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lib.rs
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#![doc(test(attr(deny(warnings))))]
#![warn(missing_docs)]
#![cfg_attr(docsrs, feature(doc_cfg))]
#![allow(deprecated)]
//! Making [`Arc`][Arc] itself atomic
//!
//! The [`ArcSwap`] type is a container for an `Arc` that can be changed atomically. Semantically,
//! it is similar to something like `Atomic<Arc<T>>` (if there was such a thing) or
//! `RwLock<Arc<T>>` (but without the need for the locking). It is optimized for read-mostly
//! scenarios, with consistent performance characteristics.
//!
//! # Motivation
//!
//! There are many situations in which one might want to have some data structure that is often
//! read and seldom updated. Some examples might be a configuration of a service, routing tables,
//! snapshot of some data that is renewed every few minutes, etc.
//!
//! In all these cases one needs:
//! * Being able to read the current value of the data structure, fast, often and concurrently from
//! many threads.
//! * Using the same version of the data structure over longer period of time ‒ a query should be
//! answered by a consistent version of data, a packet should be routed either by an old or by a
//! new version of the routing table but not by a combination, etc.
//! * Perform an update without disrupting the processing.
//!
//! The first idea would be to use [`RwLock<T>`][RwLock] and keep a read-lock for the whole time of
//! processing. Update would, however, pause all processing until done.
//!
//! Better option would be to have [`RwLock<Arc<T>>`][RwLock]. Then one would lock, clone the [Arc]
//! and unlock. This suffers from CPU-level contention (on the lock and on the reference count of
//! the [Arc]) which makes it relatively slow. Depending on the implementation, an update may be
//! blocked for arbitrary long time by a steady inflow of readers.
//!
//! ```rust
//! # use std::sync::{Arc, RwLock};
//! # use once_cell::sync::Lazy;
//! # struct RoutingTable; struct Packet; impl RoutingTable { fn route(&self, _: Packet) {} }
//! static ROUTING_TABLE: Lazy<RwLock<Arc<RoutingTable>>> = Lazy::new(|| {
//! RwLock::new(Arc::new(RoutingTable))
//! });
//!
//! fn process_packet(packet: Packet) {
//! let table = Arc::clone(&ROUTING_TABLE.read().unwrap());
//! table.route(packet);
//! }
//! # fn main() { process_packet(Packet); }
//! ```
//!
//! The [ArcSwap] can be used instead, which solves the above problems and has better performance
//! characteristics than the [RwLock], both in contended and non-contended scenarios.
//!
//! ```rust
//! # use arc_swap::ArcSwap;
//! # use once_cell::sync::Lazy;
//! # struct RoutingTable; struct Packet; impl RoutingTable { fn route(&self, _: Packet) {} }
//! static ROUTING_TABLE: Lazy<ArcSwap<RoutingTable>> = Lazy::new(|| {
//! ArcSwap::from_pointee(RoutingTable)
//! });
//!
//! fn process_packet(packet: Packet) {
//! let table = ROUTING_TABLE.load();
//! table.route(packet);
//! }
//! # fn main() { process_packet(Packet); }
//! ```
//!
//! # Crate contents
//!
//! At the heart of the crate there are [`ArcSwap`] and [`ArcSwapOption`] types, containers for an
//! [`Arc`] and [`Option<Arc>`][Option].
//!
//! Technically, these are type aliases for partial instantiations of the [`ArcSwapAny`] type. The
//! [`ArcSwapAny`] is more flexible and allows tweaking of many things (can store other things than
//! [`Arc`]s, can configure the locking [`Strategy`]). For details about the tweaking, see the
//! documentation of the [`strategy`] module and the [`RefCnt`] trait.
//!
//! The [`cache`] module provides means for speeding up read access of the contained data at the
//! cost of delayed reclamation.
//!
//! The [`access`] module can be used to do projections into the contained data to separate parts
//! of application from each other (eg. giving a component access to only its own part of
//! configuration while still having it reloaded as a whole).
//!
//! # Before using
//!
//! The data structure is a bit niche. Before using, please check the
//! [limitations and common pitfalls][docs::limitations] and the [performance
//! characteristics][docs::performance], including choosing the right [read
//! operation][docs::performance#read-operations].
//!
//! You can also get an inspiration about what's possible in the [common patterns][docs::patterns]
//! section.
//!
//! # Examples
//!
//! ```rust
//! use std::sync::Arc;
//!
//! use arc_swap::ArcSwap;
//! use crossbeam_utils::thread;
//!
//! fn main() {
//! let config = ArcSwap::from(Arc::new(String::default()));
//! thread::scope(|scope| {
//! scope.spawn(|_| {
//! let new_conf = Arc::new("New configuration".to_owned());
//! config.store(new_conf);
//! });
//! for _ in 0..10 {
//! scope.spawn(|_| {
//! loop {
//! let cfg = config.load();
//! if !cfg.is_empty() {
//! assert_eq!(**cfg, "New configuration");
//! return;
//! }
//! }
//! });
//! }
//! }).unwrap();
//! }
//! ```
//!
//! [RwLock]: https://doc.rust-lang.org/std/sync/struct.RwLock.html
pub mod access;
mod as_raw;
pub mod cache;
mod compile_fail_tests;
mod debt;
pub mod docs;
mod ref_cnt;
#[cfg(feature = "serde")]
mod serde;
pub mod strategy;
#[cfg(feature = "weak")]
mod weak;
use std::borrow::Borrow;
use std::fmt::{Debug, Display, Formatter, Result as FmtResult};
use std::marker::PhantomData;
use std::mem;
use std::ops::Deref;
use std::ptr;
use std::sync::atomic::{AtomicPtr, Ordering};
use std::sync::Arc;
use crate::access::{Access, Map};
pub use crate::as_raw::AsRaw;
pub use crate::cache::Cache;
pub use crate::ref_cnt::RefCnt;
use crate::strategy::hybrid::{DefaultConfig, HybridStrategy};
use crate::strategy::sealed::Protected;
use crate::strategy::{CaS, Strategy};
pub use crate::strategy::{DefaultStrategy, IndependentStrategy};
/// A temporary storage of the pointer.
///
/// This guard object is returned from most loading methods (with the notable exception of
/// [`load_full`](struct.ArcSwapAny.html#method.load_full)). It dereferences to the smart pointer
/// loaded, so most operations are to be done using that.
pub struct Guard<T: RefCnt, S: Strategy<T> = DefaultStrategy> {
inner: S::Protected,
}
impl<T: RefCnt, S: Strategy<T>> Guard<T, S> {
/// Converts it into the held value.
///
/// This, on occasion, may be a tiny bit faster than cloning the Arc or whatever is being held
/// inside.
// Associated function on purpose, because of deref
#[allow(clippy::wrong_self_convention)]
#[inline]
pub fn into_inner(lease: Self) -> T {
lease.inner.into_inner()
}
/// Create a guard for a given value `inner`.
///
/// This can be useful on occasion to pass a specific object to code that expects or
/// wants to store a Guard.
///
/// # Example
///
/// ```rust
/// # use arc_swap::{ArcSwap, DefaultStrategy, Guard};
/// # use std::sync::Arc;
/// # let p = ArcSwap::from_pointee(42);
/// // Create two guards pointing to the same object
/// let g1 = p.load();
/// let g2 = Guard::<_, DefaultStrategy>::from_inner(Arc::clone(&*g1));
/// # drop(g2);
/// ```
pub fn from_inner(inner: T) -> Self {
Guard {
inner: S::Protected::from_inner(inner),
}
}
}
impl<T: RefCnt, S: Strategy<T>> Deref for Guard<T, S> {
type Target = T;
#[inline]
fn deref(&self) -> &T {
self.inner.borrow()
}
}
impl<T: RefCnt, S: Strategy<T>> From<T> for Guard<T, S> {
fn from(inner: T) -> Self {
Self::from_inner(inner)
}
}
impl<T: Default + RefCnt, S: Strategy<T>> Default for Guard<T, S> {
fn default() -> Self {
Self::from(T::default())
}
}
impl<T: Debug + RefCnt, S: Strategy<T>> Debug for Guard<T, S> {
fn fmt(&self, formatter: &mut Formatter) -> FmtResult {
self.deref().fmt(formatter)
}
}
impl<T: Display + RefCnt, S: Strategy<T>> Display for Guard<T, S> {
fn fmt(&self, formatter: &mut Formatter) -> FmtResult {
self.deref().fmt(formatter)
}
}
/// Comparison of two pointer-like things.
// A and B are likely to *be* references, or thin wrappers around that. Calling that with extra
// reference is just annoying.
#[allow(clippy::needless_pass_by_value)]
fn ptr_eq<Base, A, B>(a: A, b: B) -> bool
where
A: AsRaw<Base>,
B: AsRaw<Base>,
{
let a = a.as_raw();
let b = b.as_raw();
ptr::eq(a, b)
}
/// An atomic storage for a reference counted smart pointer like [`Arc`] or `Option<Arc>`.
///
/// This is a storage where a smart pointer may live. It can be read and written atomically from
/// several threads, but doesn't act like a pointer itself.
///
/// One can be created [`from`] an [`Arc`]. To get the pointer back, use the
/// [`load`](#method.load).
///
/// # Note
///
/// This is the common generic implementation. This allows sharing the same code for storing
/// both `Arc` and `Option<Arc>` (and possibly other similar types).
///
/// In your code, you most probably want to interact with it through the
/// [`ArcSwap`](type.ArcSwap.html) and [`ArcSwapOption`](type.ArcSwapOption.html) aliases. However,
/// the methods they share are described here and are applicable to both of them. That's why the
/// examples here use `ArcSwap` ‒ but they could as well be written with `ArcSwapOption` or
/// `ArcSwapAny`.
///
/// # Type parameters
///
/// * `T`: The smart pointer to be kept inside. This crate provides implementation for `Arc<_>` and
/// `Option<Arc<_>>` (`Rc` too, but that one is not practically useful). But third party could
/// provide implementations of the [`RefCnt`] trait and plug in others.
/// * `S`: Chooses the [strategy] used to protect the data inside. They come with various
/// performance trade offs, the default [`DefaultStrategy`] is good rule of thumb for most use
/// cases.
///
/// # Examples
///
/// ```rust
/// # use std::sync::Arc;
/// # use arc_swap::ArcSwap;
/// let arc = Arc::new(42);
/// let arc_swap = ArcSwap::from(arc);
/// assert_eq!(42, **arc_swap.load());
/// // It can be read multiple times
/// assert_eq!(42, **arc_swap.load());
///
/// // Put a new one in there
/// let new_arc = Arc::new(0);
/// assert_eq!(42, *arc_swap.swap(new_arc));
/// assert_eq!(0, **arc_swap.load());
/// ```
///
/// [`Arc`]: https://doc.rust-lang.org/std/sync/struct.Arc.html
/// [`from`]: https://doc.rust-lang.org/nightly/std/convert/trait.From.html#tymethod.from
/// [`RefCnt`]: trait.RefCnt.html
pub struct ArcSwapAny<T: RefCnt, S: Strategy<T> = DefaultStrategy> {
// Notes: AtomicPtr needs Sized
/// The actual pointer, extracted from the Arc.
ptr: AtomicPtr<T::Base>,
/// We are basically an Arc in disguise. Inherit parameters from Arc by pretending to contain
/// it.
_phantom_arc: PhantomData<T>,
/// Strategy to protect the data.
strategy: S,
}
impl<T: RefCnt, S: Default + Strategy<T>> From<T> for ArcSwapAny<T, S> {
fn from(val: T) -> Self {
Self::with_strategy(val, S::default())
}
}
impl<T: RefCnt, S: Strategy<T>> Drop for ArcSwapAny<T, S> {
fn drop(&mut self) {
let ptr = *self.ptr.get_mut();
unsafe {
// To pay any possible debts
self.strategy.wait_for_readers(ptr, &self.ptr);
// We are getting rid of the one stored ref count
T::dec(ptr);
}
}
}
impl<T, S: Strategy<T>> Debug for ArcSwapAny<T, S>
where
T: Debug + RefCnt,
{
fn fmt(&self, formatter: &mut Formatter) -> FmtResult {
formatter
.debug_tuple("ArcSwapAny")
.field(&self.load())
.finish()
}
}
impl<T, S: Strategy<T>> Display for ArcSwapAny<T, S>
where
T: Display + RefCnt,
{
fn fmt(&self, formatter: &mut Formatter) -> FmtResult {
self.load().fmt(formatter)
}
}
impl<T: RefCnt + Default, S: Default + Strategy<T>> Default for ArcSwapAny<T, S> {
fn default() -> Self {
Self::new(T::default())
}
}
impl<T: RefCnt, S: Strategy<T>> ArcSwapAny<T, S> {
/// Constructs a new storage.
pub fn new(val: T) -> Self
where
S: Default,
{
Self::from(val)
}
/// Constructs a new storage while customizing the protection strategy.
pub fn with_strategy(val: T, strategy: S) -> Self {
// The AtomicPtr requires *mut in its interface. We are more like *const, so we cast it.
// However, we always go back to *const right away when we get the pointer on the other
// side, so it should be fine.
let ptr = T::into_ptr(val);
Self {
ptr: AtomicPtr::new(ptr),
_phantom_arc: PhantomData,
strategy,
}
}
/// Extracts the value inside.
pub fn into_inner(mut self) -> T {
let ptr = *self.ptr.get_mut();
// To pay all the debts
unsafe { self.strategy.wait_for_readers(ptr, &self.ptr) };
mem::forget(self);
unsafe { T::from_ptr(ptr) }
}
/// Loads the value.
///
/// This makes another copy of the held pointer and returns it, atomically (it is
/// safe even when other thread stores into the same instance at the same time).
///
/// The method is lock-free and wait-free, but usually more expensive than
/// [`load`](#method.load).
pub fn load_full(&self) -> T {
Guard::into_inner(self.load())
}
/// Provides a temporary borrow of the object inside.
///
/// This returns a proxy object allowing access to the thing held inside. However, there's
/// only limited amount of possible cheap proxies in existence for each thread ‒ if more are
/// created, it falls back to equivalent of [`load_full`](#method.load_full) internally.
///
/// This is therefore a good choice to use for eg. searching a data structure or juggling the
/// pointers around a bit, but not as something to store in larger amounts. The rule of thumb
/// is this is suited for local variables on stack, but not in long-living data structures.
///
/// # Consistency
///
/// In case multiple related operations are to be done on the loaded value, it is generally
/// recommended to call `load` just once and keep the result over calling it multiple times.
/// First, keeping it is usually faster. But more importantly, the value can change between the
/// calls to load, returning different objects, which could lead to logical inconsistency.
/// Keeping the result makes sure the same object is used.
///
/// ```rust
/// # use arc_swap::ArcSwap;
/// struct Point {
/// x: usize,
/// y: usize,
/// }
///
/// fn print_broken(p: &ArcSwap<Point>) {
/// // This is broken, because the x and y may come from different points,
/// // combining into an invalid point that never existed.
/// println!("X: {}", p.load().x);
/// // If someone changes the content now, between these two loads, we
/// // have a problem
/// println!("Y: {}", p.load().y);
/// }
///
/// fn print_correct(p: &ArcSwap<Point>) {
/// // Here we take a snapshot of one specific point so both x and y come
/// // from the same one.
/// let point = p.load();
/// println!("X: {}", point.x);
/// println!("Y: {}", point.y);
/// }
/// # let p = ArcSwap::from_pointee(Point { x: 10, y: 20 });
/// # print_correct(&p);
/// # print_broken(&p);
/// ```
#[inline]
pub fn load(&self) -> Guard<T, S> {
let protected = unsafe { self.strategy.load(&self.ptr) };
Guard { inner: protected }
}
/// Replaces the value inside this instance.
///
/// Further loads will yield the new value. Uses [`swap`](#method.swap) internally.
pub fn store(&self, val: T) {
drop(self.swap(val));
}
/// Exchanges the value inside this instance.
pub fn swap(&self, new: T) -> T {
let new = T::into_ptr(new);
// AcqRel needed to publish the target of the new pointer and get the target of the old
// one.
//
// SeqCst to synchronize the time lines with the group counters.
let old = self.ptr.swap(new, Ordering::SeqCst);
unsafe {
self.strategy.wait_for_readers(old, &self.ptr);
T::from_ptr(old)
}
}
/// Swaps the stored Arc if it equals to `current`.
///
/// If the current value of the `ArcSwapAny` equals to `current`, the `new` is stored inside.
/// If not, nothing happens.
///
/// The previous value (no matter if the swap happened or not) is returned. Therefore, if the
/// returned value is equal to `current`, the swap happened. You want to do a pointer-based
/// comparison to determine it.
///
/// In other words, if the caller „guesses“ the value of current correctly, it acts like
/// [`swap`](#method.swap), otherwise it acts like [`load_full`](#method.load_full) (including
/// the limitations).
///
/// The `current` can be specified as `&Arc`, [`Guard`](struct.Guard.html),
/// [`&Guards`](struct.Guards.html) or as a raw pointer (but _not_ owned `Arc`). See the
/// [`AsRaw`] trait.
pub fn compare_and_swap<C>(&self, current: C, new: T) -> Guard<T, S>
where
C: AsRaw<T::Base>,
S: CaS<T>,
{
let protected = unsafe { self.strategy.compare_and_swap(&self.ptr, current, new) };
Guard { inner: protected }
}
/// Read-Copy-Update of the pointer inside.
///
/// This is useful in read-heavy situations with several threads that sometimes update the data
/// pointed to. The readers can just repeatedly use [`load`](#method.load) without any locking.
/// The writer uses this method to perform the update.
///
/// In case there's only one thread that does updates or in case the next version is
/// independent of the previous one, simple [`swap`](#method.swap) or [`store`](#method.store)
/// is enough. Otherwise, it may be needed to retry the update operation if some other thread
/// made an update in between. This is what this method does.
///
/// # Examples
///
/// This will *not* work as expected, because between loading and storing, some other thread
/// might have updated the value.
///
/// ```rust
/// # use std::sync::Arc;
/// #
/// # use arc_swap::ArcSwap;
/// # use crossbeam_utils::thread;
/// #
/// let cnt = ArcSwap::from_pointee(0);
/// thread::scope(|scope| {
/// for _ in 0..10 {
/// scope.spawn(|_| {
/// let inner = cnt.load_full();
/// // Another thread might have stored some other number than what we have
/// // between the load and store.
/// cnt.store(Arc::new(*inner + 1));
/// });
/// }
/// }).unwrap();
/// // This will likely fail:
/// // assert_eq!(10, *cnt.load_full());
/// ```
///
/// This will, but it can call the closure multiple times to retry:
///
/// ```rust
/// # use arc_swap::ArcSwap;
/// # use crossbeam_utils::thread;
/// #
/// let cnt = ArcSwap::from_pointee(0);
/// thread::scope(|scope| {
/// for _ in 0..10 {
/// scope.spawn(|_| cnt.rcu(|inner| **inner + 1));
/// }
/// }).unwrap();
/// assert_eq!(10, *cnt.load_full());
/// ```
///
/// Due to the retries, you might want to perform all the expensive operations *before* the
/// rcu. As an example, if there's a cache of some computations as a map, and the map is cheap
/// to clone but the computations are not, you could do something like this:
///
/// ```rust
/// # use std::collections::HashMap;
/// #
/// # use arc_swap::ArcSwap;
/// # use once_cell::sync::Lazy;
/// #
/// fn expensive_computation(x: usize) -> usize {
/// x * 2 // Let's pretend multiplication is *really expensive expensive*
/// }
///
/// type Cache = HashMap<usize, usize>;
///
/// static CACHE: Lazy<ArcSwap<Cache>> = Lazy::new(|| ArcSwap::default());
///
/// fn cached_computation(x: usize) -> usize {
/// let cache = CACHE.load();
/// if let Some(result) = cache.get(&x) {
/// return *result;
/// }
/// // Not in cache. Compute and store.
/// // The expensive computation goes outside, so it is not retried.
/// let result = expensive_computation(x);
/// CACHE.rcu(|cache| {
/// // The cheaper clone of the cache can be retried if need be.
/// let mut cache = HashMap::clone(&cache);
/// cache.insert(x, result);
/// cache
/// });
/// result
/// }
///
/// assert_eq!(42, cached_computation(21));
/// assert_eq!(42, cached_computation(21));
/// ```
///
/// # The cost of cloning
///
/// Depending on the size of cache above, the cloning might not be as cheap. You can however
/// use persistent data structures ‒ each modification creates a new data structure, but it
/// shares most of the data with the old one (which is usually accomplished by using `Arc`s
/// inside to share the unchanged values). Something like
/// [`rpds`](https://crates.io/crates/rpds) or [`im`](https://crates.io/crates/im) might do
/// what you need.
pub fn rcu<R, F>(&self, mut f: F) -> T
where
F: FnMut(&T) -> R,
R: Into<T>,
S: CaS<T>,
{
let mut cur = self.load();
loop {
let new = f(&cur).into();
let prev = self.compare_and_swap(&*cur, new);
let swapped = ptr_eq(&*cur, &*prev);
if swapped {
return Guard::into_inner(prev);
} else {
cur = prev;
}
}
}
/// Provides an access to an up to date projection of the carried data.
///
/// # Motivation
///
/// Sometimes, an application consists of components. Each component has its own configuration
/// structure. The whole configuration contains all the smaller config parts.
///
/// For the sake of separation and abstraction, it is not desirable to pass the whole
/// configuration to each of the components. This allows the component to take only access to
/// its own part.
///
/// # Lifetimes & flexibility
///
/// This method is not the most flexible way, as the returned type borrows into the `ArcSwap`.
/// To provide access into eg. `Arc<ArcSwap<T>>`, you can create the [`Map`] type directly. See
/// the [`access`] module.
///
/// # Performance
///
/// As the provided function is called on each load from the shared storage, it should
/// generally be cheap. It is expected this will usually be just referencing of a field inside
/// the structure.
///
/// # Examples
///
/// ```rust
/// use std::sync::Arc;
///
/// use arc_swap::ArcSwap;
/// use arc_swap::access::Access;
///
/// struct Cfg {
/// value: usize,
/// }
///
/// fn print_many_times<V: Access<usize>>(value: V) {
/// for _ in 0..25 {
/// let value = value.load();
/// println!("{}", *value);
/// }
/// }
///
/// let shared = ArcSwap::from_pointee(Cfg { value: 0 });
/// let mapped = shared.map(|c: &Cfg| &c.value);
/// crossbeam_utils::thread::scope(|s| {
/// // Will print some zeroes and some twos
/// s.spawn(|_| print_many_times(mapped));
/// s.spawn(|_| shared.store(Arc::new(Cfg { value: 2 })));
/// }).expect("Something panicked in a thread");
/// ```
pub fn map<I, R, F>(&self, f: F) -> Map<&Self, I, F>
where
F: Fn(&I) -> &R + Clone,
Self: Access<I>,
{
Map::new(self, f)
}
}
/// An atomic storage for `Arc`.
///
/// This is a type alias only. Most of its methods are described on
/// [`ArcSwapAny`](struct.ArcSwapAny.html).
pub type ArcSwap<T> = ArcSwapAny<Arc<T>>;
impl<T, S: Strategy<Arc<T>>> ArcSwapAny<Arc<T>, S> {
/// A convenience constructor directly from the pointed-to value.
///
/// Direct equivalent for `ArcSwap::new(Arc::new(val))`.
pub fn from_pointee(val: T) -> Self
where
S: Default,
{
Self::from(Arc::new(val))
}
}
/// An atomic storage for `Option<Arc>`.
///
/// This is very similar to [`ArcSwap`](type.ArcSwap.html), but allows storing NULL values, which
/// is useful in some situations.
///
/// This is a type alias only. Most of the methods are described on
/// [`ArcSwapAny`](struct.ArcSwapAny.html). Even though the examples there often use `ArcSwap`,
/// they are applicable to `ArcSwapOption` with appropriate changes.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
/// use arc_swap::ArcSwapOption;
///
/// let shared = ArcSwapOption::from(None);
/// assert!(shared.load_full().is_none());
/// assert!(shared.swap(Some(Arc::new(42))).is_none());
/// assert_eq!(42, **shared.load_full().as_ref().unwrap());
/// ```
pub type ArcSwapOption<T> = ArcSwapAny<Option<Arc<T>>>;
impl<T, S: Strategy<Option<Arc<T>>>> ArcSwapAny<Option<Arc<T>>, S> {
/// A convenience constructor directly from a pointed-to value.
///
/// This just allocates the `Arc` under the hood.
///
/// # Examples
///
/// ```rust
/// use arc_swap::ArcSwapOption;
///
/// let empty: ArcSwapOption<usize> = ArcSwapOption::from_pointee(None);
/// assert!(empty.load().is_none());
/// let non_empty: ArcSwapOption<usize> = ArcSwapOption::from_pointee(42);
/// assert_eq!(42, **non_empty.load().as_ref().unwrap());
/// ```
pub fn from_pointee<V: Into<Option<T>>>(val: V) -> Self
where
S: Default,
{
Self::new(val.into().map(Arc::new))
}
/// A convenience constructor for an empty value.
///
/// This is equivalent to `ArcSwapOption::new(None)`.
pub fn empty() -> Self
where
S: Default,
{
Self::new(None)
}
}
impl<T> ArcSwapOption<T> {
/// A const-fn equivalent of [empty].
///
/// Just like [empty], this creates an `None`-holding `ArcSwapOption`. The [empty] is, however,
/// more general ‒ this is available only for the default strategy, while [empty] is for any
/// [Default]-constructible strategy (current or future one).
///
/// [empty]: ArcSwapAny::empty
///
/// # Examples
///
/// ```rust
/// # use std::sync::Arc;
/// # use arc_swap::ArcSwapOption;
/// static GLOBAL_DATA: ArcSwapOption<usize> = ArcSwapOption::const_empty();
///
/// assert!(GLOBAL_DATA.load().is_none());
/// GLOBAL_DATA.store(Some(Arc::new(42)));
/// assert_eq!(42, **GLOBAL_DATA.load().as_ref().unwrap());
/// ```
pub const fn const_empty() -> Self {
Self {
ptr: AtomicPtr::new(ptr::null_mut()),
_phantom_arc: PhantomData,
strategy: HybridStrategy {
_config: DefaultConfig,
},
}
}
}
/// An atomic storage that doesn't share the internal generation locks with others.
///
/// This makes it bigger and it also might suffer contention (on the HW level) if used from many
/// threads at once. On the other hand, it can't block writes in other instances.
///
/// See the [`IndependentStrategy`] for further details.
// Being phased out. Will deprecate once we verify in production that the new strategy works fine.
#[doc(hidden)]
pub type IndependentArcSwap<T> = ArcSwapAny<Arc<T>, IndependentStrategy>;
/// Arc swap for the [Weak] pointer.
///
/// This is similar to [ArcSwap], but it doesn't store [Arc], it stores [Weak]. It doesn't keep the
/// data alive when pointed to.
///
/// This is a type alias only. Most of the methods are described on the
/// [`ArcSwapAny`](struct.ArcSwapAny.html).
///
/// Needs the `weak` feature turned on.
///
/// [Weak]: std::sync::Weak
#[cfg(feature = "weak")]
pub type ArcSwapWeak<T> = ArcSwapAny<std::sync::Weak<T>>;
macro_rules! t {
($name: ident, $strategy: ty) => {
#[cfg(test)]
mod $name {
use std::panic;
use std::sync::atomic::{self, AtomicUsize};
use adaptive_barrier::{Barrier, PanicMode};
use crossbeam_utils::thread;
use super::*;
const ITERATIONS: usize = 10;
#[allow(deprecated)] // We use "deprecated" testing strategies in here.
type As<T> = ArcSwapAny<Arc<T>, $strategy>;
#[allow(deprecated)] // We use "deprecated" testing strategies in here.
type Aso<T> = ArcSwapAny<Option<Arc<T>>, $strategy>;
/// Similar to the one in doc tests of the lib, but more times and more intensive (we
/// want to torture it a bit).
#[test]
#[cfg_attr(miri, ignore)] // Takes like 1 or 2 infinities to run under miri
fn publish() {
const READERS: usize = 2;
for _ in 0..ITERATIONS {
let config = As::<String>::default();
let ended = AtomicUsize::new(0);
thread::scope(|scope| {
for _ in 0..READERS {
scope.spawn(|_| loop {
let cfg = config.load_full();
if !cfg.is_empty() {
assert_eq!(*cfg, "New configuration");
ended.fetch_add(1, Ordering::Relaxed);
return;
}
atomic::spin_loop_hint();
});
}
scope.spawn(|_| {
let new_conf = Arc::new("New configuration".to_owned());
config.store(new_conf);
});
})
.unwrap();
assert_eq!(READERS, ended.load(Ordering::Relaxed));
let arc = config.load_full();
assert_eq!(2, Arc::strong_count(&arc));
assert_eq!(0, Arc::weak_count(&arc));
}
}
/// Similar to the doc tests of ArcSwap, but happens more times.
#[test]
fn swap_load() {
for _ in 0..100 {
let arc = Arc::new(42);
let arc_swap = As::from(Arc::clone(&arc));
assert_eq!(42, **arc_swap.load());
// It can be read multiple times
assert_eq!(42, **arc_swap.load());
// Put a new one in there
let new_arc = Arc::new(0);
assert_eq!(42, *arc_swap.swap(Arc::clone(&new_arc)));
assert_eq!(0, **arc_swap.load());
// One loaded here, one in the arc_swap, one in new_arc
let loaded = arc_swap.load_full();
assert_eq!(3, Arc::strong_count(&loaded));
assert_eq!(0, Arc::weak_count(&loaded));
// The original got released from the arc_swap
assert_eq!(1, Arc::strong_count(&arc));
assert_eq!(0, Arc::weak_count(&arc));
}
}
/// Two different writers publish two series of values. The readers check that it is
/// always increasing in each serie.
///
/// For performance, we try to reuse the threads here.
#[test]
fn multi_writers() {
let first_value = Arc::new((0, 0));
let shared = As::from(Arc::clone(&first_value));
const WRITER_CNT: usize = 2;
const READER_CNT: usize = 3;
#[cfg(miri)]
const ITERATIONS: usize = 5;
#[cfg(not(miri))]
const ITERATIONS: usize = 100;
const SEQ: usize = 50;
let barrier = Barrier::new(PanicMode::Poison);
thread::scope(|scope| {
for w in 0..WRITER_CNT {
// We need to move w into the closure. But we want to just reference the
// other things.
let mut barrier = barrier.clone();
let shared = &shared;
let first_value = &first_value;
scope.spawn(move |_| {
for _ in 0..ITERATIONS {
barrier.wait();
shared.store(Arc::clone(&first_value));
barrier.wait();
for i in 0..SEQ {
shared.store(Arc::new((w, i + 1)));
}
}
});
}
for _ in 0..READER_CNT {
let mut barrier = barrier.clone();
let shared = &shared;
let first_value = &first_value;
scope.spawn(move |_| {
for _ in 0..ITERATIONS {
barrier.wait();
barrier.wait();
let mut previous = [0; WRITER_CNT];
let mut last = Arc::clone(&first_value);
loop {
let cur = shared.load();
if Arc::ptr_eq(&last, &cur) {
atomic::spin_loop_hint();
continue;
}
let (w, s) = **cur;
assert!(previous[w] < s, "{:?} vs {:?}", previous, cur);
previous[w] = s;
last = Guard::into_inner(cur);
if s == SEQ {
break;
}
}
}
});
}
drop(barrier);
})
.unwrap();
}
#[test]
fn load_null() {
let shared = Aso::<usize>::default();
let guard = shared.load();
assert!(guard.is_none());
shared.store(Some(Arc::new(42)));
assert_eq!(42, **shared.load().as_ref().unwrap());
}
#[test]
fn from_into() {
let a = Arc::new(42);
let shared = As::new(a);
let guard = shared.load();
let a = shared.into_inner();
assert_eq!(42, *a);
assert_eq!(2, Arc::strong_count(&a));
drop(guard);
assert_eq!(1, Arc::strong_count(&a));
}
// Note on the Relaxed order here. This should be enough, because there's that
// barrier.wait in between that should do the synchronization of happens-before for us.
// And using SeqCst would probably not help either, as there's nothing else with SeqCst
// here in this test to relate it to.
#[derive(Default)]
struct ReportDrop(Arc<AtomicUsize>);
impl Drop for ReportDrop {
fn drop(&mut self) {
self.0.fetch_add(1, Ordering::Relaxed);
}
}
/// Interaction of two threads about a guard and dropping it.
///
/// We make sure everything works in timely manner (eg. dropping of stuff) even if multiple
/// threads interact.
///
/// The idea is:
/// * Thread 1 loads a value.
/// * Thread 2 replaces the shared value. The original value is not destroyed.
/// * Thread 1 drops the guard. The value is destroyed and this is observable in both threads.
#[test]
fn guard_drop_in_thread() {
for _ in 0..ITERATIONS {
let cnt = Arc::new(AtomicUsize::new(0));
let shared = As::from_pointee(ReportDrop(cnt.clone()));
assert_eq!(cnt.load(Ordering::Relaxed), 0, "Dropped prematurely");
// We need the threads to wait for each other at places.
let sync = Barrier::new(PanicMode::Poison);
thread::scope(|scope| {
scope.spawn({
let sync = sync.clone();
|_| {
let mut sync = sync; // Move into the closure
let guard = shared.load();
sync.wait();