Keep Calm (and call Clone)

Simple shared types for multi-threaded Rust programs.

This library simplifies a number of shared-object patterns that are used in multi-threaded programs such as web-servers.

Advantages of keepcalm:

You don't need to decide on your synchronization primitives up-front. Everything is a [Shared] or [SharedMut], no matter whether it's a mutex, read/write lock, read/copy/update primitive, or a read-only shared [std::sync::Arc].
Everything is [project!]able, which means you can adjust the granularity of your locks at any time without having to refactor the whole system. If you want finer-grained locks at a later date, the code that uses the shared containers doesn't change!
Writeable containers can be turned into read-only containers, while still retaining the ability for other code to update the contents.

Container types

The following container types are available:

Container	Equivalent	Notes
SharedMut (RwLock)	`Arc<RwLock<T>>`	This is the default shared-mutable type.
SharedMut (Mutex)	`Arc<Mutex<T>>`	In some cases it may be necessary to serialize both read and writes. For example, with types that are not `Sync`.
SharedMut (read/copy/update)	`Arc<RwLock<Arc<T>`	When the write lock of an RCU container is dropped, the values written are committed to the value in the container.
Shared (default)	`Arc`	This is the default shared-immutable type. Note that this is slightly more verbose: [`Shared`] does not [`std::ops::Deref`] to the underlying type and requires calling [`Shared::read`].
Shared ([`Shared::new_unsync`])	`Arc<Mutex<T>>`	For types that are not `Sync`, a `Mutex` is used to serialize read-only access.
Shared ([`SharedMut::shared`])	n/a	This provides a read-only view into a read-write container and has no direct equivalent.

Basic syntax

The traditional Rust shared object patterns tend to be somewhat verbose, for example:

# use std::sync::{Arc, Mutex};
# fn use_string(s: &str) {}
struct Foo {
    my_string: Arc<Mutex<String>>,
    my_integer: Arc<Mutex<u16>>,
}
let foo = Foo {
    my_string: Arc::new(Mutex::new("123".to_string())),
    my_integer: Arc::new(Mutex::new(1)),
};
use_string(&*foo.my_string.lock().expect("Mutex was poisoned"));

We can reduce a some of the ceremony and verbosity with keepcalm:

# use keepcalm::*;
# fn use_string(s: &str) {}
struct Foo {
    my_string: SharedMut<String>,
    my_integer: SharedMut<u16>,
}
let foo = Foo {
    my_string: SharedMut::new("123".to_string()),
    my_integer: SharedMut::new(1),
};
use_string(&*foo.my_string.read());

SharedMut

The [SharedMut] object hides the complexity of managing Arc<Mutex<T>> or Arc<RwLock<T>> behind a single interface:

# use keepcalm::*;
let object = "123".to_string();
let shared = SharedMut::new(object);
shared.read();

By default, a [SharedMut] object uses Arc<RwLock<T>> under the hood, but you can choose the synchronization primitive at construction time. The [SharedMut] object erases the underlying primitive and you can use them interchangeably:

# use keepcalm::*;
fn use_shared(shared: SharedMut<String>) {
    shared.read();
}

let shared = SharedMut::new("123".to_string());
use_shared(shared);
let shared = SharedMut::new_with_type("123".to_string(), Implementation::Mutex);
use_shared(shared);

Managing the poison state of synchronization primitives can be challenging as well. Rust will poison a Mutex or RwLock if you hold a lock while a panic! occurs.

The SharedMut type allows you to specify a [PoisonPolicy] at construction time. By default, if a synchronization primitive is poisoned, the SharedMut will panic! on access. This can be configured so that poisoning is ignored:

# use keepcalm::*;
let shared = SharedMut::new_with_policy("123".to_string(), PoisonPolicy::Ignore);

Shared

The [Shared] object is similar to Rust's [std::sync::Arc], but adds the ability to project.

Projection

Both [Shared] and [SharedMut] allow projection into the underlying type. Projection can be used to select either a subset of a type, or to cast a type to a trait.

Note that projections are always linked to the root object!

Casting:

# use keepcalm::*;
let shared = SharedMut::new("123".to_string());
let shared_asref: SharedMut<dyn AsRef<str>> = shared.project(project_cast!(x: String => dyn AsRef<str>));

Subset of a struct/tuple:

# use keepcalm::*;
#[derive(Default)]
struct Foo {
    tuple: (String, usize)
}

let shared = SharedMut::new(Foo::default());
let shared_string: SharedMut<String> = shared.project(project!(x: Foo, x.tuple.0));

*shared_string.write() += "hello, world";
assert_eq!(shared.read().tuple.0, "hello, world");
assert_eq!(*shared_string.read(), "hello, world");

Unsized types

Both [Shared] and [SharedMut] support unsized types, but due to current limitations in the language (see [std::ops::CoerceUnsized] for details), you need to construct them in special ways.

Unsized traits are supported, but you will either need to specify Send + Sync in the shared type, or [project_cast!] the object:

# use keepcalm::*;

// In this form, `Send + Sync` are visible in the shared type
let boxed: Box<dyn AsRef<str> + Send + Sync> = Box::new("123".to_string());
let shared: SharedMut<dyn AsRef<str> + Send + Sync> = SharedMut::from_box(boxed);

// In this form, `Send + Sync` are erased via projection
let shared = SharedMut::new("123".to_string());
let shared_asref: SharedMut<dyn AsRef<str>> = shared.project(project_cast!(x: String => dyn AsRef<str>));

Unsized slices are supported using a box:

# use keepcalm::*;
let boxed: Box<[i32]> = Box::new([1, 2, 3]);
let shared: SharedMut<[i32]> = SharedMut::from_box(boxed);