Thread-safe testing multiple threads entering a function at the same time in Swift with XCTest

thread-safe-test.swift

final class Fixme {
    private var value: NSObject?
    private let lock = NSLock()

    func perform() -> NSObject {
        lock.lock()
        defer { lock.unlock() }

        if let value { return value }
        let v = NSObject()
        value = v
        return v
    }
}

final class FixmeTests: XCTestCase {
    func testRaceConditionInPerform() {
        let fixme = Fixme()
        let queue = DispatchQueue(label: #function, attributes: .concurrent)
        let group = DispatchGroup()
        let threadCount = 10
        let semaphore = DispatchSemaphore(value: 0)
        var results = Array<NSObject?>(repeating: nil, count: threadCount)

        for i in 0..<threadCount {
            group.enter()
            queue.async {
                semaphore.wait() // Line up this individual thread onto the starting line
                results[i] = fixme.perform()
                group.leave()
            }
        }

        // Starts the race
        for _ in 0..<threadCount {
            semaphore.signal()
        }

        group.wait()

        let first = results[0]!
        for result in results {
            XCTAssert(first === result!)
        }
    }
}