An implementation of OSAllocatedUnfairLock for iOS 15 and earlier

UnfairLock.swift

//
//  UnfairLock.swift
//  Created by Epps, Landon on 6/28/22.
//

import Foundation

final class UnfairLock: NSLocking {

    enum Ownership: Equatable, Hashable {
        case owner, notOwner
    }

    let _lock: UnsafeMutablePointer<os_unfair_lock>

    init() {
        _lock = .allocate(capacity: 1)
        _lock.initialize(to: os_unfair_lock())
    }

    deinit {
        _lock.deinitialize(count: 1)
        _lock.deallocate()
    }

    func lock() {
        os_unfair_lock_lock(_lock)
    }

    func lockIfAvailable() -> Bool {
        return os_unfair_lock_trylock(_lock)
    }

    func unlock() {
        os_unfair_lock_unlock(_lock)
    }

    func withLock<R>(_ body: () throws -> R) rethrows -> R {
        lock()
        defer { unlock() }
        return try body()
    }

    func withLockIfAvailable<R>(_ body: () throws -> R) rethrows -> R? {
        guard lockIfAvailable() else { return nil }
        defer { unlock() }
        return try body()
    }

    func precondition(_ condition: Ownership) {
        switch condition {
        case .owner:
            os_unfair_lock_assert_owner(_lock)
        case .notOwner:
            os_unfair_lock_assert_not_owner(_lock)
        }
    }
}