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github: stjepang

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name: Build and test
on:
push:
branches:
- master
pull_request:
jobs:
build_and_test:
runs-on: ${{ matrix.os }}
strategy:
fail-fast: false
matrix:
os: [ubuntu-latest]
rust: [nightly, beta, stable]
steps:
- uses: actions/checkout@v2
- name: Set current week of the year in environnement
if: startsWith(matrix.os, 'ubuntu') || startsWith(matrix.os, 'macOS')
run: echo "::set-env name=CURRENT_WEEK::$(date +%V)"
- name: Set current week of the year in environnement
if: startsWith(matrix.os, 'windows')
run: echo "::set-env name=CURRENT_WEEK::$(Get-Date -UFormat %V)"
- name: Install latest ${{ matrix.rust }}
uses: actions-rs/toolchain@v1
with:
toolchain: ${{ matrix.rust }}
profile: minimal
override: true
- name: Run cargo check
uses: actions-rs/cargo@v1
with:
command: check
args: --all --bins --examples --tests --all-features
- name: Run cargo check (without dev-dependencies to catch missing feature flags)
if: startsWith(matrix.rust, 'nightly')
uses: actions-rs/cargo@v1
with:
command: check
args: -Z features=dev_dep
- name: Run cargo test
uses: actions-rs/cargo@v1
with:
command: test

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name: Lint
on:
push:
branches:
- master
pull_request:
jobs:
clippy:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v2
- name: Set current week of the year in environnement
run: echo "::set-env name=CURRENT_WEEK::$(date +%V)"
- uses: actions-rs/toolchain@v1
with:
toolchain: stable
profile: minimal
components: clippy
- uses: actions-rs/clippy-check@v1
with:
token: ${{ secrets.GITHUB_TOKEN }}
args: --all-features -- -W clippy::all

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name: Security audit
on:
push:
branches:
- master
pull_request:
jobs:
security_audit:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v2
- name: Set current week of the year in environnement
run: echo "::set-env name=CURRENT_WEEK::$(date +%V)"
- uses: actions-rs/audit-check@v1
with:
token: ${{ secrets.GITHUB_TOKEN }}

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/target
Cargo.lock

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# Version 1.0.0
- Initial version

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[package]
name = "concurrent-queue"
version = "1.0.0"
authors = ["Stjepan Glavina <stjepang@gmail.com>"]
edition = "2018"
description = "Concurrent multi-producer multi-consumer queue"
license = "Apache-2.0 OR MIT"
repository = "https://github.com/stjepang/async-mutex"
homepage = "https://github.com/stjepang/async-mutex"
documentation = "https://docs.rs/async-mutex"
keywords = ["channel", "mpmc", "spsc", "spmc", "mpsc"]
categories = ["concurrency"]
readme = "README.md"
[dependencies]
cache-padded = "1.0.0"
[dev-dependencies]
easy-parallel = "2.1.0"
fastrand = "1.0.0"

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# async-mutex
[![Build](https://github.com/stjepang/async-mutex/workflows/Build%20and%20test/badge.svg)](
https://github.com/stjepang/async-mutex/actions)
[![License](https://img.shields.io/badge/license-MIT%2FApache--2.0-blue.svg)](
https://github.com/stjepang/async-mutex)
[![Cargo](https://img.shields.io/crates/v/async-mutex.svg)](
https://crates.io/crates/async-mutex)
[![Documentation](https://docs.rs/async-mutex/badge.svg)](
https://docs.rs/async-mutex)
An async mutex.
The locking mechanism uses eventual fairness to ensure locking will be fair on average without
sacrificing performance. This is done by forcing a fair lock whenever a lock operation is
starved for longer than 0.5 milliseconds.
## Examples
```rust
use async_mutex::Mutex;
use smol::Task;
use std::sync::Arc;
let m = Arc::new(Mutex::new(0));
let mut tasks = vec![];
for _ in 0..10 {
let m = m.clone();
tasks.push(Task::spawn(async move {
*m.lock().await += 1;
}));
}
for t in tasks {
t.await;
}
assert_eq!(*m.lock().await, 10);
```
## License
Licensed under either of
* Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
* MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
at your option.
#### Contribution
Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
dual licensed as above, without any additional terms or conditions.

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use std::cell::UnsafeCell;
use std::marker::PhantomData;
use std::mem::{self, MaybeUninit};
use std::sync::atomic::{self, AtomicUsize, Ordering};
use std::thread;
use cache_padded::CachePadded;
use crate::{PopError, PushError};
/// A slot in a queue.
struct Slot<T> {
/// The current stamp.
stamp: AtomicUsize,
/// The value in this slot.
value: UnsafeCell<MaybeUninit<T>>,
}
/// A bounded queue.
pub struct Bounded<T> {
/// The head of the queue.
///
/// 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.
///
/// Values are popped from the head of the queue.
head: CachePadded<AtomicUsize>,
/// The tail of the queue.
///
/// 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 queue is closed.
///
/// Values are pushed into the tail of the queue.
tail: CachePadded<AtomicUsize>,
/// The buffer holding slots.
buffer: *mut Slot<T>,
/// The queue 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 queue is closed.
mark_bit: usize,
/// Indicates that dropping an `Bounded<T>` may drop values of type `T`.
_marker: PhantomData<T>,
}
impl<T> Bounded<T> {
/// Creates a new bounded queue.
pub fn new(cap: usize) -> Bounded<T> {
assert!(cap > 0, "capacity must be positive");
// 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 = {
let mut v: Vec<Slot<T>> = (0..cap)
.map(|i| {
// Set the stamp to `{ lap: 0, mark: 0, index: i }`.
Slot {
stamp: AtomicUsize::new(i),
value: UnsafeCell::new(MaybeUninit::uninit()),
}
})
.collect();
let ptr = v.as_mut_ptr();
mem::forget(v);
ptr
};
// Compute constants `mark_bit` and `one_lap`.
let mark_bit = (cap + 1).next_power_of_two();
let one_lap = mark_bit * 2;
Bounded {
buffer,
cap,
one_lap,
mark_bit,
head: CachePadded::new(AtomicUsize::new(head)),
tail: CachePadded::new(AtomicUsize::new(tail)),
_marker: PhantomData,
}
}
/// Attempts to push an item into the queue.
pub fn push(&self, value: T) -> Result<(), PushError<T>> {
let mut tail = self.tail.load(Ordering::Relaxed);
loop {
// Check if the queue is closed.
if tail & self.mark_bit != 0 {
return Err(PushError::Closed(value));
}
// Deconstruct the tail.
let index = tail & (self.mark_bit - 1);
let lap = tail & !(self.one_lap - 1);
// Inspect the corresponding slot.
let slot = unsafe { &*self.buffer.add(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(_) => {
// Write the value into the slot and update the stamp.
unsafe {
slot.value.get().write(MaybeUninit::new(value));
}
slot.stamp.store(tail + 1, Ordering::Release);
return Ok(());
}
Err(t) => {
tail = t;
}
}
} 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 queue is full.
return Err(PushError::Full(value));
}
tail = self.tail.load(Ordering::Relaxed);
} else {
// Yield because we need to wait for the stamp to get updated.
thread::yield_now();
tail = self.tail.load(Ordering::Relaxed);
}
}
}
/// Attempts to pop an item from the queue.
pub fn pop(&self) -> Result<T, PopError> {
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.
let slot = unsafe { &*self.buffer.add(index) };
let stamp = slot.stamp.load(Ordering::Acquire);
// If the 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(_) => {
// Read the value from the slot and update the stamp.
let value = unsafe { slot.value.get().read().assume_init() };
slot.stamp
.store(head.wrapping_add(self.one_lap), Ordering::Release);
return Ok(value);
}
Err(h) => {
head = h;
}
}
} else if stamp == head {
atomic::fence(Ordering::SeqCst);
let tail = self.tail.load(Ordering::Relaxed);
// If the tail equals the head, that means the queue is empty.
if (tail & !self.mark_bit) == head {
// Check if the queue is closed.
if tail & self.mark_bit != 0 {
return Err(PopError::Closed);
} else {
return Err(PopError::Empty);
}
}
head = self.head.load(Ordering::Relaxed);
} else {
// Yield because we need to wait for the stamp to get updated.
thread::yield_now();
head = self.head.load(Ordering::Relaxed);
}
}
}
/// Returns the number of items in the queue.
pub 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 `true` if the queue is empty.
pub 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 queue was not empty, so it is safe to just return `false`.
(tail & !self.mark_bit) == head
}
/// Returns `true` if the queue is full.
pub 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 queue was not full, so it is safe to just return `false`.
head.wrapping_add(self.one_lap) == tail & !self.mark_bit
}
/// Returns the capacity of the queue.
pub fn capacity(&self) -> usize {
self.cap
}
/// Closes the queue.
///
/// Returns `true` if this call closed the queue.
pub fn close(&self) -> bool {
let tail = self.tail.fetch_or(self.mark_bit, Ordering::SeqCst);
if tail & self.mark_bit == 0 {
true
} else {
false
}
}
/// Returns `true` if the queue is closed.
pub fn is_closed(&self) -> bool {
self.tail.load(Ordering::SeqCst) & self.mark_bit != 0
}
}
impl<T> Drop for Bounded<T> {
fn drop(&mut self) {
// Get the index of the head.
let hix = self.head.load(Ordering::Relaxed) & (self.mark_bit - 1);
// Loop over all slots that hold a value and drop them.
for i in 0..self.len() {
// Compute the index of the next slot holding a value.
let index = if hix + i < self.cap {
hix + i
} else {
hix + i - self.cap
};
// Drop the value in the slot.
unsafe {
let slot = &*self.buffer.add(index);
let value = slot.value.get().read().assume_init();
drop(value);
}
}
// Finally, deallocate the buffer, but don't run any destructors.
unsafe {
Vec::from_raw_parts(self.buffer, 0, self.cap);
}
}
}

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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 crate::bounded::Bounded;
use crate::unbounded::Unbounded;
mod bounded;
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> {}
enum Inner<T> {
Bounded(Bounded<T>),
Unbounded(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> {
ConcurrentQueue(Inner::Bounded(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(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::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::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::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::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::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::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::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::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 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: 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"),
}
}
}

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use std::cell::UnsafeCell;
use std::marker::PhantomData;
use std::mem::MaybeUninit;
use std::ptr;
use std::sync::atomic::{self, AtomicPtr, AtomicUsize, Ordering};
use std::thread;
use cache_padded::CachePadded;
use crate::{PopError, PushError};
// Bits indicating the state of a slot:
// * If a value has been written into the slot, `WRITE` is set.
// * If a value has been read from the slot, `READ` is set.
// * If the block is being destroyed, `DESTROY` is set.
const WRITE: usize = 1;
const READ: usize = 2;
const DESTROY: usize = 4;
// Each block covers one "lap" of indices.
const LAP: usize = 32;
// The maximum number of items a block can hold.
const BLOCK_CAP: usize = LAP - 1;
// How many lower bits are reserved for metadata.
const SHIFT: usize = 1;
// Has two different purposes:
// * If set in head, indicates that the block is not the last one.
// * If set in tail, indicates that the queue is closed.
const MARK_BIT: usize = 1;
/// A slot in a block.
struct Slot<T> {
/// The value.
value: UnsafeCell<MaybeUninit<T>>,
/// The state of the slot.
state: AtomicUsize,
}
impl<T> Slot<T> {
const UNINIT: Slot<T> = Slot {
value: UnsafeCell::new(MaybeUninit::uninit()),
state: AtomicUsize::new(0),
};
/// Waits until a value is written into the slot.
fn wait_write(&self) {
while self.state.load(Ordering::Acquire) & WRITE == 0 {
thread::yield_now();
}
}
}
/// A block in a linked list.
///
/// Each block in the list can hold up to `BLOCK_CAP` values.
struct Block<T> {
/// The next block in the linked list.
next: AtomicPtr<Block<T>>,
/// Slots for values.
slots: [Slot<T>; BLOCK_CAP],
}
impl<T> Block<T> {
/// Creates an empty block.
fn new() -> Block<T> {
Block {
next: AtomicPtr::new(ptr::null_mut()),
slots: [Slot::UNINIT; BLOCK_CAP],
}
}
/// Waits until the next pointer is set.
fn wait_next(&self) -> *mut Block<T> {
loop {
let next = self.next.load(Ordering::Acquire);
if !next.is_null() {
return next;
}
thread::yield_now();
}
}
/// Sets the `DESTROY` bit in slots starting from `start` and destroys the block.
unsafe fn destroy(this: *mut Block<T>, start: usize) {
// It is not necessary to set the `DESTROY` bit in the last slot because that slot has
// begun destruction of the block.
for i in start..BLOCK_CAP - 1 {
let slot = (*this).slots.get_unchecked(i);
// Mark the `DESTROY` bit if a thread is still using the slot.
if slot.state.load(Ordering::Acquire) & READ == 0
&& slot.state.fetch_or(DESTROY, Ordering::AcqRel) & READ == 0
{
// If a thread is still using the slot, it will continue destruction of the block.
return;
}
}
// No thread is using the block, now it is safe to destroy it.
drop(Box::from_raw(this));
}
}
/// A position in a queue.
struct Position<T> {
/// The index in the queue.
index: AtomicUsize,
/// The block in the linked list.
block: AtomicPtr<Block<T>>,
}
/// An unbounded queue.
pub struct Unbounded<T> {
/// The head of the queue.
head: CachePadded<Position<T>>,
/// The tail of the queue.
tail: CachePadded<Position<T>>,
/// Indicates that dropping a `Unbounded<T>` may drop values of type `T`.
_marker: PhantomData<T>,
}
impl<T> Unbounded<T> {
/// Creates a new unbounded queue.
pub fn new() -> Unbounded<T> {
Unbounded {
head: CachePadded::new(Position {
block: AtomicPtr::new(ptr::null_mut()),
index: AtomicUsize::new(0),
}),
tail: CachePadded::new(Position {
block: AtomicPtr::new(ptr::null_mut()),
index: AtomicUsize::new(0),
}),
_marker: PhantomData,
}
}
/// Pushes an item into the queue.
pub fn push(&self, value: T) -> Result<(), PushError<T>> {
let mut tail = self.tail.index.load(Ordering::Acquire);
let mut block = self.tail.block.load(Ordering::Acquire);
let mut next_block = None;
loop {
// Check if the queue is closed.
if tail & MARK_BIT != 0 {
return Err(PushError::Closed(value));
}
// Calculate the offset of the index into the block.
let offset = (tail >> SHIFT) % LAP;
// If we reached the end of the block, wait until the next one is installed.
if offset == BLOCK_CAP {
thread::yield_now();
tail = self.tail.index.load(Ordering::Acquire);
block = self.tail.block.load(Ordering::Acquire);
continue;
}
// If we're going to have to install the next block, allocate it in advance in order to
// make the wait for other threads as short as possible.
if offset + 1 == BLOCK_CAP && next_block.is_none() {
next_block = Some(Box::new(Block::<T>::new()));
}
// If this is the first value to be pushed into the queue, we need to allocate the
// first block and install it.
if block.is_null() {
let new = Box::into_raw(Box::new(Block::<T>::new()));
if self
.tail
.block
.compare_and_swap(block, new, Ordering::Release)
== block
{
self.head.block.store(new, Ordering::Release);
block = new;
} else {
next_block = unsafe { Some(Box::from_raw(new)) };
tail = self.tail.index.load(Ordering::Acquire);
block = self.tail.block.load(Ordering::Acquire);
continue;
}
}
let new_tail = tail + (1 << SHIFT);
// Try advancing the tail forward.
match self.tail.index.compare_exchange_weak(
tail,
new_tail,
Ordering::SeqCst,
Ordering::Acquire,
) {
Ok(_) => unsafe {
// If we've reached the end of the block, install the next one.
if offset + 1 == BLOCK_CAP {
let next_block = Box::into_raw(next_block.unwrap());
self.tail.block.store(next_block, Ordering::Release);
self.tail.index.fetch_add(1 << SHIFT, Ordering::Release);
(*block).next.store(next_block, Ordering::Release);
}
// Write the value into the slot.
let slot = (*block).slots.get_unchecked(offset);
slot.value.get().write(MaybeUninit::new(value));
slot.state.fetch_or(WRITE, Ordering::Release);
return Ok(());
},
Err(t) => {
tail = t;
block = self.tail.block.load(Ordering::Acquire);
}
}
}
}
/// Pops an item from the queue.
pub fn pop(&self) -> Result<T, PopError> {
let mut head = self.head.index.load(Ordering::Acquire);
let mut block = self.head.block.load(Ordering::Acquire);
loop {
// Calculate the offset of the index into the block.
let offset = (head >> SHIFT) % LAP;
// If we reached the end of the block, wait until the next one is installed.
if offset == BLOCK_CAP {
thread::yield_now();
head = self.head.index.load(Ordering::Acquire);
block = self.head.block.load(Ordering::Acquire);
continue;
}
let mut new_head = head + (1 << SHIFT);
if new_head & MARK_BIT == 0 {
atomic::fence(Ordering::SeqCst);
let tail = self.tail.index.load(Ordering::Relaxed);
// If the tail equals the head, that means the queue is empty.
if head >> SHIFT == tail >> SHIFT {
// Check if the queue is closed.
if tail & MARK_BIT != 0 {
return Err(PopError::Closed);
} else {
return Err(PopError::Empty);
}
}
// If head and tail are not in the same block, set `MARK_BIT` in head.
if (head >> SHIFT) / LAP != (tail >> SHIFT) / LAP {
new_head |= MARK_BIT;
}
}
// The block can be null here only if the first push operation is in progress.
if block.is_null() {
thread::yield_now();
head = self.head.index.load(Ordering::Acquire);
block = self.head.block.load(Ordering::Acquire);
continue;
}
// Try moving the head index forward.
match self.head.index.compare_exchange_weak(
head,
new_head,
Ordering::SeqCst,
Ordering::Acquire,
) {
Ok(_) => unsafe {
// If we've reached the end of the block, move to the next one.
if offset + 1 == BLOCK_CAP {
let next = (*block).wait_next();
let mut next_index = (new_head & !MARK_BIT).wrapping_add(1 << SHIFT);
if !(*next).next.load(Ordering::Relaxed).is_null() {
next_index |= MARK_BIT;
}
self.head.block.store(next, Ordering::Release);
self.head.index.store(next_index, Ordering::Release);
}
// Read the value.
let slot = (*block).slots.get_unchecked(offset);
slot.wait_write();
let value = slot.value.get().read().assume_init();
// Destroy the block if we've reached the end, or if another thread wanted to
// destroy but couldn't because we were busy reading from the slot.
if offset + 1 == BLOCK_CAP {
Block::destroy(block, 0);
} else if slot.state.fetch_or(READ, Ordering::AcqRel) & DESTROY != 0 {
Block::destroy(block, offset + 1);
}
return Ok(value);
},
Err(h) => {
head = h;
block = self.head.block.load(Ordering::Acquire);
}
}
}
}
/// Returns the number of items in the queue.
pub fn len(&self) -> usize {
loop {
// Load the tail index, then load the head index.
let mut tail = self.tail.index.load(Ordering::SeqCst);
let mut head = self.head.index.load(Ordering::SeqCst);
// If the tail index didn't change, we've got consistent indices to work with.
if self.tail.index.load(Ordering::SeqCst) == tail {
// Erase the lower bits.
tail &= !((1 << SHIFT) - 1);
head &= !((1 << SHIFT) - 1);
// Fix up indices if they fall onto block ends.
if (tail >> SHIFT) & (LAP - 1) == LAP - 1 {
tail = tail.wrapping_add(1 << SHIFT);
}
if (head >> SHIFT) & (LAP - 1) == LAP - 1 {
head = head.wrapping_add(1 << SHIFT);
}
// Rotate indices so that head falls into the first block.
let lap = (head >> SHIFT) / LAP;
tail = tail.wrapping_sub((lap * LAP) << SHIFT);
head = head.wrapping_sub((lap * LAP) << SHIFT);
// Remove the lower bits.
tail >>= SHIFT;
head >>= SHIFT;
// Return the difference minus the number of blocks between tail and head.
return tail - head - tail / LAP;
}
}
}
/// Returns `true` if the queue is empty.
pub fn is_empty(&self) -> bool {
let head = self.head.index.load(Ordering::SeqCst);
let tail = self.tail.index.load(Ordering::SeqCst);
head >> SHIFT == tail >> SHIFT
}
/// Returns `true` if the queue is full.
pub fn is_full(&self) -> bool {
false
}
/// Closes the queue.
///
/// Returns `true` if this call closed the queue.
pub fn close(&self) -> bool {
let tail = self.tail.index.fetch_or(MARK_BIT, Ordering::SeqCst);
if tail & MARK_BIT == 0 {
true
} else {
false
}
}
/// Returns `true` if the queue is closed.
pub fn is_closed(&self) -> bool {
self.tail.index.load(Ordering::SeqCst) & MARK_BIT != 0
}
}
impl<T> Drop for Unbounded<T> {
fn drop(&mut self) {
let mut head = self.head.index.load(Ordering::Relaxed);
let mut tail = self.tail.index.load(Ordering::Relaxed);
let mut block = self.head.block.load(Ordering::Relaxed);
// Erase the lower bits.
head &= !((1 << SHIFT) - 1);
tail &= !((1 << SHIFT) - 1);
unsafe {
// Drop all values between `head` and `tail` and deallocate the heap-allocated blocks.
while head != tail {
let offset = (head >> SHIFT) % LAP;
if offset < BLOCK_CAP {
// Drop the value in the slot.
let slot = (*block).slots.get_unchecked(offset);
let value = slot.value.get().read().assume_init();
drop(value);
} else {
// Deallocate the block and move to the next one.
let next = (*block).next.load(Ordering::Relaxed);
drop(Box::from_raw(block));
block = next;
}
head = head.wrapping_add(1 << SHIFT);
}
// Deallocate the last remaining block.
if !block.is_null() {
drop(Box::from_raw(block));
}
}
}
}

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use std::sync::atomic::{AtomicUsize, Ordering};
use concurrent_queue::{ConcurrentQueue, PopError, PushError};
use easy_parallel::Parallel;
#[test]
fn smoke() {
let q = ConcurrentQueue::bounded(1);
q.push(7).unwrap();
assert_eq!(q.pop(), Ok(7));
q.push(8).unwrap();
assert_eq!(q.pop(), Ok(8));
assert!(q.pop().is_err());
}
#[test]
fn capacity() {
for i in 1..10 {
let q = ConcurrentQueue::<i32>::bounded(i);
assert_eq!(q.capacity(), Some(i));
}
}
#[test]
#[should_panic(expected = "capacity must be positive")]
fn zero_capacity() {
let _ = ConcurrentQueue::<i32>::bounded(0);
}
#[test]
fn len_empty_full() {
let q = ConcurrentQueue::bounded(2);
assert_eq!(q.len(), 0);
assert_eq!(q.is_empty(), true);
assert_eq!(q.is_full(), false);
q.push(()).unwrap();
assert_eq!(q.len(), 1);
assert_eq!(q.is_empty(), false);
assert_eq!(q.is_full(), false);
q.push(()).unwrap();
assert_eq!(q.len(), 2);
assert_eq!(q.is_empty(), false);
assert_eq!(q.is_full(), true);
q.pop().unwrap();
assert_eq!(q.len(), 1);
assert_eq!(q.is_empty(), false);
assert_eq!(q.is_full(), false);
}
#[test]
fn len() {
const COUNT: usize = 25_000;
const CAP: usize = 1000;
let q = ConcurrentQueue::bounded(CAP);
assert_eq!(q.len(), 0);
for _ in 0..CAP / 10 {
for i in 0..50 {
q.push(i).unwrap();
assert_eq!(q.len(), i + 1);
}
for i in 0..50 {
q.pop().unwrap();
assert_eq!(q.len(), 50 - i - 1);
}
}
assert_eq!(q.len(), 0);
for i in 0..CAP {
q.push(i).unwrap();
assert_eq!(q.len(), i + 1);
}
for _ in 0..CAP {
q.pop().unwrap();
}
assert_eq!(q.len(), 0);
Parallel::new()
.add(|| {
for i in 0..COUNT {
loop {
if let Ok(x) = q.pop() {
assert_eq!(x, i);
break;
}
}
let len = q.len();
assert!(len <= CAP);
}
})
.add(|| {
for i in 0..COUNT {
while q.push(i).is_err() {}
let len = q.len();
assert!(len <= CAP);
}
})
.run();
assert_eq!(q.len(), 0);
}
#[test]
fn close() {
let q = ConcurrentQueue::bounded(1);
assert_eq!(q.push(10), Ok(()));
assert!(!q.is_closed());
assert!(q.close());
assert!(q.is_closed());
assert!(!q.close());
assert_eq!(q.push(20), Err(PushError::Closed(20)));
assert_eq!(q.pop(), Ok(10));
assert_eq!(q.pop(), Err(PopError::Closed));
}
#[test]
fn spsc() {
const COUNT: usize = 100_000;
let q = ConcurrentQueue::bounded(3);
Parallel::new()
.add(|| {
for i in 0..COUNT {
loop {
if let Ok(x) = q.pop() {
assert_eq!(x, i);
break;
}
}
}
assert!(q.pop().is_err());
})
.add(|| {
for i in 0..COUNT {
while q.push(i).is_err() {}
}
})
.run();
}
#[test]
fn mpmc() {
const COUNT: usize = 25_000;
const THREADS: usize = 4;
let q = ConcurrentQueue::<usize>::bounded(3);
let v = (0..COUNT).map(|_| AtomicUsize::new(0)).collect::<Vec<_>>();
Parallel::new()
.each(0..THREADS, |_| {
for _ in 0..COUNT {
let n = loop {
if let Ok(x) = q.pop() {
break x;
}
};
v[n].fetch_add(1, Ordering::SeqCst);
}
})
.each(0..THREADS, |_| {
for i in 0..COUNT {
while q.push(i).is_err() {}
}
})
.run();
for c in v {
assert_eq!(c.load(Ordering::SeqCst), THREADS);
}
}
#[test]
fn drops() {
const RUNS: usize = 100;
static DROPS: AtomicUsize = AtomicUsize::new(0);
#[derive(Debug, PartialEq)]
struct DropCounter;
impl Drop for DropCounter {
fn drop(&mut self) {
DROPS.fetch_add(1, Ordering::SeqCst);
}
}
for _ in 0..RUNS {
let steps = fastrand::usize(..10_000);
let additional = fastrand::usize(..50);
DROPS.store(0, Ordering::SeqCst);
let q = ConcurrentQueue::bounded(50);
Parallel::new()
.add(|| {
for _ in 0..steps {
while q.pop().is_err() {}
}
})
.add(|| {
for _ in 0..steps {
while q.push(DropCounter).is_err() {
DROPS.fetch_sub(1, Ordering::SeqCst);
}
}
})
.run();
for _ in 0..additional {
q.push(DropCounter).unwrap();
}
assert_eq!(DROPS.load(Ordering::SeqCst), steps);
drop(q);
assert_eq!(DROPS.load(Ordering::SeqCst), steps + additional);
}
}
#[test]
fn linearizable() {
const COUNT: usize = 25_000;
const THREADS: usize = 4;
let q = ConcurrentQueue::bounded(THREADS);
Parallel::new()
.each(0..THREADS, |_| {
for _ in 0..COUNT {
while q.push(0).is_err() {}
q.pop().unwrap();
}
})
.run();
}

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tests/unbounded.rs Normal file
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use std::sync::atomic::{AtomicUsize, Ordering};
use concurrent_queue::{ConcurrentQueue, PopError, PushError};
use easy_parallel::Parallel;
#[test]
fn smoke() {
let q = ConcurrentQueue::unbounded();
q.push(7).unwrap();
assert_eq!(q.pop(), Ok(7));
q.push(8).unwrap();
assert_eq!(q.pop(), Ok(8));
assert!(q.pop().is_err());
}
#[test]
fn len_empty_full() {
let q = ConcurrentQueue::unbounded();
assert_eq!(q.len(), 0);
assert_eq!(q.is_empty(), true);
q.push(()).unwrap();
assert_eq!(q.len(), 1);
assert_eq!(q.is_empty(), false);
q.pop().unwrap();
assert_eq!(q.len(), 0);
assert_eq!(q.is_empty(), true);
}
#[test]
fn len() {
let q = ConcurrentQueue::unbounded();
assert_eq!(q.len(), 0);
for i in 0..50 {
q.push(i).unwrap();
assert_eq!(q.len(), i + 1);
}
for i in 0..50 {
q.pop().unwrap();
assert_eq!(q.len(), 50 - i - 1);
}
assert_eq!(q.len(), 0);
}
#[test]
fn close() {
let q = ConcurrentQueue::unbounded();
assert_eq!(q.push(10), Ok(()));
assert!(!q.is_closed());
assert!(q.close());
assert!(q.is_closed());
assert!(!q.close());
assert_eq!(q.push(20), Err(PushError::Closed(20)));
assert_eq!(q.pop(), Ok(10));
assert_eq!(q.pop(), Err(PopError::Closed));
}
#[test]
fn spsc() {
const COUNT: usize = 100_000;
let q = ConcurrentQueue::unbounded();
Parallel::new()
.add(|| {
for i in 0..COUNT {
loop {
if let Ok(x) = q.pop() {
assert_eq!(x, i);
break;
}
}
}
assert!(q.pop().is_err());
})
.add(|| {
for i in 0..COUNT {
q.push(i).unwrap();
}
})
.run();
}
#[test]
fn mpmc() {
const COUNT: usize = 25_000;
const THREADS: usize = 4;
let q = ConcurrentQueue::<usize>::unbounded();
let v = (0..COUNT).map(|_| AtomicUsize::new(0)).collect::<Vec<_>>();
Parallel::new()
.each(0..THREADS, |_| {
for _ in 0..COUNT {
let n = loop {
if let Ok(x) = q.pop() {
break x;
}
};
v[n].fetch_add(1, Ordering::SeqCst);
}
})
.each(0..THREADS, |_| {
for i in 0..COUNT {
q.push(i).unwrap();
}
})
.run();
for c in v {
assert_eq!(c.load(Ordering::SeqCst), THREADS);
}
}
#[test]
fn drops() {
static DROPS: AtomicUsize = AtomicUsize::new(0);
#[derive(Debug, PartialEq)]
struct DropCounter;
impl Drop for DropCounter {
fn drop(&mut self) {
DROPS.fetch_add(1, Ordering::SeqCst);
}
}
for _ in 0..100 {
let steps = fastrand::usize(0..10_000);
let additional = fastrand::usize(0..1000);
DROPS.store(0, Ordering::SeqCst);
let q = ConcurrentQueue::unbounded();
Parallel::new()
.add(|| {
for _ in 0..steps {
while q.pop().is_err() {}
}
})
.add(|| {
for _ in 0..steps {
q.push(DropCounter).unwrap();
}
})
.run();
for _ in 0..additional {
q.push(DropCounter).unwrap();
}
assert_eq!(DROPS.load(Ordering::SeqCst), steps);
drop(q);
assert_eq!(DROPS.load(Ordering::SeqCst), steps + additional);
}
}