Finalizers, Guardians, Phantom References

The discussion revolves around the implementation of guardians in the Whippet garbage collector, contrasting them with finalizers and their concurrency management. Guardians allow the mutator to control concurrency, unlike finalizers, which run concurrently with the mutator. The text references Java's PhantomReference and ReferenceQueue as similar concepts but notes differences in implementation. It highlights issues in Guile's implementation of guardians, where finalizers may run simultaneously with the objects they protect, potentially leading to problems like accessing already-closed file ports.

To address these issues, one proposed solution is to introduce a special guardian pass in the garbage collector to identify unreachable guarded objects before finalization occurs. This would ensure that all guarded objects are processed before any finalizers are executed. An alternative approach suggests implementing a system of finalizer priorities, allowing the garbage collector to run finalizers in a specific order, which could also accommodate constructs like Java's phantom references and weak references.

The author invites further exploration of other garbage collection constructs that could be exposed to the mutator and cannot be implemented using existing methods like ephemerons and multi-priority finalizers. The note serves as a reflection on potential improvements and considerations for future garbage collection strategies.