PieterCK 2d6426100f import-export: Rework how we write migration_status.json. ...

The current `get_migration_by_app` has a rather naive approach to
compiling the migration status of a realm, which has led to issues like
#32826. Specifically, those flaws are:

- it does not report the complete state of the migration status of the
exporting servers, only the applied migration.
- it shows both the replaced and the squashed migrations. This would be
a problem if we decide to clean up old migration files we've
squashed(replaced) and import a slightly older realm with those still in
disk. `check_migration_status` would complain of incompatibility even
though those migration files don't matter (they are replaced, after
all).
- it does not clean up ancient/stale applied migrations (for reference,
see how `check-database-compatibility` cleans those)

This commit attempts to write a better `migration_status.json` by
parsing the output of `showmigrations` instead.

This is because Django's `showmigrations` has a lot more logic and
validations baked into it than previously thought. Ones that we care
about are:

- it does validations to make sure app names are valid
- it doesn't list replaced migrations and only squashed one
- it takes into account migrations in disk(`MigrationsLoader`) vs
applied migrations (`MigrationsRecorder`)

Which would resolve the first two points highlighted above.

2025-01-24 17:08:37 -08:00

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