SQLite doesn't implement cascading flush and breaks available_apps feature.

[Simon Charette]

#30033 started performing constraint checks on schema alteration commits on SQLite and uncovered a data integrity issue of our SQLite backend flushing of data.

When TransactionTestCase.available_apps is specfied only the specified apps subset of tested model tables are flushed for performance reasons. Thing is when this happens all rows referring to flushed table rows must also be flushed as specified by the DatabaseOperations.sql_flush(allow_cascade) flag that SQLite completely ignores.

This issue has been around since foreign keys were enabled on SQLite because execute_sql_flush happens to disable constraint checks and allow foreign key integrity to be silently broken.

In order to address this issue sql_flush must consider allow_cascade and execute_sql_flush should stop disabling constraints.

More context from ​https://github.com/django/django/pull/10779#issuecomment-449483709

[Tim Graham <timograham@…>]

In ce8b65a:

Fixed #30054 -- Implemented cascaded flush on SQLite.

This is required to maintain foreign key integrity when using
TransactionTestCase.available_apps.

Refs #30033, #14204, #20483.

[Tim Graham]

This slows the Django test suite by about 30-35% on my local machine and on Jenkins.

[Simon Charette]

Is most of the time spent doing the recursive CTE or applying the generated DELETEs?

I guess we could flush all tables when tables and allow_cascade or cache relationship graphs per connection to avoid performing the CTE on each TransactionTestCase teardown.

[Tim Graham]

Looks like about 1/4 of the time increase is for the deletes and 3/4 of the time is for the CTEs.

[Simon Charette]

Significant 30-35% speed up implemented in ​https://github.com/django/django/pull/10792.

[Tim Graham <timograham@…>]

In 2b2ae4ee:

Refs #30054, #20483 -- Cached SQLite references graph retrieval on sql_flush().