simplify fetch background task

deltachat-ios/AppDelegate.swift

@@ -16,7 +16,6 @@ class AppDelegate: UIResponder, UIApplicationDelegate, UNUserNotificationCenterD
     var locationManager: LocationManager!
     var notificationManager: NotificationManager!
     private var backgroundTask: UIBackgroundTaskIdentifier = .invalid
-    private var fetchBackgroundTasks = [Double: UIBackgroundTaskIdentifier]()
     var reachability = Reachability()!
     var window: UIWindow?
     var notifyToken: String?
@@ -346,24 +345,6 @@ class AppDelegate: UIResponder, UIApplicationDelegate, UNUserNotificationCenterD
         increaseDebugCounter("notify-local-wakeup")
         performFetch(completionHandler: completionHandler)
     }
-    
-    private func unregisterFetchBackgroundTask(timestamp: Double) {
-        let backgroundTask = fetchBackgroundTasks[timestamp]
-        if let backgroundTask = backgroundTask, backgroundTask != .invalid {
-            UIApplication.shared.endBackgroundTask(backgroundTask)
-            fetchBackgroundTasks.removeValue(forKey: timestamp)
-            logger.info("⬅️ fetch background task ended and removed: \(timestamp)")
-        }
-    }
-    
-    private func registerFetchBackgroundTask(timestamp: Double) {
-        logger.info("⬅️ registering fetch background task: \(timestamp)")
-        let backgroundTask = UIApplication.shared.beginBackgroundTask { [weak self] in
-            logger.info("⬅️ fetch background expirationHandler called")
-            self?.unregisterFetchBackgroundTask(timestamp: timestamp)
-        }
-        fetchBackgroundTasks[timestamp] = backgroundTask
-    }
 
     private func performFetch(completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {
         // `didReceiveRemoteNotification` as well as `performFetchWithCompletionHandler` might be called if we're in foreground,
@@ -389,7 +370,15 @@ class AppDelegate: UIResponder, UIApplicationDelegate, UNUserNotificationCenterD
             return
         }
         bgIoTimestamp = nowTimestamp
-        registerFetchBackgroundTask(timestamp: nowTimestamp)
+
+        // make sure to balance each call to `beginBackgroundTask` with `endBackgroundTask`
+        let backgroundTask = UIApplication.shared.beginBackgroundTask {
+            // TODO: currently, this should be okay, we are not using too much background time,
+            // so that handler should never be called.
+            // the plan is to listen to an event as DC_EVENT_ENTER_IDLE and stop fetch from there.
+            // if we have such an event, we could imprive this handler and fire the event.
+            logger.info("fetch background task will end soon")
+        }
 
         // we're in background, run IO for a little time
         dcContext.maybeStartIo()
@@ -408,10 +397,13 @@ class AppDelegate: UIResponder, UIApplicationDelegate, UNUserNotificationCenterD
 
             // to avoid 0xdead10cc exceptions, scheduled jobs need to be done before we get suspended;
             // we increase the probabilty that this happens by waiting a moment before calling completionHandler()
-            DispatchQueue.main.asyncAfter(deadline: .now() + 3) { [weak self] in
+            DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
                 logger.info("⬅️ fetch done")
-                self?.unregisterFetchBackgroundTask(timestamp: nowTimestamp)
                 completionHandler(.newData)
+
+                // this line should always be reached after a background task is started
+                // and balances the call to `beginBackgroundTask` above.
+                UIApplication.shared.endBackgroundTask(backgroundTask)
             }
         }
     }