Unfair lock implementation

UnfairLock.swift

/// Low-level lock that allows waiters to block efficiently on contention.
///
/// Based on `os_unfair_lock_s`
public class UnfairLock {
    // MARK: - Instance variables

    private var unfairLock = os_unfair_lock_s()

    // MARK: - Public

    public init() {
    }

    public func lock() {
        os_unfair_lock_lock(&unfairLock)
    }

    public func unlock() {
        os_unfair_lock_unlock(&unfairLock)
    }

    public func fetchLock<T>(block: () throws -> T) rethrows -> T {
        lock()
        defer {
            unlock()
        }
        return try block()
    }

    public func lock(block: () throws -> Void) rethrows {
        lock()
        defer {
            unlock()
        }
        try block()
    }
}