Cache return value of `swappable_setting`

[Marten Kenbeek]

Field.swappable_setting is called very often during model rendering. It loops over all models until it finds a match or otherwise returns None. Most models aren't swappable, so in most cases it never breaks out of the loop.

This method should be cached to speed up rendering in migrations. The Field instances are constantly copied, so that's not a good place, but it always uses django.apps.apps. Moving this looping over models to Apps.get_swappable_setting and using an lru_cache speeds up rendering quite a bit.

Both AUTH_USER_MODEL and the models contained in the main apps instance should never change, except for testing scenario's. The cache should never have to be cleared in non-testing scenario's, which results in another very significant speed-up, for a total of about 35% faster rendering in my benchmarks.

[Marten Kenbeek]

PR: ​https://github.com/django/django/pull/4459

[Markus Holtermann]

I updated the existing PR: ​https://github.com/django/django/pull/5176

[Markus Holtermann <info@…>]

In e1427cc6:

Fixed #24590 -- Cached calls to swappable_setting.

Moved the lookup in Field.swappable_setting to Apps, and added
an lru_cache to cache the results.

Refs #24743

Thanks Marten Kenbeek for the initial work on the patch. Thanks Aymeric
Augustin and Tim Graham for the review.

[Tim Graham]

Looks like the new tests aren't isolated as they fail in reverse: $ ./tests/runtests.py --reverse migrations

[Markus Holtermann]

This change isolates the tests in migrations that rely on a replaced swappable model: ​https://github.com/django/django/pull/5199

[Markus Holtermann <info@…>]

In 8e631a3:

Refs #24590 -- Ensured isolation between autodetector tests

Fixed a regression introduced in e1427cc609fa6ab247501b101cfb3c0092aba55b when running tests in reverse order.