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//! A concurrent multi-producer multi-consumer queue.
//!
//! There are two kinds of queues:
//!
//! 1. [Bounded] queue with limited capacity.
//! 2. [Unbounded] queue with unlimited capacity.
//!
//! Queues also have the capability to get [closed] at any point. When closed, no more items can be
//! pushed into the queue, although the remaining items can still be popped.
//!
//! These features make it easy to build channels similar to [`std::sync::mpsc`] on top of this
//! crate.
//!
//! # Examples
//!
//! ```
//! use concurrent_queue::ConcurrentQueue;
//!
//! let q = ConcurrentQueue::unbounded();
//! q.push(1).unwrap();
//! q.push(2).unwrap();
//!
//! assert_eq!(q.pop(), Ok(1));
//! assert_eq!(q.pop(), Ok(2));
//! ```
//!
//! [Bounded]: `ConcurrentQueue::bounded()`
//! [Unbounded]: `ConcurrentQueue::unbounded()`
//! [closed]: `ConcurrentQueue::close()`
#![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]
use std::error;
use std::fmt;
use std::panic::{RefUnwindSafe, UnwindSafe};
use std::sync::atomic::{self, AtomicUsize, Ordering};
use crate::bounded::Bounded;
use crate::single::Single;
use crate::unbounded::Unbounded;
mod bounded;
mod single;
mod unbounded;
/// A concurrent queue.
///
/// # Examples
///
/// ```
/// use concurrent_queue::{ConcurrentQueue, PopError, PushError};
///
/// let q = ConcurrentQueue::bounded(2);
///
/// assert_eq!(q.push('a'), Ok(()));
/// assert_eq!(q.push('b'), Ok(()));
/// assert_eq!(q.push('c'), Err(PushError::Full('c')));
///
/// assert_eq!(q.pop(), Ok('a'));
/// assert_eq!(q.pop(), Ok('b'));
/// assert_eq!(q.pop(), Err(PopError::Empty));
/// ```
pub struct ConcurrentQueue<T>(Inner<T>);
unsafe impl<T: Send> Send for ConcurrentQueue<T> {}
unsafe impl<T: Send> Sync for ConcurrentQueue<T> {}
impl<T> UnwindSafe for ConcurrentQueue<T> {}
impl<T> RefUnwindSafe for ConcurrentQueue<T> {}
enum Inner<T> {
Single(Single<T>),
Bounded(Box<Bounded<T>>),
Unbounded(Box<Unbounded<T>>),
}
impl<T> ConcurrentQueue<T> {
/// Creates a new bounded queue.
///
/// The queue allocates enough space for `cap` items.
///
/// # Panics
///
/// If the capacity is zero, this constructor will panic.
///
/// # Examples
///
/// ```
/// use concurrent_queue::ConcurrentQueue;
///
/// let q = ConcurrentQueue::<i32>::bounded(100);
/// ```
pub fn bounded(cap: usize) -> ConcurrentQueue<T> {
if cap == 1 {
ConcurrentQueue(Inner::Single(Single::new()))
} else {
ConcurrentQueue(Inner::Bounded(Box::new(Bounded::new(cap))))
}
}
/// Creates a new unbounded queue.
///
/// # Examples
///
/// ```
/// use concurrent_queue::ConcurrentQueue;
///
/// let q = ConcurrentQueue::<i32>::unbounded();
/// ```
pub fn unbounded() -> ConcurrentQueue<T> {
ConcurrentQueue(Inner::Unbounded(Box::new(Unbounded::new())))
}
/// Attempts to push an item into the queue.
///
/// If the queue is full or closed, the item is returned back as an error.
///
/// # Examples
///
/// ```
/// use concurrent_queue::{ConcurrentQueue, PushError};
///
/// let q = ConcurrentQueue::bounded(1);
///
/// // Push succeeds because there is space in the queue.
/// assert_eq!(q.push(10), Ok(()));
///
/// // Push errors because the queue is now full.
/// assert_eq!(q.push(20), Err(PushError::Full(20)));
///
/// // Close the queue, which will prevent further pushes.
/// q.close();
///
/// // Pushing now errors indicating the queue is closed.
/// assert_eq!(q.push(20), Err(PushError::Closed(20)));
///
/// // Pop the single item in the queue.
/// assert_eq!(q.pop(), Ok(10));
///
/// // Even though there is space, no more items can be pushed.
/// assert_eq!(q.push(20), Err(PushError::Closed(20)));
/// ```
pub fn push(&self, value: T) -> Result<(), PushError<T>> {
match &self.0 {
Inner::Single(q) => q.push(value),
Inner::Bounded(q) => q.push(value),
Inner::Unbounded(q) => q.push(value),
}
}
/// Attempts to pop an item from the queue.
///
/// If the queue is empty, an error is returned.
///
/// # Examples
///
/// ```
/// use concurrent_queue::{ConcurrentQueue, PopError};
///
/// let q = ConcurrentQueue::bounded(1);
///
/// // Pop errors when the queue is empty.
/// assert_eq!(q.pop(), Err(PopError::Empty));
///
/// // Push one item and close the queue.
/// assert_eq!(q.push(10), Ok(()));
/// q.close();
///
/// // Remaining items can be popped.
/// assert_eq!(q.pop(), Ok(10));
///
/// // Again, pop errors when the queue is empty,
/// // but now also indicates that the queue is closed.
/// assert_eq!(q.pop(), Err(PopError::Closed));
/// ```
pub fn pop(&self) -> Result<T, PopError> {
match &self.0 {
Inner::Single(q) => q.pop(),
Inner::Bounded(q) => q.pop(),
Inner::Unbounded(q) => q.pop(),
}
}
/// Returns `true` if the queue is empty.
///
/// # Examples
///
/// ```
/// use concurrent_queue::ConcurrentQueue;
///
/// let q = ConcurrentQueue::<i32>::unbounded();
///
/// assert!(q.is_empty());
/// q.push(1).unwrap();
/// assert!(!q.is_empty());
/// ```
pub fn is_empty(&self) -> bool {
match &self.0 {
Inner::Single(q) => q.is_empty(),
Inner::Bounded(q) => q.is_empty(),
Inner::Unbounded(q) => q.is_empty(),
}
}
/// Returns `true` if the queue is full.
///
/// An unbounded queue is never full.
///
/// # Examples
///
/// ```
/// use concurrent_queue::ConcurrentQueue;
///
/// let q = ConcurrentQueue::bounded(1);
///
/// assert!(!q.is_full());
/// q.push(1).unwrap();
/// assert!(q.is_full());
/// ```
pub fn is_full(&self) -> bool {
match &self.0 {
Inner::Single(q) => q.is_full(),
Inner::Bounded(q) => q.is_full(),
Inner::Unbounded(q) => q.is_full(),
}
}
/// Returns the number of items in the queue.
///
/// # Examples
///
/// ```
/// use concurrent_queue::ConcurrentQueue;
///
/// let q = ConcurrentQueue::unbounded();
/// assert_eq!(q.len(), 0);
///
/// assert_eq!(q.push(10), Ok(()));
/// assert_eq!(q.len(), 1);
///
/// assert_eq!(q.push(20), Ok(()));
/// assert_eq!(q.len(), 2);
/// ```
pub fn len(&self) -> usize {
match &self.0 {
Inner::Single(q) => q.len(),
Inner::Bounded(q) => q.len(),
Inner::Unbounded(q) => q.len(),
}
}
/// Returns the capacity of the queue.
///
/// Unbounded queues have infinite capacity, represented as [`None`].
///
/// # Examples
///
/// ```
/// use concurrent_queue::ConcurrentQueue;
///
/// let q = ConcurrentQueue::<i32>::bounded(7);
/// assert_eq!(q.capacity(), Some(7));
///
/// let q = ConcurrentQueue::<i32>::unbounded();
/// assert_eq!(q.capacity(), None);
/// ```
pub fn capacity(&self) -> Option<usize> {
match &self.0 {
Inner::Single(_) => Some(1),
Inner::Bounded(q) => Some(q.capacity()),
Inner::Unbounded(_) => None,
}
}
/// Closes the queue.
///
/// Returns `true` if this call closed the queue, or `false` if it was already closed.
///
/// When a queue is closed, no more items can be pushed but the remaining items can still be
/// popped.
///
/// # Examples
///
/// ```
/// use concurrent_queue::{ConcurrentQueue, PopError, PushError};
///
/// let q = ConcurrentQueue::unbounded();
/// assert_eq!(q.push(10), Ok(()));
///
/// assert!(q.close()); // `true` because this call closes the queue.
/// assert!(!q.close()); // `false` because the queue is already closed.
///
/// // Cannot push any more items when closed.
/// assert_eq!(q.push(20), Err(PushError::Closed(20)));
///
/// // Remaining items can still be popped.
/// assert_eq!(q.pop(), Ok(10));
///
/// // When no more items are present, the error is `Closed`.
/// assert_eq!(q.pop(), Err(PopError::Closed));
/// ```
pub fn close(&self) -> bool {
match &self.0 {
Inner::Single(q) => q.close(),
Inner::Bounded(q) => q.close(),
Inner::Unbounded(q) => q.close(),
}
}
/// Returns `true` if the queue is closed.
///
/// # Examples
///
/// ```
/// use concurrent_queue::ConcurrentQueue;
///
/// let q = ConcurrentQueue::<i32>::unbounded();
///
/// assert!(!q.is_closed());
/// q.close();
/// assert!(q.is_closed());
/// ```
pub fn is_closed(&self) -> bool {
match &self.0 {
Inner::Single(q) => q.is_closed(),
Inner::Bounded(q) => q.is_closed(),
Inner::Unbounded(q) => q.is_closed(),
}
}
}
impl<T> fmt::Debug for ConcurrentQueue<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("ConcurrentQueue")
.field("len", &self.len())
.field("capacity", &self.capacity())
.field("is_closed", &self.is_closed())
.finish()
}
}
/// Error which occurs when popping from an empty queue.
#[derive(Clone, Copy, Eq, PartialEq)]
pub enum PopError {
/// The queue is empty but not closed.
Empty,
/// The queue is empty and closed.
Closed,
}
impl PopError {
/// Returns `true` if the queue is empty but not closed.
pub fn is_empty(&self) -> bool {
match self {
PopError::Empty => true,
PopError::Closed => false,
}
}
/// Returns `true` if the queue is empty and closed.
pub fn is_closed(&self) -> bool {
match self {
PopError::Empty => false,
PopError::Closed => true,
}
}
}
impl error::Error for PopError {}
impl fmt::Debug for PopError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self {
PopError::Empty => write!(f, "Empty"),
PopError::Closed => write!(f, "Closed"),
}
}
}
impl fmt::Display for PopError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self {
PopError::Empty => write!(f, "Empty"),
PopError::Closed => write!(f, "Closed"),
}
}
}
/// Error which occurs when pushing into a full or closed queue.
#[derive(Clone, Copy, Eq, PartialEq)]
pub enum PushError<T> {
/// The queue is full but not closed.
Full(T),
/// The queue is closed.
Closed(T),
}
impl<T> PushError<T> {
/// Unwraps the item that couldn't be pushed.
pub fn into_inner(self) -> T {
match self {
PushError::Full(t) => t,
PushError::Closed(t) => t,
}
}
/// Returns `true` if the queue is full but not closed.
pub fn is_full(&self) -> bool {
match self {
PushError::Full(_) => true,
PushError::Closed(_) => false,
}
}
/// Returns `true` if the queue is closed.
pub fn is_closed(&self) -> bool {
match self {
PushError::Full(_) => false,
PushError::Closed(_) => true,
}
}
}
impl<T: fmt::Debug> error::Error for PushError<T> {}
impl<T: fmt::Debug> fmt::Debug for PushError<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self {
PushError::Full(t) => f.debug_tuple("Full").field(t).finish(),
PushError::Closed(t) => f.debug_tuple("Closed").field(t).finish(),
}
}
}
impl<T> fmt::Display for PushError<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self {
PushError::Full(_) => write!(f, "Full"),
PushError::Closed(_) => write!(f, "Closed"),
}
}
}
/// Equivalent to `atomic::fence(Ordering::SeqCst)`, but in some cases faster.
#[inline]
fn full_fence() {
if cfg!(any(target_arch = "x86", target_arch = "x86_64")) {
// HACK(stjepang): On x86 architectures there are two different ways of executing
// a `SeqCst` fence.
//
// 1. `atomic::fence(SeqCst)`, which compiles into a `mfence` instruction.
// 2. `_.compare_and_swap(_, _, SeqCst)`, which compiles into a `lock cmpxchg` instruction.
//
// Both instructions have the effect of a full barrier, but empirical benchmarks have shown
// that the second one is sometimes a bit faster.
//
// The ideal solution here would be to use inline assembly, but we're instead creating a
// temporary atomic variable and compare-and-exchanging its value. No sane compiler to
// x86 platforms is going to optimize this away.
let a = AtomicUsize::new(0);
a.compare_and_swap(0, 1, Ordering::SeqCst);
} else {
atomic::fence(Ordering::SeqCst);
}
}