Core Data migration fails silently

This can only be reproduced if the app is migrating from a database with preexisting validation errors like the database described in https://github.com/wordpress-mobile/WordPress-iOS/issues/12028#issuecomment-557591559 where some of the AbstractPost rows have no blog. The blog is a required property in the Core Data model.

Expected behavior

I'm not entirely sure how to approach this. I only have questions. Should we stop the app from running? Should we show an error to the user? What are the user's options if an error happens?

Actual behavior

The app runs normally after the migration. However, when inspecting the database, I found that some parts of the migration were not executed. The following are some examples.

Heavyweight migrations are not executed

BlogToBlogMigration87to88 prefills the Blog.mobileEditor property with either “aztec“ or “gutenberg”. The values just end up being NULL:

Custom mappings are not executed

The WordPress 92 migration creates an AbstractPost.statusAfterSync property. It has a custom mapping that copies the existing values of AbstractPost.status to statusAfterSync. That doesn't happen. The values are just NULL:

Curiously, the AbstractPost.statusAfterSync property was created.

Steps to reproduce the behavior

The steps involve migrating from WPiOS 12.7 to the most recent develop at the time of this writing, 1523fd2683. There is a working test/working-12.7 branch which compiles on Xcode 10.3. WPiOS 12.7 uses the WordPress 87 Core Data model.

1. Using test/working-12.7 and Xcode 10.3, run the app on iPhone XS 12.4 simulator. Make sure to delete any existing apps beforehand.
2. Log in and browse a Post List to download AbstractPost records.
3. Stop the app.
4. Open the app's WordPress.sqlite database and set one of the ZBASEPOST row's ZBLOG value to NULL. Save the database changes.
5. Using develop and the recent Xcode, run the app on the same simulator. This will automatically migrate the database from 87 to the recent version (93 at the time of this writing).
6. Stop the app.
7. Open the app's WordPress.sqlite database and investigate. You may run the SQL statements shown above.

Notes

#12995 could help us get some more information if this happens.

FYI @astralbodies @koke I'd like to hear your opinion on how we should proceed with this. 🙂

Reproduce the migration using the test/working-12.7 branch and the steps in this issue, then inspect BlogToBlogMigration87to88.swift, MIGRATIONS.md, and the behavior noted in #12995. Establish what should happen when required Core Data values are invalid, and verify that heavyweight and custom mappings either complete or surface an actionable failure.