SubscriptionStore::get_next_pending_version() returns the next subscription that needs to be sent to the server with a snapshot version past the upload cursor. Empty uploads may be acknowledged during bootstraps and so that will advance the upload cursor past the snapshot version of the subscription. If the subscription bootstrap is interrupted, it is not picked up by the sync client the next time get_next_pending_version() is called (and as a side effect a notification on that subscription will no be fulfilled).