open(FileManager.default.fileSystemRepresentation(withPath: path), O_EVTONLY) returns -1

Question

I am using SKQueue to monitor some folders in the mac filesystem. As per the documentation, I have added the directory paths to the queues but I noticed that while adding the path, the following line of code in SKQueue is returning -1 and hence it is unable to monitor my folder.

This is the SKQueue Documentation.

The following is code from the documentation, written in the controller class.

import SKQueue

class SomeClass: SKQueueDelegate {
  func receivedNotification(_ notification: SKQueueNotification, path: String, queue: SKQueue) {
    print("\(notification.toStrings().map { $0.rawValue }) @ \(path)")
  }
}

let delegate = SomeClass()
let queue = SKQueue(delegate: delegate)!

queue.addPath("/Users/steve/Documents")
queue.addPath("/Users/steve/Documents/dog.jpg")

The following is code inside SKQueue dependency.

  public func addPath(_ path: String, notifyingAbout notification: SKQueueNotification = SKQueueNotification.Default) {
    var fileDescriptor: Int32! = watchedPaths[path]
    if fileDescriptor == nil {
      fileDescriptor = open(FileManager.default.fileSystemRepresentation(withPath: path), O_EVTONLY)
      guard fileDescriptor >= 0 else { return }
      watchedPaths[path] = fileDescriptor
    }

fileDescriptor = open(FileManager.default.fileSystemRepresentation(withPath: path), O_EVTONLY)

The above code is returning -1 and hence it is failing.

Answer 1

I was getting the same -1 return code and couldn't understand why. Whilst looking for a solution I stumbled upon SwiftFolderMonitor at https://github.com/MartinJNash/SwiftFolderMonitor. This class worked so I knew it wasn't a permission problem.

SwiftFolderMonitor uses DispatchSource.makeFileSystemObjectSource rather than kevent, but it also takes a URL parameter rather than a String path. I amended SKQueue to take a URL instead of a String and it works.

Here's my amended addPath:

public func addPath(url: URL, notifyingAbout notification: SKQueueNotification = SKQueueNotification.Default) {
    let path = url.absoluteString
    var fileDescriptor: Int32! = watchedPaths[path]
    if fileDescriptor == nil {
        fileDescriptor = open((url as NSURL).fileSystemRepresentation, O_EVTONLY)
        guard fileDescriptor >= 0 else { return }
        watchedPaths[path] = fileDescriptor
    }

    var edit = kevent(
        ident: UInt(fileDescriptor),
        filter: Int16(EVFILT_VNODE),
        flags: UInt16(EV_ADD | EV_CLEAR),
        fflags: notification.rawValue,
        data: 0,
        udata: nil
    )
    kevent(kqueueId, &edit, 1, nil, 0, nil)

    if !keepWatcherThreadRunning {
        keepWatcherThreadRunning = true
        DispatchQueue.global().async(execute: watcherThread)
    }
}

I don't know why this works, perhaps someone else can shed some light on this.

I'm still playing with both solutions but it looks like SwiftFolderMonitor does all I need (I just need to know when a specific file has changed) and it's code is clean and minimal so I think I'll use it over SKQueue.

I hope this helps.