Cross-process (distributed) lock.

DistributedLock.swift

/// Distributed lock based on file locking
final class DistributedLock: NSLocking {
    
    enum Error: Swift.Error {
        case failToAquireLock
    }
    
    private let fileURL: URL
    private var fileDescriptor: Int32 = -1
    
    init(sharedDirectory: URL, name: String) {
        self.fileURL = sharedDirectory.appendingPathComponent("\(name).lock")
    }
    
    func tryLock() -> Bool {
        fileDescriptor = open(fileURL.path, O_CREAT | O_RDWR, S_IRUSR | S_IWUSR)
        let lockResult = flock(fileDescriptor, LOCK_EX)
        return lockResult == 0
    }
    
    func lock() {
        guard tryLock() else {
            assertionFailure("Failed to aquire file lock, this is not expected.")
            return
        }
    }
    
    func unlock() {
        if fileDescriptor != -1 {
            close(fileDescriptor)
            fileDescriptor = -1
        }
    }
    
    func withAsyncLock<T>(_ closure: @escaping (@escaping() -> Void) throws -> T) throws -> T {
        guard tryLock() else {
            throw Error.failToAquireLock
        }
        return try closure(unlock)
    }
}