Epoch-Based Memory Reclamation in Rust

Modern systems often require efficient memory management, especially in concurrent environments. Manual memory management can lead to errors like use-after-free bugs, while garbage collection can introduce unpredictable pauses. Epoch-based reclamation is a lock-free memory reclamation technique that aims to provide a balance, allowing for concurrent access without the overhead of locks and avoiding the unpredictability of traditional garbage collectors.

This challenge asks you to implement a simplified epoch-based reclamation system in Rust. You will create a mechanism to safely deallocate memory that might still be referenced by concurrent threads.

Problem Description

Your task is to implement an EpochReclamation struct in Rust that manages a collection of objects for epoch-based reclamation. The system should allow threads to register for epochs, mark objects as "to be retired" (meaning they are no longer actively used and can potentially be deallocated), and then signal when they have finished their current epoch. The reclamation mechanism should then safely deallocate objects that are no longer reachable by any active thread.

Key Requirements:

Epochs: The system must support distinct epochs. Threads operate within a specific epoch.
Retirement: Threads should be able to "retire" objects. Retiring an object means marking it for potential deallocation.
Epoch Advancement: Threads must be able to advance to the next epoch, signaling that they have completed their work in the current epoch.
Reclamation: When a thread advances to a new epoch, the system should check if any retired objects can be safely deallocated. An object can be deallocated if no thread is currently in an epoch less than or equal to the epoch in which the object was retired.
Thread Safety: All operations on the EpochReclamation system must be thread-safe.
Memory Safety: The implementation must prevent use-after-free bugs.

Expected Behavior:

When an object is retired, it should be safely stored until it can be deallocated.
When a thread advances its epoch, it should release its hold on previous epochs.
The reclamation process should happen implicitly as threads advance their epochs.
Objects retired in epoch E should only be deallocated once all threads have advanced beyond epoch E.

Edge Cases:

Threads retiring objects and then immediately advancing epochs.
Many threads retiring many objects concurrently.
Threads that never advance epochs (though this is less of a primary concern for correctness, it's good to consider).
What happens if a thread tries to retire an object after it has already been deallocated (this should be prevented by the system's design).

Examples

Let's conceptualize how this might work with a simple, illustrative example. Actual Rust code will involve Arc, AtomicUsize, and potentially unsafe for deallocation.

Example 1: Simple Retirement and Reclamation

Imagine we have an object ObjA.

Thread 1 is in Epoch 0. It retires ObjA. ObjA is now awaiting reclamation.
Thread 1 advances to Epoch 1.
Thread 2 was also in Epoch 0 and has advanced to Epoch 1.
Now that all threads have advanced past Epoch 0, ObjA can be deallocated.

Example 2: Delayed Reclamation

Thread 1 is in Epoch 0 and retires ObjB.
Thread 2 is in Epoch 0 and retires ObjC.
Thread 1 advances to Epoch 1.
Thread 3 is still in Epoch 0. ObjB and ObjC cannot be deallocated yet because Thread 3 is still in Epoch 0.
Thread 3 advances to Epoch 1.
Thread 2 advances to Epoch 2.
Now all threads have advanced beyond Epoch 0. ObjB and ObjC can be deallocated.

Example 3: Multiple Epochs and Concurrent Operations

Thread 1 (Epoch 0) retires ObjD.
Thread 2 (Epoch 0) retires ObjE.
Thread 1 advances to Epoch 1.
Thread 3 (Epoch 0) retires ObjF.
Thread 2 advances to Epoch 1.
Thread 3 advances to Epoch 2.
Thread 1 advances to Epoch 2.
Now all threads are in Epoch 2 or higher. ObjD and ObjE (retired in Epoch 0) can be deallocated.
Thread 2 advances to Epoch 3.
Thread 3 advances to Epoch 3.
Now all threads are in Epoch 3 or higher. ObjF (retired in Epoch 0) can be deallocated.

Constraints

The system should handle at least 100 concurrent threads.
The number of retired objects can be up to 1,000,000 before reclamation.
Retirement and epoch advancement operations should ideally have minimal overhead, aiming for amortized O(1) complexity.
The solution must be written in Rust.
You are allowed to use unsafe code for the final deallocation of memory, as this is a core aspect of epoch-based reclamation where ownership is managed manually. However, the interface exposed by EpochReclamation should be safe.

Notes

This is a challenging problem that requires a good understanding of concurrency primitives in Rust (like AtomicUsize, Arc, Mutex if absolutely necessary, though lock-free is preferred).
You will need a way to track the current epoch of each thread. An AtomicUsize for the global epoch and thread-local storage for individual thread epochs could be a starting point.
A common approach is to use a linked list or a similar structure to store retired objects, categorized by the epoch they were retired in.
When threads advance their epoch, they should also "clean up" retired objects from epochs that all threads have passed.
The actual deallocation of memory will likely involve Box::from_raw or similar mechanisms when you are certain an object is no longer reachable.
Consider using a data structure to hold retired objects that allows efficient iteration and removal.
Think about how to prevent a thread from accessing memory that another thread has retired and is about to deallocate. This is where the "epoch" concept is crucial.