This crate provides the struct Prison, an arena data structure that allows simultaneous interior mutability to each and every element by providing .visit() methods that take closures that are passed mutable references to the values.

This documentation describes the usage of Prison, how its methods differ from those found on a [Vec], how to use its unusual .visit() methods, and how it achieves memory safety.

Quick Look

• Uses an underlying [Vec] to store items of the same type
• Acts primarily as a Generational Arena, where each element is accessed using a [CellKey] that differentiates two values that may have been located at the same index but represent fundamentally separate data
• Can also be indexed with a plain [usize] for simple use cases
• Provides safe (needs verification) interior mutability by only providing mutable references to values using closures to define strict scopes where they are valid and hide the setup/teardown for safety checks
• Uses [bool] locks on each element and a master [usize] counter to track the number/location of active references and prevent mutable reference aliasing and disallow scenarios that could invalidate existing references
• [CellKey] uses a [usize] index and [usize] generation to match an index to the context in which it was created and prevent two unrelated values that both at some point lived at the same index from being mistaken as equal
• All methods return an [AccessError] where the scenario would cause a panic if not caught

NOTE

This package is still UNSTABLE and may go through several iterations before I consider it good enough to set in stone See changelog

Motivation

I wanted a data structure that met these criteria:

• Backed by a [Vec] (or similar) for cache efficiency
• Allowed interior mutability to each of its elements
• Was fully memory safe (needs verification)
• Always returned a relevant error instead of panicking
• Was easier to reason about when and where it might error than reference counting

Usage

This crate is on crates.io

First, add this crate as a dependency to your project:

[dependencies]
grit-data-prison = "0.2"

Then import [AccessError] and [CellKey] from the crate root, along with the relevant version you wish to use in the file where it is needed (right now only one flavor is available, [single_threaded]):

use grit_data_prison::{AccessError, CellKey, single_threaded::Prison};

Create a Prison and add your data to it using one of the insert() type methods

Note the following quirks:

• A Prison does not need to be declared mut to mutate it
• insert() and its variants return a [Result]<[CellKey], [AccessError]> that you need to handle
• You can ignore the [CellKey] and simply look up the value by index if you wish (shown later)

let prison: Prison<String> = Prison::new();
let key_hello = prison.insert(String::from("Hello, "))?;
prison.insert(String::from("World!"))?;

You can then use one of the .visit() methods to access a mutable reference to your data from within a closure

prison.visit_idx(1, |val_at_idx_1| {
       *val_at_idx_1 = String::from("Rust!!");
       Ok(())
});

Visiting multiple values at the same time can be done by nesting .visit() calls, or by using one of the batch .visit() methods

prison.visit(key_hello, |val_0| {
    prison.visit_idx(1, |val_1| {
        println!("{}{}", *val_0, *val_1); // Prints "Hello, Rust!!"
        Ok(())
    });
    Ok(())
});
prison.visit_many_idx(&[0, 1], |vals| {
    println!("{}{}", vals[0], vals[1]); // Also prints "Hello, Rust!!"
    Ok(())
});

Full Example Code

use grit_data_prison::{AccessError, CellKey, single_threaded::Prison};

fn main() -> Result<(), AccessError> {
    let prison: Prison<String> = Prison::new();
    let key_hello = prison.insert(String::from("Hello, "))?;
    prison.insert(String::from("World!"))?;
    prison.visit_idx(1, |val_at_idx_1| {
        *val_at_idx_1 = String::from("Rust!!");
        Ok(())
    });
    prison.visit(key_hello, |val_0| {
        prison.visit_idx(1, |val_1| {
            println!("{}{}", *val_0, *val_1); // Prints "Hello, Rust!!"
            Ok(())
        });
        Ok(())
    });
    prison.visit_many_idx(&[0, 1], |vals| {
        println!("{}{}", vals[0], vals[1]); // Also prints "Hello, Rust!!"
        Ok(())
    });
    Ok(())
}

Operations that affect the underlying [Vec] can also be done from within .visit() closures as long as none of the following rules are violated:

• The operation does not remove, read, or modify any element that is currently being visited
• The operation does not cause a re-allocation of the entire [Vec] (or otherwise cause the entire [Vec] to relocate to another memory address)

let prison: Prison<u64> = Prison::with_capacity(10);
prison.insert(0)?;
prison.insert(10)?;
prison.insert(20)?;
prison.insert(30)?;
prison.insert(42)?;
let mut accidental_val: u64 = 0;
prison.visit_idx(3, |val| {
    accidental_val = prison.remove_idx(4)?;
    prison.insert_at(4, 40);
    Ok(())
});

For more examples, see the specific documentation for the relevant type/method

Why this strange syntax?

Closures provide a safe sandbox to access mutable references, as they cant be moved out of the closure, and because the visit() functions that take the closures handle all of the safety and housekeeping needed before and after.

Since closures use generics the rust compiler can inline them in many/most/all? cases.

How is this safe?!

The short answer is: it should be mostly safe. I welcome any feedback and analysis showing otherwise so I can fix it or revise my methodology.

Prison follows a few simple rules:

• One and ONLY one reference to any element can be in scope at any given time
• Because we are only allowing one reference, that one reference will always be a mutable reference
• Any method that would or could read, modify, or delete any element cannot be performed while that element is currently being visited
• Any method that would or could cause the underlying [Vec] to relocate to a different spot in memory cannot be performed while even ONE visit is in progress In addition, it provides the functionality of a Generational Arena with these additional rules:
• The Prison has a master generation counter to track the largest generation of any element inside it
• Every valid element has a generation attatched to it, and insert() operations return a [CellKey] that pairs the element index with the current largest generation value
• Any operation that removes or overwrites a valid element with a genreation counter that is equal to the largest generation causes the master generation counter to increase by one

It achieves all of the above with a few lightweight sentinel values:

• A single UnsafeCell to hold all of the Prison internals and provide interior mutability
• A master visit_count [usize] on Prison itself to track whether any visit is in progress
• A master generation [usize] on Prison itself to track largest generation
• Each element is either a Cell or Free variant:
  • A Free Simply contains the value of the next free index after this one is filled
  • A locked [bool] on each Cell that prevents getting 2 mutable references to the same element
  • A generation [usize] on each Cell to use when matching to the [CellKey] used to access the index

(see performance for more info on specifics)

Attempting to perform an action that would violate any of these rules will either be prevented from compiling or return an [AccessError] that describes why it was an error, and should never panic.

Example: compile-time safety

let prison: Prison<String> = Prison::new();
prison.insert(String::from("cannot be stolen"));
let mut steal_mut_ref: &mut String;
let mut steal_prison: Prison<String>;
prison.visit_idx(0, |mut_ref| {
    // will not compile: (error[E0521]: borrowed data escapes outside of closure)
    steal_mut_ref = mut_ref;
    // will not compile: (error[E0505]: cannot move out of `prison` because it is borrowed)
    steal_prison = prison;
    Ok(())
});

Example: run-time safety

struct MyStruct(u32);

fn main() -> Result<(), AccessError> {
    let prison: Prison<MyStruct> = Prison::with_capacity(2); // Note this prison can only hold 2 elements
    let key_0 = prison.insert(MyStruct(1))?;
    prison.insert(MyStruct(2))?;
    prison.visit(key_0, |val_0| {
        assert!(prison.visit(key_0, |val_0_again| Ok(())).is_err());
        assert!(prison.visit_idx(0, |val_0_again| Ok(())).is_err());
        assert!(prison.visit_idx(3, |val_3_out_of_bounds| Ok(())).is_err());
        prison.visit_idx(1, |val_1| {
            assert!(prison.remove_idx(1).is_err()); // would delete memory referenced by val_1
            assert!(prison.remove(key_0).is_err()); // would delete memory referenced by val_0
            assert!(prison.insert(MyStruct(3)).is_err()); // would cause reallocation and invalidate any current references
            Ok(())
        });
        Ok(())
    });
    Ok(())
}

Performance

Speed

(Benchmarks are Coming Soon™)

Size

Prison has 4 [usize] house-keeping values in addition to a [Vec<CellOrFree>]

Each element in [Vec<CellOrFree>] is Either a Cell variant or Free variant, so the marker is only a [u8] Free variant only contains a single [usize], so it is not the limiting variant Cell variant contains a [usize] generation counter, [bool] access lock, and a value of type T

Therefore the total additional size compared to a [Vec], per element, is 16 bytes

How this crate may change in the future

This crate is very much UNSTABLE, meaning that not every error condition may have a test, methods may return different errors/values as my understanding of how they should be properly implemented evolves, I may add/remove methods altogether, etc.

Possible future additions may include:

• Single-thread safe Prison
• More public methods (as long as they make sense and don't bloat the API)
• Multi-thread safe AtomicPrison<T>
• ? Single standalone value version, JailCell<T>
• ? Multi-thread safe standalone value version, AtomicJailCell<T>
• ?? Completely unchecked and unsafe version UnPrison<T>
• ??? Multi-thread safe unsafe version AtomicUnPrison<T>

How to Help/Contribute

This crate is on crates.io The repo is on github

Feel free to leave feedback, or fork/branch the project and submit fixes/optimisations!

If you can give me concrete examples that definitely violate memory-safety, meaning that the provided mutable references can be made to point to invalid/illegal memory (without the use of additional unsafe :P), or otherwise cause unsafe conditions (for example changing an expected enum variant to another where the compiler doesnt expect it to be possible), I'd love to fix, further restrict, or rethink the crate entirely.

Changelog

• Version 0.2.x has a different API than version 0.1.x and is a move from a plain Vec to a Generational Arena
• Version 0.1.x: first version, plain old [Vec] with [usize] indexing