Fix #2049: sharedSyncDevice race conditions. (#2050)

Shared sync device was kept as a static variable. The thing is that, NSManagedObjects are not thread safe.

This changes the implementation so its thread safe object id is being kept and used to access the object.

Data/models/Device.swift

@@ -10,7 +10,7 @@ private let log = Logger.browserLogger
 public final class Device: NSManagedObject, Syncable, CRUD {
 
     // Check if this can be nested inside the method
-    static var sharedCurrentDevice: Device?
+    static var sharedCurrentDeviceId: NSManagedObjectID?
 
     // Assign on parent model via CD
     @NSManaged public var isSynced: Bool
@@ -56,14 +56,14 @@ extension Device {
 
     /// Returns a current device and assings it to a shared variable.
     static func currentDevice(context: NSManagedObjectContext = DataController.viewContext) -> Device? {
-        if sharedCurrentDevice == nil {
+        guard let deviceId = sharedCurrentDeviceId else {
             let predicate = NSPredicate(format: "isCurrentDevice == true")
-            sharedCurrentDevice = first(where: predicate, context: context)
-        } else if let sharedDevice = sharedCurrentDevice {
-            sharedCurrentDevice = context.object(with: sharedDevice.objectID) as? Device
+            let device = first(where: predicate, context: context)
+            sharedCurrentDeviceId = device?.objectID
+            return device
         }
 
-        return sharedCurrentDevice
+        return context.object(with: deviceId) as? Device
     }
 
     class func add(name: String?, isCurrent: Bool = false) {

Data/sync/Sync.swift

@@ -181,7 +181,7 @@ public class Sync: JSInjector {
                 self.sendSyncRecords(action: .delete, records: [device], context: context)
             }
 
-            Device.sharedCurrentDevice = nil
+            Device.sharedCurrentDeviceId = nil
             Device.deleteAll(context: .existing(context))
         }
 

DataTests/CoreDataTestCase.swift

@@ -21,7 +21,7 @@ class CoreDataTestCase: XCTestCase {
     override func tearDown() {
         NotificationCenter.default.removeObserver(self)
         DataController.viewContext.reset()
-        Device.sharedCurrentDevice = nil
+        Device.sharedCurrentDeviceId = nil
         contextSaveCompletion = nil
         super.tearDown()
     }