ios - 调度计时器源中的数据争用

ThreadSanitizer在以下在macOS上运行的Swift程序中检测到数据争用：

import Dispatch


class Foo<T> {
    var value: T?
    let queue = DispatchQueue(label: "Foo syncQueue")
    init(){}
    func complete(value: T) {
        queue.sync {
            self.value = value
        }
    }

    static func completeAfter(_ delay: Double, value: T) -> Foo<T> {
        let returnedFoo = Foo<T>()
        let queue = DispatchQueue(label: "timerEventHandler")
        let timer = DispatchSource.makeTimerSource(queue: queue)
        timer.setEventHandler {
            returnedFoo.complete(value: value)
            timer.cancel()
        }
        timer.scheduleOneshot(deadline: .now() + delay)
        timer.resume()
        return returnedFoo
    }
}



func testCompleteAfter() {

    let foo = Foo<Int>.completeAfter(0.1, value: 1)
    sleep(10)
}

testCompleteAfter()

在iOS Simulator上运行时，ThreadSanitizer不会检测到种族。

ThreadSanitizer输出：

WARNING: ThreadSanitizer: data race (pid=71596)
  Read of size 8 at 0x7d0c0000eb48 by thread T2:
    #0 block_destroy_helper.5 main.swift (DispatchTimerSourceDataRace+0x0001000040fb)
    #1 _Block_release <null>:38 (libsystem_blocks.dylib+0x000000000951)

  Previous write of size 8 at 0x7d0c0000eb48 by main thread:
    #0 block_copy_helper.4 main.swift (DispatchTimerSourceDataRace+0x0001000040b0)
    #1 _Block_copy <null>:38 (libsystem_blocks.dylib+0x0000000008b2)
    #2 testCompleteAfter() -> () main.swift:40 (DispatchTimerSourceDataRace+0x000100003981)
    #3 main main.swift:44 (DispatchTimerSourceDataRace+0x000100002250)

  Location is heap block of size 48 at 0x7d0c0000eb20 allocated by main thread:
    #0 malloc <null>:144 (libclang_rt.tsan_osx_dynamic.dylib+0x00000004188a)
    #1 _Block_copy <null>:38 (libsystem_blocks.dylib+0x000000000873)
    #2 testCompleteAfter() -> () main.swift:40 (DispatchTimerSourceDataRace+0x000100003981)
    #3 main main.swift:44 (DispatchTimerSourceDataRace+0x000100002250)

  Thread T2 (tid=3107318, running) created by thread T-1
    [failed to restore the stack]

SUMMARY: ThreadSanitizer: data race main.swift in block_destroy_helper.5

代码有什么可疑的吗？

最佳答案

@Rob的评论让我重新考虑了这个问题。我想出了对静态函数completeAfter的以下修改-ThreadSanitizer满意*）：

static func completeAfter(_ delay: Double, value: T) -> Foo<T> {
    let returnedFoo = Foo<T>()
    let queue = DispatchQueue(label: "timerEventHandler")
    queue.async {
        let timer = DispatchSource.makeTimerSource(queue: queue)
        timer.setEventHandler {
            returnedFoo.complete(value: value)
            timer.cancel()
        }
        timer.scheduleOneshot(deadline: .now() + delay)
        timer.resume()
    }
    return returnedFoo
}

此更改确保对timer的所有访问都将在队列queue中执行，该队列尝试以这种方式同步计时器。即使在我的“真实”代码中使用相同的解决方案，也无法使用该解决方案，这可能是由于其他外部因素造成的。



*）我们永远不要认为，我们的代码没有种族，只是因为ThreadSanitizer不会检测到种族。可能有一些外部因素恰好“擦除”了潜在的数据争用（例如，dispatch lib恰好在同一线程上执行两个具有冲突访问的块-并且不会发生数据争用）