Parallel testing fails reporting unit test combining subTest and assertNumQueries

[Jean Bvt]

Updating from django 5.1.8 to 5.2.0, our test suit fails to correctly reports failed test.

python manage.py test app.module.tests --parallel 8
Found 9 test(s).
[...]




Destroying test database for alias 'default'...
Destroying test database for alias 'default'...
Destroying test database for alias 'default'...
Traceback (most recent call last):
  File "/app/manage.py", line 23, in <module>
    main()
  File "/app/manage.py", line 19, in main
    execute_from_command_line(sys.argv)
  File "/app/.venv/lib/python3.12/site-packages/django/core/management/__init__.py", line 442, in execute_from_command_line
    utility.execute()
  File "/app/.venv/lib/python3.12/site-packages/django/core/management/__init__.py", line 436, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/app/.venv/lib/python3.12/site-packages/django/core/management/commands/test.py", line 24, in run_from_argv
    super().run_from_argv(argv)
  File "/app/.venv/lib/python3.12/site-packages/django/core/management/base.py", line 416, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/app/.venv/lib/python3.12/site-packages/django/core/management/base.py", line 460, in execute
    output = self.handle(*args, **options)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.12/site-packages/django/core/management/commands/test.py", line 63, in handle
    failures = test_runner.run_tests(test_labels)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.12/site-packages/django/test/runner.py", line 1099, in run_tests
    result = self.run_suite(suite)
             ^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.12/site-packages/django/test/runner.py", line 1026, in run_suite
    return runner.run(suite)
           ^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/unittest/runner.py", line 240, in run
    test(result)
  File "/usr/local/lib/python3.12/unittest/suite.py", line 84, in __call__
    return self.run(*args, **kwds)
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.12/site-packages/django/test/runner.py", line 553, in run
    subsuite_index, events = test_results.next(timeout=0.1)
                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/multiprocessing/pool.py", line 873, in next
    raise value
multiprocessing.pool.MaybeEncodingError: Error sending result: '(17, [('startTest', 0), ('addDuration', 0, 0.02817973299897858), ('addSuccess', 0), ('stopTest', 0), ('startTest', 1), ('addDuration', 1, 0.02015996399859432), ('addSuccess', 1), ('stopTest', 1), ('startTest', 2), ('addDuration', 2, 0.024580582001362927), ('addSuccess', 2), ('stopTest', 2), ('startTest', 3), ('addSubTest', 3, <unittest.case._SubTest testMethod=runTest>, (<class 'AssertionError'>, AssertionError('3 != 2 : 3 queries executed, 2 expected\nCaptured queries were:\n1. SELECT "app_tokenapi"."id", "app_tokenapi"."user_id", "app_tokenapi"."key", "app_tokenapi"."name", "app_tokenapi"."scope_level", "app_tokenapi"."created_at", "app_tokenapi"."expired_at", "app_tokenapi"."expire_reason", "auth_user"."id", "auth_user"."password", "auth_user"."last_login", "auth_user"."is_superuser", "auth_user"."username", "auth_user"."first_name", "auth_user"."last_name", "auth_user"."email", "auth_user"."is_staff", "auth_user"."is_active", "auth_user"."date_joined" FROM "app_tokenapi" INNER JOIN "auth_user" ON ("app_tokenapi"."user_id" = "auth_user"."id") WHERE "app_tokenapi"."key" = \'abcdabcd-abcd-abcd-abcd-abcdabcd\' ORDER BY "app_tokenapi"."id" ASC LIMIT 1\n2. SELECT "my_models_entite"."id", "my_models_entite"."deleted", "my_models_entite"."deleted_by_cascade", "my_models_entite"."some_id", "my_models_entite"."parent_id", "my_models_entite"."type_id", "my_models_entite"."code", "my_models_entite"."libelle", "my_models_entite"."libelle_court", "my_models_entite"."url_logo", "my_models_entite"."begin_date", "my_models_entite"."end_date", "my_models_entite"."created_at", "my_models_entite"."modified_at", "my_models_entite"."json", "my_models_entite"."allow_externe_id", "my_models_entitetype"."id", "my_models_entitetype"."name", "my_models_entitetype"."id_rfu_link", "my_models_entitetype"."id_imotep_link", "my_models_entitetype"."created_at", "my_models_entitetype"."modified_at" FROM "my_models_entite" INNER JOIN "my_models_entitetype" ON ("my_models_entite"."type_id" = "my_models_entitetype"."id") WHERE "my_models_entite"."deleted" IS NULL ORDER BY "my_models_entite"."libelle" ASC\n3. SELECT "my_models_entite"."id", "my_models_entite"."deleted", "my_models_entite"."deleted_by_cascade", "my_models_entite"."some_id", "my_models_entite"."parent_id", "my_models_entite"."type_id", "my_models_entite"."code", "my_models_entite"."libelle", "my_models_entite"."libelle_court", "my_models_entite"."url_logo", "my_models_entite"."begin_date", "my_models_entite"."end_date", "my_models_entite"."created_at", "my_models_entite"."modified_at", "my_models_entite"."json", "my_models_entite"."allow_externe_id" FROM "my_models_entite" WHERE ("my_models_entite"."id") IN ((54), (NULL), (59), (62), (56), (55), (58), (57))'), <traceback object at 0x744f93f72e80>)), ('addDuration', 3, 0.032738704001531005), ('stopTest', 3), ('startTest', 4), ('addDuration', 4, 0.01990954599750694), ('addSuccess', 4), ('stopTest', 4), ('startTest', 5), ('addDuration', 5, 0.10901878799995757), ('addSuccess', 5), ('stopTest', 5), ('startTest', 6), ('addDuration', 6, 0.01989884100112249), ('addSuccess', 6), ('stopTest', 6)])'. Reason: 'AttributeError("Can't get local object 'convert_exception_to_response.<locals>.inner'")'
6998993075), ('addSuccess', 6), ('stopTest', 6)])'. Reason: 'AttributeError("Can't get local object 'convert_exception_to_response.<locals>.inner'")'

(I've renamed some name in this SQL queries for privacy reason)
# the culprit test

    def test_queryset_performances(self):
        EntiteFactory.create()
        with self.subTest("sub test"):
            with self.assertNumQueries(2):
                response = self.client.get(self.url)

# Reproductibility
This errors does not happen if

• We call only this test or only its TestCase (9 tests)
• self.client.get(self.url) is not called, assertNumQueries still raise AssertionError but reporting works fine.
• replacing self.client.get(...) with a simpler QuerySet call still raise AssertionError but reporting works fine.
• we remove self.subTest(...)

The problem does not appear in django 5.1.8 because the test succeeded in 5.1.8, it seems that 5.2 unexpectedly raised the number of query from 2 to 3 (notice the third query look redundant with the second query) so the core issue with reporting may be older than 5.2.