improve Android background service reliability (#603)

This change greatly reduces the chance that a backup is not performed
when a new photo/video is made.
Instead of combining the change trigger and additonal constraints (wifi
or charging) into a single worker, these aspects are now separated.
Thus, it is now reliably possible to take pictures while the wifi
constraint is not satisfied and upload them hours/days later once
connected to wifi without taking a new photo.
As a positive side effect, this simplifies the error/retry handling
by directly leveraging Android's WorkManager without workarounds.
The separation also allows to notify the currently running BackupWorker
that new assets were added while backing up other assets to also upload
those newly added assets.
Further, a new tiny service checks if the app is killed, to reschedule
the content change worker and allow to detect the first new photo.
Bonus: The home screen now shows backup as enabled if background backup
is active.

* use separate worker/task for listening on changed/added assets
* use separate worker/task for performing the backup
* content observer worker enqueues backup worker on each new asset
* wifi/charging constraints only apply to backup worker
* backupworker is notified of assets added while running to re-run
* new service to catch app being killed to workaround WorkManager issue

mobile/lib/modules/backup/background_service/background.service.dart

@@ -4,7 +4,6 @@ import 'dart:io';
 import 'dart:isolate';
 import 'dart:ui' show IsolateNameServer, PluginUtilities;
 import 'package:cancellation_token_http/http.dart';
-import 'package:collection/collection.dart';
 import 'package:easy_localization/easy_localization.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
@@ -33,7 +32,6 @@ class BackgroundService {
       MethodChannel('immich/foregroundChannel');
   static const MethodChannel _backgroundChannel =
       MethodChannel('immich/backgroundChannel');
-  bool _isForegroundInitialized = false;
   bool _isBackgroundInitialized = false;
   CancellationToken? _cancellationToken;
   bool _canceledBySystem = false;
@@ -43,32 +41,34 @@ class BackgroundService {
   ReceivePort? _rp;
   bool _errorGracePeriodExceeded = true;
 
-  bool get isForegroundInitialized {
-    return _isForegroundInitialized;
-  }
-
   bool get isBackgroundInitialized {
     return _isBackgroundInitialized;
   }
 
-  Future<bool> _initialize() async {
-    final callback = PluginUtilities.getCallbackHandle(_nativeEntry)!;
-    var result = await _foregroundChannel
-        .invokeMethod('initialize', [callback.toRawHandle()]);
-    _isForegroundInitialized = true;
-    return result;
-  }
-
   /// Ensures that the background service is enqueued if enabled in settings
   Future<bool> resumeServiceIfEnabled() async {
-    return await isBackgroundBackupEnabled() &&
-        await startService(keepExisting: true);
+    return await isBackgroundBackupEnabled() && await enableService();
   }
 
   /// Enqueues the background service
-  Future<bool> startService({
-    bool immediate = false,
-    bool keepExisting = false,
+  Future<bool> enableService({bool immediate = false}) async {
+    if (!Platform.isAndroid) {
+      return true;
+    }
+    try {
+      final callback = PluginUtilities.getCallbackHandle(_nativeEntry)!;
+      final String title =
+          "backup_background_service_default_notification".tr();
+      final bool ok = await _foregroundChannel
+          .invokeMethod('enable', [callback.toRawHandle(), title, immediate]);
+      return ok;
+    } catch (error) {
+      return false;
+    }
+  }
+
+  /// Configures the background service
+  Future<bool> configureService({
     bool requireUnmetered = true,
     bool requireCharging = false,
   }) async {
@@ -76,14 +76,9 @@ class BackgroundService {
       return true;
     }
     try {
-      if (!_isForegroundInitialized) {
-        await _initialize();
-      }
-      final String title =
-          "backup_background_service_default_notification".tr();
       final bool ok = await _foregroundChannel.invokeMethod(
-        'start',
-        [immediate, keepExisting, requireUnmetered, requireCharging, title],
+        'configure',
+        [requireUnmetered, requireCharging],
       );
       return ok;
     } catch (error) {
@@ -92,15 +87,12 @@ class BackgroundService {
   }
 
   /// Cancels the background service (if currently running) and removes it from work queue
-  Future<bool> stopService() async {
+  Future<bool> disableService() async {
     if (!Platform.isAndroid) {
       return true;
     }
     try {
-      if (!_isForegroundInitialized) {
-        await _initialize();
-      }
-      final ok = await _foregroundChannel.invokeMethod('stop');
+      final ok = await _foregroundChannel.invokeMethod('disable');
       return ok;
     } catch (error) {
       return false;
@@ -113,9 +105,6 @@ class BackgroundService {
       return false;
     }
     try {
-      if (!_isForegroundInitialized) {
-        await _initialize();
-      }
       return await _foregroundChannel.invokeMethod("isEnabled");
     } catch (error) {
       return false;
@@ -128,9 +117,6 @@ class BackgroundService {
       return true;
     }
     try {
-      if (!_isForegroundInitialized) {
-        await _initialize();
-      }
       return await _foregroundChannel
           .invokeMethod('isIgnoringBatteryOptimizations');
     } catch (error) {
@@ -289,18 +275,11 @@ class BackgroundService {
         try {
           final bool hasAccess = await acquireLock();
           if (!hasAccess) {
-            debugPrint("[_callHandler] could acquire lock, exiting");
+            debugPrint("[_callHandler] could not acquire lock, exiting");
             return false;
           }
           await translationsLoaded;
           final bool ok = await _onAssetsChanged();
-          if (ok) {
-            Hive.box(backgroundBackupInfoBox).delete(backupFailedSince);
-          } else if (Hive.box(backgroundBackupInfoBox).get(backupFailedSince) ==
-              null) {
-            Hive.box(backgroundBackupInfoBox)
-                .put(backupFailedSince, DateTime.now());
-          }
           return ok;
         } catch (error) {
           debugPrint(error.toString());
@@ -343,6 +322,29 @@ class BackgroundService {
     }
 
     await PhotoManager.setIgnorePermissionCheck(true);
+
+    do {
+      final bool backupOk = await _runBackup(backupService, backupAlbumInfo);
+      if (backupOk) {
+        await Hive.box(backgroundBackupInfoBox).delete(backupFailedSince);
+        await box.put(
+          backupInfoKey,
+          backupAlbumInfo,
+        );
+      } else if (Hive.box(backgroundBackupInfoBox).get(backupFailedSince) ==
+          null) {
+        Hive.box(backgroundBackupInfoBox)
+            .put(backupFailedSince, DateTime.now());
+        return false;
+      }
+      // check for new assets added while performing backup
+    } while (true ==
+        await _backgroundChannel.invokeMethod<bool>("hasContentChanged"));
+    return true;
+  }
+
+  Future<bool> _runBackup(
+      BackupService backupService, HiveBackupAlbums backupAlbumInfo) async {
     _errorGracePeriodExceeded = _isErrorGracePeriodExceeded();
 
     if (_canceledBySystem) {
@@ -382,10 +384,6 @@ class BackgroundService {
     );
     if (ok) {
       _clearErrorNotifications();
-      await box.put(
-        backupInfoKey,
-        backupAlbumInfo,
-      );
     } else {
       _showErrorNotification(
         title: "backup_background_service_error_title".tr(),