Struct arc_swap::ArcSwapAny

pub struct ArcSwapAny<T: RefCnt, S: LockStorage = Global> { /* private fields */ }

An atomic storage for a 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. But to general access to the data, lease may be more appropriate.

Note

This is the generic low-level 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 and ArcSwapOption 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<_>>. But third party could provide implementations of the RefCnt trait and plug in others.
• S: This describes where the generation lock is stored and how it works.

Examples

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());

Implementations

impl<T: RefCnt, S: LockStorage> ArcSwapAny<T, S>

pub fn new(val: T) -> Self

Constructs a new value.

pub fn into_inner(self) -> T

Extracts the value inside.

pub fn load(&self) -> T

Loads the value.

This makes another copy (reference) 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 lease.

Signal safety

The method is not async-signal-safe. Use peek_signal_safe for that.

pub fn peek(&self) -> Guard<'_, T, S>

Provides a peek inside the held value.

This returns a temporary borrow of the object currently held inside. This is slightly faster than load, but it is not suitable for holding onto for longer periods of time.

If you discover later on that you need to hold onto it for longer, you can Guard::upgrade it.

Warning

This currently prevents the pointer inside from being replaced. Any swap, store or rcu will busy-loop while waiting for the proxy object to be destroyed, even on unrelated objects. Therefore, this is suitable only for things like reading a (reasonably small) configuration value, but not for eg. computations on the held values.

If you want to do anything non-trivial, prefer lease.

If you are not sure what is better, benchmarking is recommended.

Signal safety

For an async-signal-safe version, use peek_signal_safe.

pub fn peek_signal_safe(&self) -> Guard<'_, T, S>

An async-signal-safe version of peek

This method uses only restricted set of primitives to be async-signal-safe, at a slight performance hit in a contended scenario (signals should be rare, so it shouldn’t be a problem in practice).

As the returned guard prevents the value inside to be dropped, the value can be used during the signal handler. Unless it is upgraded (which is not recommended in a signal handler), there’s also no way the signal handler would have to drop the pointed to value.

The same performance warning about writer methods of peek applies, so it is recommended not to spend too much time holding the returned guard.

pub fn lease(&self) -> Lease<T>

Provides a temporary borrow of the object inside.

This returns a proxy object allowing access to the thing held inside and it is usually as fast as peek. However, there’s only limited amount of possible cheap proxies in existence for each thread ‒ if more are created, it falls back to load 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 structures.

pub fn store(&self, val: T)

Replaces the value inside this instance.

Further loads will yield the new value. Uses swap internally.

pub fn swap(&self, new: T) -> T

Exchanges the value inside this instance.

While multiple swaps can run concurrently and won’t block each other, each one needs to wait for all the loads and peek Guards that have seen the old value to finish before returning. This is in a way similar to locking ‒ a living Guard can prevent this from finishing. However, unlike RwLock, a steady stream of readers will not block writers and if each guard is held only for a short period of time, writers will progress too.

However, it is also possible to cause a deadlock (eg. this is an example of broken code):

let shared = ArcSwap::from(Arc::new(42));
let guard = shared.peek();
// This will deadlock, because the guard is still active here and swap
// can't pull the value from under its feet.
shared.swap(Arc::new(0));

pub fn compare_and_swap<C: AsRaw<T::Base>>(
    &self,
    current: C,
    new: T
) -> Lease<T>

Swaps the stored Arc if it is equal to current.

If the current value of the ArcSwapAny is equal 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 (like Arc::ptr_eq).

In other words, if the caller „guesses“ the value of current correctly, it acts like swap, otherwise it acts like load (including the limitations).

The current can be specified as &Arc, Guard, &Lease or as a raw pointer.

pub fn rcu<R, F>(&self, f: F) -> T where
    F: FnMut(&Lease<T>) -> R,
    R: Into<T>, 

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 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 or 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.

extern crate arc_swap;
extern crate crossbeam_utils;

use std::sync::Arc;

use arc_swap::ArcSwap;
use crossbeam_utils::thread;

fn main() {
    let cnt = ArcSwap::from(Arc::new(0));
    thread::scope(|scope| {
        for _ in 0..10 {
            scope.spawn(|_| {
                let inner = cnt.load();
                // 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());
}

This will, but it can call the closure multiple times to do retries:

extern crate arc_swap;
extern crate crossbeam_utils;

use std::sync::Arc;

use arc_swap::ArcSwap;
use crossbeam_utils::thread;

fn main() {
    let cnt = ArcSwap::from(Arc::new(0));
    thread::scope(|scope| {
        for _ in 0..10 {
            scope.spawn(|_| cnt.rcu(|inner| **inner + 1));
        }
    }).unwrap();
    assert_eq!(10, *cnt.load());
}

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:

extern crate arc_swap;
extern crate crossbeam_utils;
#[macro_use]
extern crate lazy_static;

use std::collections::HashMap;
use std::sync::Arc;

use arc_swap::ArcSwap;

fn expensive_computation(x: usize) -> usize {
    x * 2 // Let's pretend multiplication is really expensive
}

type Cache = HashMap<usize, usize>;

lazy_static! {
    static ref CACHE: ArcSwap<Cache> = ArcSwap::from(Arc::new(HashMap::new()));
}

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
}

fn main() {
    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 Arcs inside to share the unchanged values). Something like rpds or im might do what you need.

impl<T, S: LockStorage> ArcSwapAny<Arc<T>, S>

pub fn from_pointee(val: T) -> Self

A convenience constructor directly from the pointed-to value.

Direct equivalent for ArcSwap::new(Arc::new(val)).

pub fn rcu_unwrap<R, F>(&self, f: F) -> T where
    F: FnMut(&T) -> R,
    R: Into<Arc<T>>, 

An rcu which waits to be the sole owner of the original value and unwraps it.

This one works the same way as the rcu method, but works on the inner type instead of Arc. After replacing the original, it waits until there are no other owners of the arc and unwraps it.

Possible use case might be an RCU with a structure that is rather slow to drop ‒ if it was left to random reader (the last one to hold the old value), it could cause a timeout or jitter in a query time. With this, the deallocation is done in the updater thread, therefore outside of the hot path.

Warning

Note that if you store a copy of the Arc somewhere except the ArcSwap itself for extended period of time, this’ll busy-wait the whole time. Unless you need the assurance the Arc is deconstructed here, prefer rcu.

impl<T, S: LockStorage> ArcSwapAny<Option<Arc<T>>, S>

pub fn from_pointee<V: Into<Option<T>>>(val: V) -> Self

A convenience constructor directly from a pointed-to value.

This just allocates the Arc under the hood.

Examples

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().unwrap());

pub fn empty() -> Self

A convenience constructor for an empty value.

This is equivalent to ArcSwapOption::new(None).

Trait Implementations

impl<T: RefCnt, S: LockStorage> Clone for ArcSwapAny<T, S>

fn clone(&self) -> Self

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T, S: LockStorage> Debug for ArcSwapAny<T, S> where
    T: RefCnt,
    T::Base: Debug, 

fn fmt(&self, formatter: &mut Formatter<'_>) -> FmtResult

Formats the value using the given formatter.

impl<T: RefCnt + Default, S: LockStorage> Default for ArcSwapAny<T, S>

fn default() -> Self

Returns the “default value” for a type.

impl<T, S: LockStorage> Display for ArcSwapAny<T, S> where
    T: NonNull,
    T::Base: Display, 

fn fmt(&self, formatter: &mut Formatter<'_>) -> FmtResult

Formats the value using the given formatter.

impl<T: RefCnt, S: LockStorage> Drop for ArcSwapAny<T, S>

fn drop(&mut self)

Executes the destructor for this type.

impl<T: RefCnt, S: LockStorage> From<T> for ArcSwapAny<T, S>

fn from(val: T) -> Self

Performs the conversion.