// Error out indicating this
if buffer_entry.is_set() {
return Err((data, EnqueueError::Full));
}
// The Node is not already set meaning that we can simply store the
// given Data into the Node
buffer_entry.store(data);
// Advance the current Head, where we insert the Elements, onto the
// next Position
self.head = next_element(self.head, self.buffer.len());
// Return Ok to indicate a successful enqueue operation
Ok(())
}
/// A blocking enqueue Operation. This is obviously not lock-free anymore
/// and will simply spin while trying to enqueue the Data until it works
pub fn enqueue(&mut self, mut data: T) -> Result<(), (T, EnqueueError)> {
loop {
match self.try_enqueue(data) {
Ok(_) => return Ok(()),
Err((d, e)) => match e {
EnqueueError::Full => {
data = d;
}
EnqueueError::Closed => return Err((d, EnqueueError::Closed)),
},
};
}
}
/// Checks if the current Queue is full
pub fn is_full(&self) -> bool {
// If the Node where we would insert the next Element is already set
// that means, that there is currently no room for new Elements in the
// Queue, meaning that the Queue is currently full
self.buffer[self.head].is_set()
}
}
impl<T> Debug for BoundedSender<T> {
fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
write!(f, "BoundedSender ()")
}
}
impl<T> Drop for BoundedSender<T> {
fn drop(&mut self) {
self.closed.store(true, atomic::Ordering::Release);
}
}
unsafe impl<T> Send for BoundedSender<T> {}
unsafe impl<T> Sync for BoundedSender<T> {}
impl<T> BoundedReceiver<T> {
/// Checks if the Queue has been closed by the Producer
///
/// # Note
/// Even when this indicates that the Queue has been closed, there might
/// still be Items in the Queue left that should first be dequeued by the
/// Consumer before discarding the entire Queue
///
/// # Example
/// ```
/// # use nolock::queues::spsc::bounded;
/// let (rx, tx) = bounded::queue::<usize>(3);
///
/// // Drop the Producer and therefore also close the Queue
/// drop(tx);
///
/// assert_eq!(true, rx.is_closed());
/// ```
pub fn is_closed(&self) -> bool {
self.closed.load(atomic::Ordering::Acquire)
}
/// Attempts to Dequeue a single Element from the Queue
///
/// # Example
/// There was something to dequeu
/// ```
/// # use nolock::queues::spsc::bounded;
/// // Create a new Queue with the Capacity for 16-Elements
/// let (mut rx, mut tx) = bounded::queue::<usize>(16);
///
/// // Enqueue the Element
/// tx.try_enqueue(13);
///
/// // Dequeue the Element again
/// assert_eq!(Ok(13), rx.try_dequeue());
/// ```
///
/// The Queue is empty and therefore nothing could be dequeued
/// ```
/// # use nolock::queues::spsc::bounded;
/// # use nolock::queues::DequeueError;
/// // Create a new Queue with the Capacity for 16-Elements
/// let (mut rx, mut tx) = bounded::queue::<usize>(16);
///
/// // Dequeue the Element again
/// assert_eq!(Err(DequeueError::Empty), rx.try_dequeue());
///
/// # drop(tx);
/// ```
pub fn try_dequeue(&mut self) -> Result<T, DequeueError> {
// Get the Node where would read the next Item from
let buffer_entry = unsafe { self.buffer.get_unchecked(self.tail) };
// If the Node is not set, we should return an Error as the Queue is
// empty and there is nothing for us to return in this Operation
if !buffer_entry.is_set() {
// Check if the Queue has been marked as closed
if self.is_closed() {
// We need to recheck the current Node, because it may have
// been set in the mean time and then the closed flag was
// updated
if !buffer_entry.is_set() {
return Err(DequeueError::Closed);
}
}
return Err(DequeueError::Empty);
}
// If the Node is set, we can load the Data out of the Node itself
let data = buffer_entry.load();
// Advance the current Tail, indicating where we should read the next
// Element from, onto the next Node in the Buffer
self.tail = next_element(self.tail, self.buffer.len());
// Return `Ok` with the Data
Ok(data)
}
/// A blocking dequeue operations. This is not lock-free anymore and simply
/// spins while trying to dequeue until it works.
pub fn dequeue(&mut self) -> Option<T> {
loop {
match self.try_dequeue() {
Ok(d) => return Some(d),
Err(e) => match e {
DequeueError::Empty => {}
DequeueError::Closed => return None,
},
};
}
}
/// Checks if the current queue is empty
pub fn is_empty(&self) -> bool {
// If the current Node where would dequeue the next Item from is not
// marked as being set, the Node contains no `set` Nodes and therefore
// the Queue is currently empty
!self.buffer[self.tail].is_set()
}
}
impl<T> Debug for BoundedReceiver<T> {
fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
write!(f, "BoundedReceiver ()")
}
}
impl<T> Drop for BoundedReceiver<T> {
fn drop(&mut self) {
self.closed.store(true, atomic::Ordering::Release);
}
}
unsafe impl<T> Send for BoundedReceiver<T> {}
unsafe impl<T> Sync for BoundedReceiver<T> {}
/// Creates a new Bounded-Queue with the given Capacity and returns the
/// corresponding Handles ([`BoundedReceiver`], [`BoundedSender`])
pub fn queue<T>(capacity: usize) -> (BoundedReceiver<T>, BoundedSender<T>) {
// Create the underlying Buffer of Nodes and fill it up with empty Nodes
// as the initial Configuration
let mut raw_buffer = Vec::with_capacity(capacity);
for _ in 0..capacity {
raw_buffer.push(Node::new());
}
let closed = Arc::new(atomic::AtomicBool::new(false));
let buffer = Arc::new(raw_buffer);
(
BoundedReceiver {
closed: closed.clone(),
buffer: buffer.clone(),
tail: 0,
},
BoundedSender {
closed,
buffer,
head: 0,
},
)
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn enqueue_dequeue() {
let (mut rx, mut tx) = queue(10);
assert_eq!(Ok(()), tx.try_enqueue(13));
assert_eq!(Ok(13), rx.try_dequeue());
}
#[test]
fn enqueue_will_block() {
let (rx, mut tx) = queue(1);
assert_eq!(Ok(()), tx.try_enqueue(13));
assert_eq!(Err((14, EnqueueError::Full)), tx.try_enqueue(14));
drop(rx);
}
#[test]
fn dequeue_will_block() {
let (mut rx, tx) = queue::<usize>(1);
assert_eq!(Err(DequeueError::Empty), rx.try_dequeue());
drop(tx);
}
#[test]
fn enqueue_dequeue_full_buffer() {
let (mut rx, mut tx) = queue(3);
for i in 0..4 {
assert_eq!(Ok(()), tx.try_enqueue(i));
assert_eq!(Ok(i), rx.try_dequeue());
}
}
#[test]
fn enqueue_is_closed() {
let (rx, mut tx) = queue(3);
drop(rx);
assert_eq!(Err((13, EnqueueError::Closed)), tx.try_enqueue(13));
}
#[test]
fn dequeue_is_closed() {
let (mut rx, tx) = queue::<usize>(3);
drop(tx);
assert_eq!(Err(DequeueError::Closed), rx.try_dequeue());
}
#[test]
fn enqueue_dequeue_is_closed() {
let (mut rx, mut tx) = queue::<usize>(3);
tx.try_enqueue(13).unwrap();
drop(tx);
assert_eq!(Ok(13), rx.try_dequeue());
assert_eq!(Err(DequeueError::Closed), rx.try_dequeue());
}
#[test]
fn blocking_enqueue_closed() {
let (rx, mut tx) = queue::<usize>(3);
drop(rx);
assert_eq!(Err((13, EnqueueError::Closed)), tx.enqueue(13));
}
#[test]
fn blocking_dequeue_closed() {
let (mut rx, tx) = queue::<usize>(3);
drop(tx);
assert_eq!(None, rx.dequeue());
}
#[test]
fn is_empty() {
let (mut rx, mut tx) = queue::<usize>(3);
assert!(rx.is_empty());
tx.try_enqueue(13).unwrap();
assert!(!rx.is_empty());
rx.try_dequeue().unwrap();
assert!(rx.is_empty());
}
#[test]
fn is_full() {
let (mut rx, mut tx) = queue::<usize>(1);
assert!(!tx.is_full());
tx.try_enqueue(13).unwrap();
assert!(tx.is_full());
rx.try_dequeue().unwrap();
assert!(!tx.is_full());
}
}