The push extension sometimes breaks e2e

[manuroe]

We mitigate OTK duplicates (#3721) with matrix-org/matrix-ios-sdk#922 without solving the root issue which is a race condition between the app and the NotificationServiceExtension.

Since iOS 13 (June 2020), pushes cannot be managed anymore by the main app in the background. They must be managed by a separated extension, NotificationServiceExtension, which is a background service that has its own life cycle.

To be able to decrypt message content, the notification extension needs to have a minimal set of crypto available. Most of things are done in an atomic way but not everything which is a problem when the app is running in parallel.

One issue is that both modules have their own instance of OLMAccount loaded in memory. They can both decide to store their OLMAccount instance in the DB at anytime, overwriting what the other stored.

The NotificationServiceExtension does it when receiving a new inbound olm session. When it happens, there is a high risk that it will store an older version of the olm account that the app has. This newly stored olm account will ignore OTK that was sent by the app for example.

There is possibly other issues related to this multi-process access but we need to make MXOlmDevice.olmAccount atomic.