Core Data: Multithreading violation when setting blogging reminders

[staskus]

Core Data: Multithreading violation when setting blogging reminders.

Could be the cause of crashes in the production: #20840

Steps to reproduce the behavior

1. Pass -com.apple.CoreData.ConcurrencyDebug 1 arguments on launch
2. Open Jetpack app and log in
3. Site Settings
4. Reminders
5. Set Reminders
6. Observe Multithreading Violation

Thread 41 Queue : com.apple.usernotifications.UNUserNotificationServiceConnection.call-out (serial)
#0	0x00000001845323a0 in +[NSManagedObjectContext __Multithreading_Violation_AllThatIsLeftToUsIsHonor__] ()
#1	0x0000000184531f7c in _PFAssertSafeMultiThreadedAccess_impl ()
#2	0x0000000184520074 in -[NSManagedObject willAccessValueForKey:] ()
#3	0x0000000104df6f74 in -[Blog dotComID] at /Users/povilasstaskus/Projects/WordPress-iOS/WordPress/Classes/Models/Blog.m:749
#4	0x0000000106195b04 in PromptRemindersScheduler.unschedule(for:) at /Users/povilasstaskus/Projects/WordPress-iOS/WordPress/Classes/Utility/Blogging Prompts/PromptRemindersScheduler.swift:70
#5	0x000000010619504c in PromptRemindersScheduler.processSchedule(_:blog:time:completion:) at /Users/povilasstaskus/Projects/WordPress-iOS/WordPress/Classes/Utility/Blogging Prompts/PromptRemindersScheduler.swift:139
#6	0x00000001061958f0 in closure #1 in PromptRemindersScheduler.schedule(_:for:time:completion:) at /Users/povilasstaskus/Projects/WordPress-iOS/WordPress/Classes/Utility/Blogging Prompts/PromptRemindersScheduler.swift:62
#7	0x0000000105def2f8 in closure #1 in InteractiveNotificationsManager.requestAuthorization(completion:) at /Users/povilasstaskus/Projects/WordPress-iOS/WordPress/Classes/Utility/InteractiveNotificationsManager.swift:72
#8	0x00000001054facc4 in thunk for @escaping @callee_guaranteed (@unowned Bool, @guaranteed Error?) -> () ()

Screen.Recording.2023-06-13.at.10.30.39.mov

Jetpack iOS 22.6

[dvdchr]

This should be related to #20848.

[staskus]

This should be related to #20848.

They are related but could also require fixes in 2 places. This ticket crashes when accessing [Blog dotComID] in PromptRemindersScheduler.unschedule, while #20848 crashes when accessing WPAccount.lookup in BloggingPromptsService.init.

[crazytonyli]

This crash is caused by the processSchedule function accessing a blog instance in an incorrect thread. This PR has changed all the call sites of processSchedule function, to ensure it's called from the blog's context queue, which should resolve this issue.

I'll close this issue as resolved, please let me know if you have any questions.