This crate provides the struct Prison, a generational 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, or by using the .guard() methods to obtain a guarded mutable reference to the value.

This documentation describes the usage of Prison, how its methods differ from those found on a [Vec], how to access the data contained in it, and how it achieves memory safety.

Project Links

grit-data-prison on Crates.io
grit-data-prison on Lib.rs
grit-data-prison on Github
grit-data-prison on Docs.rs

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 doing reference counting on each element to adhere to Rust's memory safety rules
• Uses [usize] refernce counters 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

• Version 0.4.x is a (very minor) breaking api change for 0.3.x and older, but it shouldn't affect most users and is an easy update
• ALSO: Version 0.2.x and older were discovered to have a soft memory leak when using insert_at() and overwrite(), see changelog for details

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 = "0.3"

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!"))?;

From here there are 2 main ways to access the values contained in the Prison

Visiting the values in prison

You can use one of the .visit() methods to access a mutable reference to your data from within a closure, either mutably or immutably

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

The rules for mutable or immutable references are the same as Rust's rules for normal variable referencing:

• ONLY one mutable reference
• OR any number of immutable references

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

Full Visit 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_mut_idx(1, |val_at_idx_1| {
        *val_at_idx_1 = String::from("Rust!!");
        Ok(())
    });
    prison.visit_ref(key_hello, |val_0| {
        prison.visit_ref_idx(1, |val_1| {
            println!("{}{}", *val_0, *val_1); // Prints "Hello, Rust!!"
            Ok(())
        });
        Ok(())
    });
    prison.visit_many_ref_idx(&[0, 1], |vals| {
        println!("{}{}", vals[0], vals[1]); // Also prints "Hello, Rust!!"
        Ok(())
    });
    Ok(())
}

Guarding values with wrapper structs

You can also use one of the .guard() methods to obtain a guarded wrapper around your data, keeping the value marked as referenced as long as the wrapper remains in scope.

First you need to import one of PrisonValueMut, PrisonValueRef, PrisonSliceMut, or PrisonSliceRef from the same module as Prison

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

Then obtain a guarded wrapper by using the corresponding .guard() method

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

As long as the referencing rules aren't violated, you can guard (or visit) that value, even when other values from the same prison are being visited or guarded. The guarded wrappers (for example PrisonValueRef) keep the element(s) marked with the appropriate form of referencing until they go out of scope. This can be done by wrapping the area it is used in a code block, or by manually passing it to the associated ::unguard() function on the wrapper type to immediately drop it out of scope and update the reference count.

The guarded wrapper types all implement [Deref], [AsRef], and [Borrow], while the mutable versions also implement [DerefMut], [AsMut], and [BorrowMut] to provide transparent access to their inner values

{
    let grd_hello = prison.guard_ref(key_hello)?;
    let grd_world = prison.guard_ref_idx(1)?;
    println!("{}{}", *grd_hello, *grd_world); // Prints "Hello, World!"
}
// block ends, both guards go out of scope and their reference counts return to what they were before
let mut grd_world_to_rust = prison.guard_mut_idx(1)?;
*grd_world_to_rust = String::from("Rust!!");
PrisonValueMut::unguard(grd_world_to_rust); // index one is no longer marked mutably referenced
let grd_both = prison.guard_many_ref_idx(&[0, 1])?;
println!("{}{}", grd_both[0], grd_both[1]); // Prints "Hello, Rust!!"

Full Guard Example Code

use grit_data_prison::{AccessError, CellKey, single_threaded::{Prison, PrisonValueRef, PrisonValueMut, PrisonSliceRef}};

fn main() -> Result<(), AccessError> {
    let prison: Prison<String> = Prison::new();
    let key_hello = prison.insert(String::from("Hello, "))?;
    prison.insert(String::from("World!"))?;
    {
        let grd_hello = prison.guard_ref(key_hello)?;
        let grd_world = prison.guard_ref_idx(1)?;
        println!("{}{}", *grd_hello, *grd_world); // Prints "Hello, World!"
    }
    // block ends, both guards go out of scope and their reference counts return to what they were before
    let mut grd_world_to_rust = prison.guard_mut_idx(1)?;
    *grd_world_to_rust = String::from("Rust!!");
    PrisonValueMut::unguard(grd_world_to_rust); // index one is no longer marked mutably referenced
    let grd_both = prison.guard_many_ref_idx(&[0, 1])?;
    println!("{}{}", grd_both[0], grd_both[1]); // Prints "Hello, Rust!!"
    Ok(())
}

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

• The operation does not remove, read, or modify any element that is currently being referenced
• 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(5);
prison.insert(0)?;
prison.insert(10)?;
prison.insert(20)?;
prison.insert(30)?;
prison.insert(42)?;
let mut accidental_val: u64 = 0;
let mut grd_0 = prison.guard_mut_idx(0)?;
prison.visit_ref_idx(3, |val| {
    accidental_val = prison.remove_idx(4)?;
    prison.insert(40)?;
    Ok(())
});
*grd_0 = 80;
PrisonValueMut::unguard(grd_0);
// No values are actively referenced here so we can perform
// an action that would cause re-allocation safely
for i in 0..100u64 {
    prison.insert(i + 100)?;
}

Also provided is a quick shortcut to clone values out of the Prison when type T implements [Clone]. Because cloning values does not alter the original or presume any precondition regarding the content of the value, it is safe (in a single-threaded context) to clone values that are currently being guarded or visited.

Example

let prison: Prison<String> = Prison::new();
let key_0 = prison.insert(String::from("Foo"))?;
prison.insert(String::from("Bar"))?;
let cloned_foo = prison.clone_val(key_0)?;
let cloned_bar = prison.clone_val_idx(1)?;

For more examples, see the specific documentation for the relevant types/methods

JailCell

Also included is the struct JailCell, which acts as a stand-alone version of a Prison, but with no generation counter.

JailCell includes the same basic interface as Prison and also employs reference counting, but with a much simpler set of safety checks

use grit_data_prison::{AccessError, CellKey, single_threaded::{JailCell, JailValueRef}};

fn main() -> Result<(), AccessError> {
    let string_jail: JailCell<String> = JailCell::new(String::from("'Bad-Guy' Bert"));
    string_jail.visit_mut(|criminal| {
        let bigger_bad = String::from("Dr. Lego-Step");
        println!("Breaking News: {} to be set free to make room for {}", *criminal, bigger_bad);
        *criminal = bigger_bad;
        Ok(())
    })?;
    let guarded_criminal = string_jail.guard_ref()?;
    println!("{} will now be paraded around town for public shaming", *guarded_criminal);
    assert_eq!(*guarded_criminal, String::from("Dr. Lego-Step"));
    JailValueRef::unguard(guarded_criminal);
    Ok(())
}

See the documentation on JailCell for more info

Why this strange syntax?

For the visit() methodology, 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.

The guard() methodology requires the values not be able to leak, alias, or never reset their reference counts, so they are wrapped in structs that provide limited access to the references and know how to automatically reset the reference counter for the value when they go out of scope

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:

• You can only get an immutable reference if the value has zero references or only immutable references
• You can only get a mutable reference is the value has zero references of any type
• Any method that would or could read, modify, or delete any element cannot be performed while that element is currently being referenced
• Any method that would or could cause the underlying [Vec] to relocate to a different spot in memory cannot be performed while even ONE reference to ANY element in the [Vec] is still in scope

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 access_count [usize] on Prison itself to track whether any reference is in active
• Each element is (basically) a Cell or Free variant:
  • Free elements act as nodes in a doubly linked list that tracks free indexes
    • One [usize] that points to the previous free index before this one was made free
    • One [usize] that points to the next free index after this one is filled
  • Cell
    • A ref_count [usize] that tracks both mutable and immutable references
    • A generation [usize] to use when matching to the [CellKey] used to access the index
    • A value of type T

(see performance for more info on the actual 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_mut_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))?;
    let grd_0 = prison.guard_mut(key_0)?;
    assert!(prison.guard_mut(key_0).is_err());
    assert!(prison.guard_ref_idx(0).is_err());
    PrisonValueMut::unguard(grd_0);
    prison.visit_mut(key_0, |val_0| {
        assert!(prison.visit_mut(key_0, |val_0_again| Ok(())).is_err());
        assert!(prison.visit_ref(key_0, |val_0_again| Ok(())).is_err());
        assert!(prison.visit_mut_idx(0, |val_0_again| Ok(())).is_err());
        assert!(prison.visit_ref_idx(3, |val_3_out_of_bounds| Ok(())).is_err());
        assert!(prison.guard_mut(key_0).is_err());
        assert!(prison.guard_ref(key_0).is_err());
        assert!(prison.guard_ref_idx(3).is_err());
        prison.visit_ref_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(())
}

Crate Features

no_std: This crate can be used with the no_std feature to use only imports from the core library instead of the std library

Major Malfunctions:
this crate can be passed one of three (optional) features that define how the library handles behavior that is DEFINITELY un-intended and should be considered a bug in the library itself. It defaults to major_malf_is_err if none are specified:

• major_malf_is_err: major malfunctions will be returned as an [AccessError::MAJOR_MALFUNCTION(msg)], this is the default even if not specified
• major_malf_is_panic: major malfunctions will result in a call to panic(msg) describing the unexpected behavior
• major_malf_is_undefined: branches where a major malfunction would nomally be are replaced with [unreachable_unchecked()], possibly allowing them to be removed from compilation entirely

Performance

Speed

(Benchmarks are Coming Soon™)

Size

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

Although the abstract of each PrisonCell<T> is as described as found in How is This Safe?!, the truth of the matter it that Rust was not optimising the memory footprint where it could have done so using Enums, so I had to roll my own type of enum:

• Each element is a struct with a custom-enforced a Cell or Free variant, with the variant tracked in the top bit of one of its fields:
  • field refs_or_next holds a [usize] that holds either the reference count in Cell variant or the next free in Free variant
  • field d_gen_or_prev holds a [usize] that holds either the generation count in Cell variant or the prev free in Free variant
    • In addition, the most significant bit of d_gen_or_prev is reserved for marking the variant of the PrisonCell (the d is for discriminant). This means the ACTUAL maximum generation count is isize::MAX, but the prev index is unafected because a [Vec] cannot have more than isize::MAX elements anyway...
  • field val is a [MaybeUninit<T>] that is always assumed uninitialized when the element is in Free state, and always assumed initialized when it is in Cell state.

Therefore the total additional size compared to a [Vec] on a 64-bit system is 32 bytes flat + 16 bytes per element, and these values are validated in the test suite with an optional test that checks mem::size_of for several types of T

How this crate may change in the future

This crate is very much UNSTABLE, meaning that not every error condition may be tested, 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
• Guard api for a more Rust-idiomatic way to access values
• Switch to reference counting with same memory footprint
• Const Generic bounds to customize the size of internal utility values
• 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
• ? 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 references can be made to point to invalid/illegal memory or violate aliasing rules (without the use of additional unsafe :P), leak memory, 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.

The best way to do this would be to follow these steps:

• make a bug/something or issue/something branch off of the dev branch
• create a new test that demonstrates the current failing of the library
• then do one of the following:
  • solve the problem in your branch and create a pull request into the dev branch with a message explaining everything
  • create a pull request with only the test proving the failure point with a message describing why it is a failure and that this pull request does not solve the problem

Changelog

• Version 0.4.0: BREAKING change: change peek_ref() and peek_ref_idx() to return [Result<T, AccessError>] instead of [Option], and add peek_ref() to JailCell
  • I know it's a very small difference, but breaking is breaking, sorry! It should have been a Result from the beginning to match the existing API and allow easy error propogation inside functions that expect AccessErrors without a bunch of boilerplate testing for Some/None just to return a AccessError::ValueDeleted anyway
• Version 0.3.1: Non-Breaking feature: peek_ref() and peek_ref_idx(), UNSAFE methods that allow the caller to get a reference to a value while bypassing reference counting and other safety checks
• Version 0.3.0: MAJOR BREAKING change to API:
  • Switch to reference counting instead of [bool] locks: the memory footprint is the same (in most cases) and the safety logic is almost the same. Reference counting gives more flexibility and finer grained control with no real penalty compared to using a [bool]
  • escort() methods renamed to guard() methods
  • visit() and guard() methods split into _ref() and _mut() variants
  • [AccessError] variants renamed and changed to be more clear
  • Addition of 3 crate features: major_malf_is_err, major_malf_is_panic, major_malf_is_undefined that allow conditional compilation choices for behavior that is certainly a bug in the library
  • Version 0.2.x and older discovered to have a soft memory leak and should be avoided: when using insert_at() and overwrite() on indexes that werent the 'top' free in the stack, all other free indexes above them in the stack would be forgotten and never re-used. However, they should be freed when the entire Prison is freed. Sorry!
• Version 0.2.3: Non-Breaking feature: clone_val() methods to shortcut cloning a value when T implements [Clone]
• Version 0.2.2: Non-Breaking update to PrisonValue and PrisonSlice to reduce their memory footprint
• Version 0.2.1: Non-breaking addition of escort() api function (why didnt I think of this earlier?)
• 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