import Dispatch

func makeSource(fd: Int32, queue: DispatchQueue, handler: DispatchWorkItem) -> DispatchSourceRead {
    let s = DispatchSource.makeReadSource(fileDescriptor: Int32(fd), queue: queue)
    s.setEventHandler(handler: handler)
    s.resume()
    return s
}

let q = DispatchQueue(label: "some q", attributes: .concurrent)
let numFakeSources = 32
var fakeSources: [DispatchSourceRead] = []
let fd: Int32 = 0
let sem = DispatchSemaphore(value: 0)

for f in 0..<numFakeSources {
    fakeSources.append(makeSource(fd: fd, queue: q, handler: DispatchWorkItem(block: {
        print("fake source A \(f) fired")
    })))
}

let realSource = makeSource(fd: fd, queue: q, handler: DispatchWorkItem(block: {
    print("real source fired")
    sem.signal()
}))

for f in 0..<numFakeSources {
    fakeSources.append(makeSource(fd: fd, queue: q, handler: DispatchWorkItem(block: {
        print("fake source B \(f) fired")
    })))
}

_ = fakeSources.map { $0.cancel() }

sem.wait()