An error in TestCase.setUpTestData() leaks data on databases without transactions

[Tim Graham]

While working on ​a backend for MongoDB, I found that an exception on ​the fourth line of expressions.tests.ExpressionsNumericTests.setUpTestData left Number rows in the database such that a later test failed like this:

======================================================================
ERROR: test_F_reuse (expressions.tests.ExpressionsTests.test_F_reuse)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/tim/code/django/tests/expressions/tests.py", line 1217, in test_F_reuse
    self.assertEqual(n_qs.get(), n)
                     ^^^^^^^^^^
  File "/home/tim/code/django/django/db/models/query.py", line 652, in get
    raise self.model.MultipleObjectsReturned(
expressions.models.Number.MultipleObjectsReturned: get() returned more than one Number -- it returned more than 20!

This is the same problem as #25176 but for databases with DatabaseFeatures.supports_transactions = False.

[Simon Charette]

I think the following should do and also avoid a lack of rolling back transaction when they are supported

django/test/testcases.py

@@ -7 +7 @@
 import threading
 import unittest
 from collections import Counter
-from contextlib import contextmanager
+from contextlib import contextmanager, suppress
 from copy import copy, deepcopy
 from difflib import get_close_matches
 from functools import wraps
@@ -1125 +1125 @@
         try:
             self._fixture_setup()
         except Exception:
+            # Attempt to teardown fixtures on exception during setup as
+            # `_post_teardown` won't be triggered to cleanup state when an
+            # an exception is surfaced to `SimpleTestCase._pre_setup`.
+            with suppress(Exception):
+                self._fixture_teardown()
             if self.available_apps is not None:
                 apps.unset_available_apps()
                 setting_changed.send(

Only lightly tested though.

[Tim Graham]

The ​PR lacks any tests. Perhaps inspiration could be taken from 0abb06930fc0686cb35079934e5bb40df66f5691.