When the app is updated while in the background, the system
cold-launches your process and immediately fires your background task.
Because your initialization (mail user session setup) isn't yet
complete, the background task's work ends too soon and the system
thinks you've finished all background activity.
That violates the expectation that no further background work should
occur once the task reports completion.
Scenario
1. App is installed and user is logged in.
2. User goes background.
3. App is updated.
4. System cold-runs the updated app to execute scheduled background
tasks.
5. Background task starts together with session initialization.
Background task finsihes first and we let the system know that all
background execution is completed - which is not true and usually
cause crash, in our case `0xdead10cc` - deadlock. The reason is we
didn't have enough of time to release file locks.
Jacek Krasiukianis
Xavi Gil
Maciej Gomółka