tracing-rc

Cycle collecting reference counted pointers for Rust with a safe, simple api.

The Gc type implemented by this crate provides a cycle-aware smart pointer in the style of std::rc::Rc, the major difference being that you cannot have weak references to a Gc. This is useful if you have some number of interlinked data structures - e.g. GraphNode, LuaTable, etc. - which don't have clear lifetimes or ownership.

Gc is probably the wrong choice for many usecases:

• If you have a data structure with clear parent-child relationships (like a doubly-linked list or tree), you should probably just use std::rc::Rc and Weak.
• If your data is only used for a certain scope and discarded wholesale, you will probably benefit from a crate like typed-arena or bumpalo
• If you have a single type and want to add/remove values, something like generational-arena is probably best.

Usage of Unsafe

The library has two usages of unsafe where it drops the inner data of the Gc and Agc types. In the case of Gc, this unsafe is protected by a borrow_mut call to a RefCell which is intentionally leaked after dropping the data, making the inner (dropped) data inaccessible. Agc similarly leaks a WriteMutexGuard to its inner data post-drop.

Support for Concurrent Collection

This library has experimental support for concurrent collection behind the sync flag. Enabling the sync flag will provide access to an atomic Agc type as well as concurrent collect implementations.

Soundness & Rc Collector Design Considerations

Because any implementation of the Trace trait and custom Drop implementations for objects owned by a garbage collected pointer can run arbitrary code, they may attempt to create new copies or drop existing traced objects (even already traced ones) in the middle of collection. In addition, due to bugs in client code, Trace may report more items as children than it actually owns (reporting fewer is trivially safe, as it will simply leak).

In order for this crate to be sound and present a safe Trace trait, the collector must not cause undefined behavior in any of the scenarios outlined. In order to accomplish this, the collector does the following:

1. Items that are waiting for collection or have been visited during tracing are given an extra strong or weak reference to make sure the memory remains allocated and the data it contains remains valid even if strong references are dropped during traversal.
2. Reference counts for traced items are not decremented by the collector during traversal (this is a difference from e.g. Bacon-Rajan which originally inspired this crate). The collector instead keeps a count of the number of times it reached each pointer through tracing, and then compares that count against the strong count for each pointer it traced to decide if the item is dead.
3. Before dropping any values, the collector marks the object dead. During this process, bugs in the Trace implementation may cause the collector to believe a dead value is still alive (causing a leak) or a live value is dead (making it inaccessible, but leaving the gc pointer valid). Safe code is unable to access dead values (it will panic or return Option::None), and cannot restore the live state of a dead object. Values are never temporarily dead, the collector only marks them dead after it has fully determined that it is a valid candidate for collection (all strong refs are from members of its cycle).
4. After the full set of traced objects has been marked, the collector begins dropping the inner data of objects it believes to be dead, after checking if there are any oustanding borrows (immutable or mutable). This drop does not and can not free the memory for the gc pointer, nor does it make the reference count or liveness inaccessible. The borrow check ensures that data isn't dropped out from underneath active borrows.
5. After it has completed dropping of the inner data of dead values, the collector releases its strong/weak reference. If the cycle was broken, that reference will be the last remaining reference to the Gc value, and its memory will be cleaned up. If there are remaining references, its memory will be cleaned up when the oustanding references fall out of scope.

There are a decent number number of tests designed to exercise each of these scenarios included, and all of these tests pass miri (barring leaks for intentionally misbehaved code).

Example

use tracing_rc::{
    rc::{
        collect_full,
        Gc,
    },
    Trace,
};

#[derive(Trace)]
struct GraphNode<T: 'static> {
    #[trace(ignore)]
    data: T,

    edge: Option<Gc<GraphNode<T>>>,
}

fn main() {
    {
        let node_a = Gc::new(GraphNode {
            data: 10,
            edge: None,
        });
        let node_b = Gc::new(GraphNode {
            data: 11,
            edge: None,
        });
        let node_c = Gc::new(GraphNode {
            data: 12,
            edge: Some(node_a.clone()),
        });

        node_a.borrow_mut().edge = Some(node_b.clone());
        node_b.borrow_mut().edge = Some(node_c);

        let a = node_a.borrow();
        let b = a.edge.as_ref().unwrap().borrow();
        let c = b.edge.as_ref().unwrap().borrow();

        assert_eq!(a.data, c.edge.as_ref().unwrap().borrow().data);
        // all of the nodes go out of scope at this point and would normally be leaked.
    }

    // In this simple example, we always have cycles and our program is complete after this,
    // so we can't take advantage of the young generation picking up acyclic pointers without
    // tracing.
    collect_full();

    // All leaked nodes have been cleaned up!
}

Other GC Implementations

There are a number of excellent implementations of garbage collection in Rust, some of which inspired this crate.

• bacon_rajan_cc
  • Like this crate, it is based on the Bacon-Rajan algorithm and provides a Trace trait which is used for cycle tracing. It was very helpful in understanding how the original paper works, although this crate takes a very different approach.
• gc-arena
  • A very neat implementation of garbage collection relying on lifetimes and arenas rather than cycle tracing.
• cactusref
  • Another take on cycle collection which is pretty novel.
• rust-gc
• shredder
• shifgrethor