Fix concurrent index migrations with schema-driven multitenancy #610

[RylandBangerter85]

Addressed @zachdaniel’s feedback: migration_requires_no_transaction? now relies only on the compiled module’s metadata via mod.__migration__(), and no longer reads the migration file.

Solution

Updated AshPostgres.MultiTenancy.migrate_tenant/3 to:

1. Detect migrations that require no transaction (via @disable_ddl_transaction in the compiled module metadata)
2. Run those migrations outside any transaction using Ecto.Adapters.SQL.checkout/3
3. Keep backward compatibility for regular migrations

Changes

• lib/multitenancy.ex: Added migration_requires_no_transaction?/1 and run_migration_without_transaction/4
• test/concurrent_index_multitenancy_test.exs: Added regression tests for:
  • Concurrent index migrations with create_tenant!
  • Regular migrations (backward compatibility)
  • Mixed migrations (concurrent + non-concurrent)
  • Multiple concurrent indexes in one migration

Technical notes

• Uses the compiled module’s __migration__/0 metadata for disable_ddl_transaction instead of scanning the file
• Covers attributes set programmatically via Module.put_attribute/3

Contributor checklist

Leave anything that you believe does not apply unchecked.

• I accept the AI Policy, or AI was not used in the creation of this PR.
• Bug fixes include regression tests
• Chores
• Documentation changes
• Features include unit/acceptance tests
• Refactoring
• Update dependencies

[zachdaniel]

I've actually just realized unfortunately a fundamental issue with this approach that I didn't consider the first time. Specifically, migrations often depend on each other, i.e one migration adds a table, another migration adds an index to that table. But in postgres, DDL migrations are transactional, meaning they won't be visible to migrations that are running outside of a transaction 😢

I think our only actual answer here is instead to check if we are running inside of a transaction while doing this and provide the user with guidance that they should disable the transaction on the action they are running. I don't think its possible to solve this in reality 😓

[RylandBangerter85]

@zachdaniel so you just want me to have it output a warning?

[zachdaniel]

Yes, unfortunately that is all that we can really do.