Download failing immediately doesn't update UI

[Chralu]

Describe the bug

When requesting a download that immediately fails, UI stays stucked on the "download in progress" state.

It might happen using iOS when episode URLs are non-SSL http://. There might be other cases where download fails as soon as it starts.

To Reproduce

1. Using iOS
2. add the following podcast : https://feeds.feedburner.com/LaperoDuCaptain
3. request any episode download.
4. Episode in list keeps displayed as a download in progress. But download is actually stopped because OS refused http request.

Analyse

mobile_download_service.dart

  @override
  Future<bool> downloadEpisode(Episode episode) async {
 // ....
        final taskId = await downloadManager.enqueueTask(episode.contentUrl, downloadPath, filename);

        // Update the episode with download data
        episode.filepath = episodePath;
        episode.filename = filename;
        episode.downloadTaskId = taskId;
        episode.downloadState = DownloadState.downloading;
        episode.downloadPercentage = 0;

        await repository.saveEpisode(episode);
// ...
  }

Starting a download sequentially :

1. Enqueues a download task in FlutterDownloader
2. Gets the FlutterDownloader task id -> persists it in sembast Episode.downloadTaskId
3. On download update, DownloadService
   1. looks for matching Episode.downloadTaskId in Sembast
   2. If episode is found, it is updated with new download progress.

In the case where download immediately fails, the download status changes before step 2. is done. Then, a download update event arrives but DownloadService is unable to find the matching Episode in sembast. => Sembast episode is not updated => Repository.episodeListener is not updated => PodcastBloc is not updated

Screenshots

https://user-images.githubusercontent.com/375723/222785098-f3ee7f5b-48b9-4c38-a986-0cfda6ae84c6.mp4

Smartphone (please complete the following information):

• Device: iPhone11
• OS: iOS 16

[Chralu]

A solution would be to make step 2 synchronous. That way, we would be certain that Episode.downloadTaskId is changed on time.

I see two ways of implementing that :

1. Replace Sembast by Isar which makes all read/write operations synchronous (by lazily doing actual persistence).
2. Add a hand written memory cache on SembastRepository to make read/write operations synchronous (just like Isar does).

[amugofjava]

I will give this issue some thought. I have been thinking about db solutions as I feel that Anytime is starting to outgrow Sembast. Sembast is an excellent package and has made it very easy to incorporate a pure Dart data store solution, but its strengths are now starting to limit Anytime: I have to be very conscious of what data we store with Sembast being an in-memory store and with it not being cross isolate safe implementing background data fetching will be challenging. It may be that Sembast is replaced or complemented with another solution such as Isar, Objectbox or Realm. Replacing the entire DB layer is no small task.

[Chralu]

I might be helpful on this kind of task.

a thought : Is null-safety migration prior to SGBD migration ?