[swift-users] ThreadSanitizer: data race

[Thierry Passeron]

Hi everyone,

I don’t know if it’s the good place to ask for this, so if it’s not, please be kind enough to tell me where I should post this question.

I’m having a hard time figuring out why, since I activated ThreadSanitizer to my Xcode 9 scheme, I keep seeing race conditions when using OperationQueue and a custom BlockOperation subclass I made to deal with Asynchronous operations.

If I refer to the documentation on Operation, it says that isCancelled, isFinished, isExecuting properties must be thread safe since you never know from which thread they can be invoked. So I decided to go for a pthread_mutex_lock/unlock to protect the critical parts. For instance isFinished looks like this:

class AsyncBlockOperation : BlockOperation {

  enum State: String {
    case  ready = "Ready",
          executing = "Executing",
          finished = "Finished"
    fileprivate var keyPath: String {
      return "is" + self.rawValue
    }
  }

  private var state: State = .ready
  {
    willSet {
      willChangeValue(forKey: state.keyPath)
      willChangeValue(forKey: newValue.keyPath)
    }
    didSet {
      didChangeValue(forKey: oldValue.keyPath)
      didChangeValue(forKey: state.keyPath)
    }
  }
  
  private var mutex = pthread_mutex_t()

  override var isFinished: Bool {
    pthread_mutex_lock(&mutex)
    defer { pthread_mutex_unlock(&mutex) }
    return state == .finished
  }

/* Same goes for isCancelled, and isExecuting… */
}

I thought that the problem of « thread-safe » would be solved with these lock/unlock mutex but when an other thread accesses one of the these properties I get a data race warning by the ThreadSanitizer.

What do I do wrong?
Is it possible that ThreadSanitizer reports a false positive? How should I debug this kind of issue? 

Any help would be much appreciated. I can provide a Xcode test project with a running example of this issue.

Thanks in advance.

Best regards,
Thierry