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Implement a Multi-Producer, Multi-Consumer (MPMC) Queue in Rust

This challenge focuses on building a fundamental concurrent data structure: a Multi-Producer, Multi-Consumer (MPMC) queue. MPMC queues are essential for high-performance inter-thread communication, enabling multiple threads to safely send data to and receive data from a shared queue without data races or deadlocks.

Problem Description

Your task is to implement a thread-safe MPMC queue in Rust. This queue should allow any number of threads to enqueue (push) elements and any number of other threads to dequeue (pop) elements concurrently. The implementation should be efficient and avoid common pitfalls associated with concurrent programming.

Key Requirements:

  • Thread Safety: The queue must be safe to use from multiple threads simultaneously for both enqueuing and dequeuing operations.
  • MPMC Semantics: Multiple threads should be able to push elements, and multiple threads should be able to pop elements.
  • Blocking Behavior:
    • If a consumer attempts to pop from an empty queue, it should block until an element becomes available.
    • If the queue has a bounded capacity and a producer attempts to push to a full queue, it should block until space becomes available.
  • Generic Type: The queue should be generic over the type of elements it stores (T).
  • No Data Races: The implementation must prevent data races when accessing shared queue state.
  • No Deadlocks: The blocking mechanisms should be designed to prevent deadlocks.

Expected Behavior:

Producers will call an enqueue method, adding elements to the queue. Consumers will call a dequeue method, retrieving elements from the queue. When the queue is empty, dequeue will block. When the queue is full (if bounded), enqueue will block.

Important Edge Cases:

  • Handling a queue with zero capacity (if the bounded implementation is chosen).
  • Correctly waking up blocked threads when the queue state changes (e.g., an element is added or removed).
  • Graceful shutdown or handling of dropped producers/consumers (though explicit handling of this might be outside the scope of the initial implementation).

Examples

Example 1: Basic Enqueue and Dequeue

// Assume a bounded queue with capacity 5
let queue = MpmcQueue::new(5);

// Thread 1 (Producer)
queue.enqueue(10);
queue.enqueue(20);

// Thread 2 (Consumer)
let val1 = queue.dequeue(); // Should be Some(10)
let val2 = queue.dequeue(); // Should be Some(20)
let val3 = queue.dequeue(); // Should block

Output:

val1 will be Some(10). val2 will be Some(20). val3 will eventually become Some(30) once another producer enqueues an element.

Explanation: The producers add two elements. The consumers successfully retrieve them. The third dequeue call blocks because the queue is empty.

Example 2: Multiple Producers and Consumers

// Assume a bounded queue with capacity 3
let queue = MpmcQueue::new(3);

// Thread 1 (Producer) enqueues 1, 2
// Thread 2 (Producer) enqueues 3, 4
// Thread 3 (Consumer) dequeues
// Thread 4 (Consumer) dequeues

// After all enqueues: 1, 2, 3, 4 are in the queue (or attempting to be enqueued).
// Consumers will receive elements in the order they were successfully enqueued,
// interleaved based on thread execution.

// Example possible outcome:
// Consumer 3 gets 1
// Consumer 4 gets 2
// Consumer 3 gets 3
// Consumer 4 gets 4

Output: The consumers will eventually retrieve all four elements. The exact interleaving of which consumer gets which element depends on thread scheduling.

Explanation: Multiple producers and consumers operate on the queue. The thread-safe nature ensures that elements are not lost or corrupted, and consumers receive all enqueued items.

Example 3: Bounded Queue Fullness

// Assume a bounded queue with capacity 2
let queue = MpmcQueue::new(2);

// Thread 1 (Producer)
queue.enqueue(1);
queue.enqueue(2); // Queue is now full

// Thread 2 (Producer) attempts to enqueue 3 - this should block
// Thread 3 (Consumer) dequeues 1
// Thread 4 (Producer) now has space and enqueues 3

let val1 = queue.dequeue(); // Should be Some(2)
let val2 = queue.dequeue(); // Should be Some(3)

Output: val1 will be Some(2). val2 will be Some(3).

Explanation: The first producer fills the queue. The second producer blocks. A consumer frees up space, allowing the second producer's blocked enqueue to complete.

Constraints

  • The MPMC queue should be implemented using standard Rust concurrency primitives (e.g., std::sync::Mutex, std::sync::Condvar, std::sync::atomic types). You may also use channels from std::sync::mpsc as building blocks if you can demonstrate how they are adapted to an MPMC scenario, but a direct implementation of the MPMC logic is preferred.
  • The queue should support a bounded capacity. An unbounded queue can be considered an extension or a separate problem if difficulty is a concern.
  • The average enqueue and dequeue operations should ideally have an amortized time complexity close to O(1), assuming sufficient contention.
  • The implementation should be tested with multiple threads to verify correctness under concurrent access.

Notes

  • Consider using a ring buffer or a linked list for the underlying data structure. Each approach has its own trade-offs in terms of complexity and performance.
  • std::sync::Condvar is crucial for implementing efficient blocking and waking mechanisms.
  • Carefully manage the state transitions of the queue (empty, partially full, full) to ensure correct signaling between producers and consumers.
  • Think about how to handle the case where all producers might exit before consumers have consumed all items, or vice-versa, though explicit shutdown logic is not strictly required for the core implementation.
  • You will need to wrap your queue implementation in Arc to share it safely across threads.
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