ProjectSyncWorker should skip projects that have a broken Gitaly shard
One interesting outcome from gitlab-com/migration#205 (comment 59228979) is that even though RepositoryShardSyncWorker
only schedules healthy shards, ProjectSyncWorker
doesn't pay attention to unhealthy shards. If an event comes from a project on a unhealthy shard, we'll attempt to sync it.
It's probably best to mark the project as unsynched and skip it.