Reader-writer with a barrier: four interviewer questions

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Task

A candidate wrote a thread-safe store. What does the interviewer ask next?

final class ThreadSafeStore<Key: Hashable, Value> {
    private var storage: [Key: Value] = [:]
    private let queue = DispatchQueue(label: "store.queue", attributes: .concurrent)

    func get(_ key: Key) -> Value? {
        queue.sync { storage[key] }
    }

    func set(_ value: Value?, for key: Key) {
        queue.async(flags: .barrier) {
            self.storage[key] = value
        }
    }
}