id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
37077	TransactionTestCase.serialized_rollback doesn't run tests when database includes a mirror in parallel	Jake Howard	Jake Howard	"In a very specific situation, the test runner skips running tests, and just terminates (successfully). Instead of running the tests, the database is set up, cloned for each process, then destroyed without running any tests. This is related to the changes from #35967

With a `TransactionTestCase` with `serialized_rollback=True`, and a project with multiple `DATABASES` where one is a mirror of the other, where both connections are needed for the test (eg `databases = ""__all__""). If a `TransactionTestCase` isn't going to be run, this behaviour isn't seen.

The cause is that in `ParallelTestSuite.initialize_suite`, an `AttributeError` is raised when accessing `connections[alias]._test_serialized_contents`, because contents is never set.

This could be resolved in 2 ways (at least that I've found):

1. In `django.test.utils.setup_databases`. `serialize_connections` is never added to for mirrored aliases, meaning the contents is never serialized.
2. In `DiscoveryRunner.run_tests`. `suite.serialized_aliases` is set to all databases, and doesn't exclude mirrors (where serialization isn't necessary).

I've written (crude) fixes for both, and both resolve the issue, but I'm not sure which makes the most sense (maybe both?). I suspect the correct answer is option 2, since serializing (and restoring) mirrors makes little sense, but interested in some more opinions.

I've tested this issue is also present on 5.2 (requires `spawn` multiprocessing mode)."	Uncategorized	assigned	Database layer (models, ORM)	5.2	Normal				Unreviewed	0	0	0	0	0	0