fix: run reload within a mutex

mobile/lib/domain/services/timeline.service.dart

@@ -108,6 +108,11 @@ class TimelineFactory {
 class TimelineService {
   final TimelineAssetSource _assetSource;
   final TimelineBucketSource _bucketSource;
+  final AsyncMutex _mutex = AsyncMutex();
+  int _bufferOffset = 0;
+  List<BaseAsset> _buffer = [];
+  StreamSubscription? _bucketSubscription;
+
   int _totalAssets = 0;
   int get totalAssets => _totalAssets;
 
@@ -117,24 +122,41 @@ class TimelineService {
   })  : _assetSource = assetSource,
         _bucketSource = bucketSource {
     _bucketSubscription = _bucketSource().listen((buckets) {
-      _totalAssets =
+      _mutex.run(() async {
+        final totalAssets =
             buckets.fold<int>(0, (acc, bucket) => acc + bucket.assetCount);
-      unawaited(_reloadBucket());
+
+        if (totalAssets == 0) {
+          _bufferOffset = 0;
+          _buffer.clear();
+          // When the buffer is empty or the old bufferOffset is greater than the new total assets,
+          // we need to reset the buffer and load the first batch of assets.
+        } else {
+          final int offset;
+          final int count;
+          if (_bufferOffset >= totalAssets || _buffer.isEmpty) {
+            offset = 0;
+            count = kTimelineAssetLoadBatchSize;
+          } else {
+            offset = _bufferOffset;
+            count = math.min(
+              _buffer.length,
+              totalAssets - _bufferOffset,
+            );
+          }
+          _buffer = await _assetSource(offset, count);
+          _bufferOffset = offset;
+        }
+
+        EventStream.shared.emit(const TimelineReloadEvent());
+        // change the state's total assets count only after the buffer is reloaded
+        _totalAssets = totalAssets;
+      });
     });
   }
 
-  final AsyncMutex _mutex = AsyncMutex();
-  int _bufferOffset = 0;
-  List<BaseAsset> _buffer = [];
-  StreamSubscription? _bucketSubscription;
-
   Stream<List<Bucket>> Function() get watchBuckets => _bucketSource;
 
-  Future<void> _reloadBucket() => _mutex.run(() async {
-        _buffer = await _assetSource(_bufferOffset, _buffer.length);
-        EventStream.shared.emit(const TimelineReloadEvent());
-      });
-
   Future<List<BaseAsset>> loadAssets(int index, int count) =>
       _mutex.run(() => _loadAssets(index, count));
 
@@ -163,18 +185,20 @@ class TimelineService {
           : (len > kTimelineAssetLoadBatchSize ? index : index + count - len),
     );
 
-    final assets = await _assetSource(start, len);
-    _buffer = assets;
+    _buffer = await _assetSource(start, len);
     _bufferOffset = start;
 
     return getAssets(index, count);
   }
 
   bool hasRange(int index, int count) =>
-      index >= _bufferOffset && index + count <= _bufferOffset + _buffer.length;
+      index >= 0 &&
+      index < _totalAssets &&
+      index >= _bufferOffset &&
+      index + count <= _bufferOffset + _buffer.length &&
+      index + count <= _totalAssets;
 
   List<BaseAsset> getAssets(int index, int count) {
-    assert(index + count <= totalAssets);
     if (!hasRange(index, count)) {
       throw RangeError('TimelineService::getAssets Index out of range');
     }
@@ -184,11 +208,13 @@ class TimelineService {
 
   // Pre-cache assets around the given index for asset viewer
   Future<void> preCacheAssets(int index) =>
-      _mutex.run(() => _loadAssets(index, 5));
+      _mutex.run(() => _loadAssets(index, math.min(5, _totalAssets - index)));
 
   BaseAsset getAsset(int index) {
     if (!hasRange(index, 1)) {
-      throw RangeError('TimelineService::getAsset Index out of range');
+      throw RangeError(
+        'TimelineService::getAsset Index $index not in buffer range [$_bufferOffset, ${_bufferOffset + _buffer.length})',
+      );
     }
     return _buffer.elementAt(index - _bufferOffset);
   }