Optimize post-migrate permission creation

[Adam Johnson]

I have often seen django.contrib.auth.management.create_permissions() take a significant amount of time in test run profiles. It can be optimized by batching more of its operations, including making ContentTypeManager.get_for_models() use batch creation.

For a comparison, I profiled 1518 of Django’s 1518 tests in modules called “models”:

$ python -m cProfile -o profile runtests.py --parallel 1 *model*

$ python -m pstats profile <<< 'sort cumtime
stats 10000' | less

Before optimization stats:

• Total 11,938,857 function calls taking 5.349 seconds.
• 88 calls to create_permissions() take 456ms, ~8.5% of the total time.

After optimization stats:

• Total 11,359,071 function calls taking 5.035 seconds.
• 88 calls to create_permissions() now take 239ms, ~4.7% of the toal time.
• 217ms and 579,786 function calls saved.

Optimization is limited because the post_migrate signal runs once per migrated app config, so there’s no chance to bulk create *all* content types and permissions at once. If we introduced a new “all migrated apps” signal, that could reduce runtime further by batching all creation.