# Keep Calm (and call Clone)
[](https://docs.rs/keepcalm)
[](https://crates.io/crates/keepcalm)
Simple shared types for multi-threaded Rust programs: `keepcalm` gives you permission to simplify your synchronization code in concurrent Rust applications.
Name inspired by @luser's [Keep Calm and Call Clone](https://github.com/luser/keep-calm-and-call-clone).
## Overview
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.
* Read and write guards are `Send` thanks to the `parking_lot` crate.
* Each synchronization primitive transparently manages the poisoned state (if code `panic!`s while the lock is being held). If you don't want to
poison on `panic!`, constructors are available to disable this option entirely.
* `static` Globally-scoped containers for both `Sync` and `!Sync` objects are easily constructed using [`SharedGlobal`], and can provide [`Shared`]
containers. Mutable global containers can similarly be constructed with [`SharedGlobalMut`]. ***NOTE**: This requires the `--feature global_experimental` flag*
## Container types
The following container types are available:
| [`SharedMut::new`] | `Arc<RwLock<T>>` | This is the default shared-mutable type.
| [`SharedMut::new_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::new_rcu`] | `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::new`] | `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::new_mutex`] | `Arc<Mutex<T>>` | For types that are not `Sync`, a `Mutex` is used to serialize read-only access.
| [`SharedMut::shared`] | n/a | This provides a read-only view into a read-write container and has no direct equivalent.
The following global container types are available:
| [`SharedGlobal::new`] | `static T` | This is a global `const`-style object, for types that are `Send` + `Sync`.
| [`SharedGlobal::new_lazy`] | `static Lazy<T>` | This is a lazily-initialized global `const`-style object, for types that are `Send` + `Sync`.
| [`SharedGlobal::new_mutex`] | `static Mutex<T>` | This is a global `const`-style object, for types that are `Send` but not necessarily `Sync`
| [`SharedGlobalMut::new`] | `static RwLock<T>` | This is a global mutable object, for types that are `Send` + `Sync`.
| [`SharedGlobalMut::new_lazy`] | `static Lazy<RwLock<T>>` | This is a lazily-initialized global mutable object, for types that are `Send` + `Sync`.
| [`SharedGlobalMut::new_mutex`] | `static Mutex<T>` | This is a global mutable object, for types that are `Send` but not necessarily `Sync`.
## Basic syntax
The traditional Rust shared object patterns tend to be somewhat verbose and repetitive, for example:
```rust
# 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"));
```
If we want to switch our shared fields from [`std::sync::Mutex`] to [`std::sync::RwLock`], we need to change four lines just for types, and
switch the `lock` method for a `read` method.
We can increase flexibility, and reduce some of the ceremony and verbosity with `keepcalm`:
```rust
# 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());
```
If we want to use a `Mutex` instead of the default `RwLock` that [`SharedMut`] uses under the hood, we only need to change [`SharedMut::new`] to
[`SharedMut::new_mutex`]!
## SharedMut
The [`SharedMut`] object hides the complexity of managing `Arc<Mutex<T>>`, `Arc<RwLock<T>>`, and other synchronization types
behind a single interface:
```rust
# 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:
```rust
# use keepcalm::*;
fn use_shared(shared: SharedMut<String>) {
shared.read();
}
let shared = SharedMut::new("123".to_string());
use_shared(shared);
let shared = SharedMut::new_mutex("123".to_string());
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:
```rust
# use keepcalm::*;
let shared = SharedMut::new_with_policy("123".to_string(), PoisonPolicy::Ignore);
```
## Shared
The default [`Shared`] object is similar to Rust's [`std::sync::Arc`], but adds the ability to project. [`Shared`] objects may also be
constructed as a `Mutex`, or may be a read-only view into a [`SharedMut`].
Note that because of this flexibility, the [`Shared`] object is slightly more complex than a traditional [`std::sync::Arc`], as all accesses
must be performed through the [`Shared::read`] accessor.
## Globals
***NOTE**: This requires the `--feature global_experimental` flag*
While `static` globals may often be an anti-pattern in Rust, this library also offers easily-to-use alternatives that are compatible with
the [`Shared`] and [`SharedMut`] types.
Global [`Shared`] references can be created using [`SharedGlobal`]:
```rust
# use keepcalm::*;
# #[cfg(feature="global_experimental")]
static GLOBAL: SharedGlobal<usize> = SharedGlobal::new(1);
# #[cfg(feature="global_experimental")]
fn use_global() {
let shared: Shared<usize> = GLOBAL.shared();
assert_eq!(shared.read(), 1);
}
```
Similarly, global [`SharedMut`] references can be created using [`SharedGlobalMut`]:
```rust
# use keepcalm::*;
# #[cfg(feature="global_experimental")]
static GLOBAL: SharedGlobalMut<usize> = SharedGlobalMut::new(1);
# #[cfg(feature="global_experimental")]
fn use_global() {
let shared: SharedMut<usize> = GLOBAL.shared_mut();
*shared.write() = 12;
assert_eq!(shared.read(), 12);
}
```
Both [`SharedGlobal`] and [`SharedGlobalMut`] offer a `new_lazy` constructor that allows initialization to be deferred to first
access:
```rust
# use keepcalm::*;
# use std::collections::HashMap;
# #[cfg(feature="global_experimental")]
static GLOBAL_LAZY: SharedGlobalMut<HashMap<&str, usize>> =
SharedGlobalMut::new_lazy(|| HashMap::from_iter([("a", 1), ("b", 2)]));
```
## 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. The [`project!`] and [`project_cast!`] macros can simplify
this code.
Note that projections are always linked to the root object! If a projection is locked, the root object is locked.
Casting:
```rust
# 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:
```rust
# 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:
```rust
# 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:
```rust
# use keepcalm::*;
let boxed: Box<[i32]> = Box::new([1, 2, 3]);
let shared: SharedMut<[i32]> = SharedMut::from_box(boxed);
```