https://stackoverflow.com/a/72384174/1271826

WorkerThreadLimit.swift

func foo(threads: Int) {
    print("starting \(threads) threads")
    
    let semaphore = DispatchSemaphore(value: 0)
    let group = DispatchGroup()
    
    for i in 0 ..< threads {
        DispatchQueue.global().async(group: group) {
            semaphore.wait()
            print("done waiting \(i)")
        }
    }
    
    group.notify(queue: .main) {
        print("all \(threads) threads are done")
    }
    
    DispatchQueue.global().async {
        print("signaling")
        for _ in 0 ..< threads {
            semaphore.signal()
        }
    }
}

foo(threads: 63) // this won't deadlock

foo(threads: 64) // this will deadlock