Fixed #32178 -- Allowed database backends to skip tests and mark expected failures.

Co-authored-by: Tim Graham <timograham@gmail.com>

29 files changed, 248 insertions, 87 deletions

diff --git a/django/contrib/gis/db/backends/mysql/features.py b/django/contrib/gis/db/backends/mysql/features.py
index 63e2fa1dd3..72e59d2278 100644
--- a/django/contrib/gis/db/backends/mysql/features.py
+++ b/django/contrib/gis/db/backends/mysql/features.py
@@ -24,3 +24,17 @@ class DatabaseFeatures(BaseSpatialFeatures, MySQLDatabaseFeatures):
     def supports_geometry_field_unique_index(self):
         # Not supported in MySQL since https://dev.mysql.com/worklog/task/?id=11808
         return self.connection.mysql_is_mariadb
+
+    @cached_property
+    def django_test_skips(self):
+        skips = super().django_test_skips
+        if (
+            not self.connection.mysql_is_mariadb and
+            self.connection.mysql_version < (8, 0, 0)
+        ):
+            skips.update({
+                'MySQL < 8 gives different results.': {
+                    'gis_tests.geoapp.tests.GeoLookupTest.test_disjoint_lookup',
+                },
+            })
+        return skips
diff --git a/django/contrib/gis/db/backends/spatialite/features.py b/django/contrib/gis/db/backends/spatialite/features.py
index 6e5bd95179..947d874b30 100644
--- a/django/contrib/gis/db/backends/spatialite/features.py
+++ b/django/contrib/gis/db/backends/spatialite/features.py
@@ -12,3 +12,13 @@ class DatabaseFeatures(BaseSpatialFeatures, SQLiteDatabaseFeatures):
     @cached_property
     def supports_area_geodetic(self):
         return bool(self.connection.ops.lwgeom_version())
+
+    @cached_property
+    def django_test_skips(self):
+        skips = super().django_test_skips
+        skips.update({
+            "SpatiaLite doesn't support distance lookups with Distance objects.": {
+                'gis_tests.geogapp.tests.GeographyTest.test02_distance_lookup',
+            },
+        })
+        return skips
diff --git a/django/db/backends/base/creation.py b/django/db/backends/base/creation.py
index 0099dd18bb..81cb34bd9f 100644
--- a/django/db/backends/base/creation.py
+++ b/django/db/backends/base/creation.py
@@ -1,12 +1,14 @@
 import os
 import sys
 from io import StringIO
+from unittest import expectedFailure, skip
 
 from django.apps import apps
 from django.conf import settings
 from django.core import serializers
 from django.db import router
 from django.db.transaction import atomic
+from django.utils.module_loading import import_string
 
 # The prefix to put on the default database name when creating
 # the test database.
@@ -92,6 +94,9 @@ class BaseDatabaseCreation:
         # Ensure a connection for the side effect of initializing the test database.
         self.connection.ensure_connection()
 
+        if os.environ.get('RUNNING_DJANGOS_TEST_SUITE') == 'true':
+            self.mark_expected_failures_and_skips()
+
         return test_database_name
 
     def set_as_test_mirror(self, primary_settings_dict):
@@ -293,6 +298,29 @@ class BaseDatabaseCreation:
             cursor.execute("DROP DATABASE %s"
                            % self.connection.ops.quote_name(test_database_name))
 
+    def mark_expected_failures_and_skips(self):
+        """
+        Mark tests in Django's test suite which are expected failures on this
+        database and test which should be skipped on this database.
+        """
+        for test_name in self.connection.features.django_test_expected_failures:
+            test_case_name, _, test_method_name = test_name.rpartition('.')
+            test_app = test_name.split('.')[0]
+            # Importing a test app that isn't installed raises RuntimeError.
+            if test_app in settings.INSTALLED_APPS:
+                test_case = import_string(test_case_name)
+                test_method = getattr(test_case, test_method_name)
+                setattr(test_case, test_method_name, expectedFailure(test_method))
+        for reason, tests in self.connection.features.django_test_skips.items():
+            for test_name in tests:
+                test_case_name, _, test_method_name = test_name.rpartition('.')
+                test_app = test_name.split('.')[0]
+                # Importing a test app that isn't installed raises RuntimeError.
+                if test_app in settings.INSTALLED_APPS:
+                    test_case = import_string(test_case_name)
+                    test_method = getattr(test_case, test_method_name)
+                    setattr(test_case, test_method_name, skip(reason)(test_method))
+
     def sql_table_creation_suffix(self):
         """
         SQL to append to the end of the test table creation statements.
diff --git a/django/db/backends/base/features.py b/django/db/backends/base/features.py
index 125d52a21f..885033361a 100644
--- a/django/db/backends/base/features.py
+++ b/django/db/backends/base/features.py
@@ -323,6 +323,13 @@ class BaseDatabaseFeatures:
         'swedish_ci': None  # Swedish case-insensitive.
     }
 
+    # A set of dotted paths to tests in Django's test suite that are expected
+    # to fail on this database.
+    django_test_expected_failures = {}
+    # A map of reasons to sets of dotted paths to tests in Django's test suite
+    # that should be skipped for this database.
+    django_test_skips = {}
+
     def __init__(self, connection):
         self.connection = connection
 
diff --git a/django/db/backends/mysql/features.py b/django/db/backends/mysql/features.py
index 3713df549b..72fff72648 100644
--- a/django/db/backends/mysql/features.py
+++ b/django/db/backends/mysql/features.py
@@ -51,6 +51,52 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     }
 
     @cached_property
+    def django_test_skips(self):
+        skips = {
+            "This doesn't work on MySQL.": {
+                'db_functions.comparison.test_greatest.GreatestTests.test_coalesce_workaround',
+                'db_functions.comparison.test_least.LeastTests.test_coalesce_workaround',
+            },
+            'Running on MySQL requires utf8mb4 encoding (#18392).': {
+                'model_fields.test_textfield.TextFieldTests.test_emoji',
+                'model_fields.test_charfield.TestCharField.test_emoji',
+            },
+        }
+        if 'ONLY_FULL_GROUP_BY' in self.connection.sql_mode:
+            skips.update({
+                'GROUP BY optimization does not work properly when '
+                'ONLY_FULL_GROUP_BY mode is enabled on MySQL, see #31331.': {
+                    'aggregation.tests.AggregateTestCase.test_aggregation_subquery_annotation_multivalued',
+                    'annotations.tests.NonAggregateAnnotationTestCase.test_annotation_aggregate_with_m2o',
+                },
+            })
+        if (
+            self.connection.mysql_is_mariadb and
+            (10, 4, 3) < self.connection.mysql_version < (10, 5, 2)
+        ):
+            skips.update({
+                'https://jira.mariadb.org/browse/MDEV-19598': {
+                    'schema.tests.SchemaTests.test_alter_not_unique_field_to_primary_key',
+                },
+            })
+        if (
+            self.connection.mysql_is_mariadb and
+            (10, 4, 12) < self.connection.mysql_version < (10, 5)
+        ):
+            skips.update({
+                'https://jira.mariadb.org/browse/MDEV-22775': {
+                    'schema.tests.SchemaTests.test_alter_pk_with_self_referential_field',
+                },
+            })
+        if not self.supports_explain_analyze:
+            skips.update({
+                'MariaDB and MySQL >= 8.0.18 specific.': {
+                    'queries.test_explain.ExplainTests.test_mysql_analyze',
+                },
+            })
+        return skips
+
+    @cached_property
     def _mysql_storage_engine(self):
         "Internal method used in Django tests. Don't rely on this from your code"
         return self.connection.mysql_server_data['default_storage_engine']
diff --git a/django/db/backends/oracle/features.py b/django/db/backends/oracle/features.py
index e66c5da09f..2570675809 100644
--- a/django/db/backends/oracle/features.py
+++ b/django/db/backends/oracle/features.py
@@ -72,6 +72,28 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         'swedish_ci': 'SWEDISH_CI',
     }
 
+    django_test_skips = {
+        "Oracle doesn't support SHA224.": {
+            'db_functions.text.test_sha224.SHA224Tests.test_basic',
+            'db_functions.text.test_sha224.SHA224Tests.test_transform',
+        },
+        "Oracle doesn't support bitwise XOR.": {
+            'expressions.tests.ExpressionOperatorTests.test_lefthand_bitwise_xor',
+            'expressions.tests.ExpressionOperatorTests.test_lefthand_bitwise_xor_null',
+        },
+        "Oracle requires ORDER BY in row_number, ANSI:SQL doesn't.": {
+            'expressions_window.tests.WindowFunctionTests.test_row_number_no_ordering',
+        },
+        'Raises ORA-00600: internal error code on Oracle 18.': {
+            'model_fields.test_jsonfield.TestQuerying.test_usage_in_subquery',
+        },
+    }
+    django_test_expected_failures = {
+        # A bug in Django/cx_Oracle with respect to string handling (#23843).
+        'annotations.tests.NonAggregateAnnotationTestCase.test_custom_functions',
+        'annotations.tests.NonAggregateAnnotationTestCase.test_custom_functions_can_ref_other_functions',
+    }
+
     @cached_property
     def introspected_field_types(self):
         return {
diff --git a/django/db/backends/postgresql/features.py b/django/db/backends/postgresql/features.py
index f5f844ca42..84259c0c19 100644
--- a/django/db/backends/postgresql/features.py
+++ b/django/db/backends/postgresql/features.py
@@ -59,6 +59,12 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     has_json_operators = True
     json_key_contains_list_matching_requires_list = True
 
+    django_test_skips = {
+        'opclasses are PostgreSQL only.': {
+            'indexes.tests.SchemaIndexesNotPostgreSQLTests.test_create_index_ignores_opclasses',
+        },
+    }
+
     @cached_property
     def test_collations(self):
         # PostgreSQL < 10 doesn't support ICU collations.
diff --git a/django/db/backends/sqlite3/features.py b/django/db/backends/sqlite3/features.py
index 880d2e50dd..3348256c74 100644
--- a/django/db/backends/sqlite3/features.py
+++ b/django/db/backends/sqlite3/features.py
@@ -51,6 +51,37 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     }
 
     @cached_property
+    def django_test_skips(self):
+        skips = {
+            'SQLite stores values rounded to 15 significant digits.': {
+                'model_fields.test_decimalfield.DecimalFieldTests.test_fetch_from_db_without_float_rounding',
+            },
+            'SQLite naively remakes the table on field alteration.': {
+                'schema.tests.SchemaTests.test_unique_no_unnecessary_fk_drops',
+                'schema.tests.SchemaTests.test_unique_and_reverse_m2m',
+                'schema.tests.SchemaTests.test_alter_field_default_doesnt_perform_queries',
+                'schema.tests.SchemaTests.test_rename_column_renames_deferred_sql_references',
+            },
+            "SQLite doesn't have a constraint.": {
+                'model_fields.test_integerfield.PositiveIntegerFieldTests.test_negative_values',
+            },
+        }
+        if Database.sqlite_version_info < (3, 27):
+            skips.update({
+                'Nondeterministic failure on SQLite < 3.27.': {
+                    'expressions_window.tests.WindowFunctionTests.test_subquery_row_range_rank',
+                },
+            })
+        if self.connection.is_in_memory_db():
+            skips.update({
+                "the sqlite backend's close() method is a no-op when using an "
+                "in-memory database": {
+                    'servers.test_liveserverthread.LiveServerThreadTest.test_closes_connections',
+                },
+            })
+        return skips
+
+    @cached_property
     def supports_atomic_references_rename(self):
         # SQLite 3.28.0 bundled with MacOS 10.15 does not support renaming
         # references atomically.
diff --git a/docs/releases/3.2.txt b/docs/releases/3.2.txt
index d5047dfe60..b155f37e00 100644
--- a/docs/releases/3.2.txt
+++ b/docs/releases/3.2.txt
@@ -197,6 +197,14 @@ CSRF
 
 * ...
 
+Database backends
+~~~~~~~~~~~~~~~~~
+
+* Third-party database backends can now skip or mark as expected failures
+  tests in Django's test suite using the new
+  ``DatabaseFeatures.django_test_skips`` and
+  ``django_test_expected_failures`` attributes.
+
 Decorators
 ~~~~~~~~~~
 
diff --git a/tests/aggregation/tests.py b/tests/aggregation/tests.py
index ad4b94a7f9..0bced06ce9 100644
--- a/tests/aggregation/tests.py
+++ b/tests/aggregation/tests.py
@@ -1218,11 +1218,6 @@ class AggregateTestCase(TestCase):
         Subquery annotations must be included in the GROUP BY if they use
         potentially multivalued relations (contain the LOOKUP_SEP).
         """
-        if connection.vendor == 'mysql' and 'ONLY_FULL_GROUP_BY' in connection.sql_mode:
-            self.skipTest(
-                'GROUP BY optimization does not work properly when '
-                'ONLY_FULL_GROUP_BY mode is enabled on MySQL, see #31331.'
-            )
         subquery_qs = Author.objects.filter(
             pk=OuterRef('pk'),
             book__name=OuterRef('book__name'),
diff --git a/tests/annotations/tests.py b/tests/annotations/tests.py
index cc9bd82656..8337f344ed 100644
--- a/tests/annotations/tests.py
+++ b/tests/annotations/tests.py
@@ -2,7 +2,6 @@ import datetime
 from decimal import Decimal
 
 from django.core.exceptions import FieldDoesNotExist, FieldError
-from django.db import connection
 from django.db.models import (
     BooleanField, Case, CharField, Count, DateTimeField, DecimalField, Exists,
     ExpressionWrapper, F, FloatField, Func, IntegerField, Max,
@@ -20,19 +19,6 @@ from .models import (
 )
 
 
-def cxOracle_py3_bug(func):
-    """
-    There's a bug in Django/cx_Oracle with respect to string handling under
-    Python 3 (essentially, they treat Python 3 strings as Python 2 strings
-    rather than unicode). This makes some tests here fail under Python 3, so
-    we mark them as expected failures until someone fixes them in #23843.
-    """
-    from unittest import expectedFailure
-
-    from django.db import connection
-    return expectedFailure(func) if connection.vendor == 'oracle' else func
-
-
 class NonAggregateAnnotationTestCase(TestCase):
 
     @classmethod
@@ -590,7 +576,6 @@ class NonAggregateAnnotationTestCase(TestCase):
                 e.id, e.first_name, e.manager, e.random_value, e.last_name, e.age,
                 e.salary, e.store.name, e.annotated_value))
 
-    @cxOracle_py3_bug
     def test_custom_functions(self):
         Company(name='Apple', motto=None, ticker_name='APPL', description='Beautiful Devices').save()
         Company(name='Django Software Foundation', motto=None, ticker_name=None, description=None).save()
@@ -617,7 +602,6 @@ class NonAggregateAnnotationTestCase(TestCase):
             lambda c: (c.name, c.tagline)
         )
 
-    @cxOracle_py3_bug
     def test_custom_functions_can_ref_other_functions(self):
         Company(name='Apple', motto=None, ticker_name='APPL', description='Beautiful Devices').save()
         Company(name='Django Software Foundation', motto=None, ticker_name=None, description=None).save()
@@ -770,11 +754,6 @@ class NonAggregateAnnotationTestCase(TestCase):
         ])
 
     def test_annotation_aggregate_with_m2o(self):
-        if connection.vendor == 'mysql' and 'ONLY_FULL_GROUP_BY' in connection.sql_mode:
-            self.skipTest(
-                'GROUP BY optimization does not work properly when '
-                'ONLY_FULL_GROUP_BY mode is enabled on MySQL, see #31331.'
-            )
         qs = Author.objects.filter(age__lt=30).annotate(
             max_pages=Case(
                 When(book_contact_set__isnull=True, then=Value(0)),
diff --git a/tests/backends/base/test_creation.py b/tests/backends/base/test_creation.py
index 3e29961d21..6c98556f1e 100644
--- a/tests/backends/base/test_creation.py
+++ b/tests/backends/base/test_creation.py
@@ -1,5 +1,6 @@
 import copy
 import datetime
+import os
 from unittest import mock
 
 from django.db import DEFAULT_DB_ALIAS, connection, connections
@@ -107,6 +108,22 @@ class TestDbCreationTests(SimpleTestCase):
             with mock.patch.object(creation, '_destroy_test_db'):
                 creation.destroy_test_db(old_database_name, verbosity=0)
 
+    @mock.patch.dict(os.environ, {'RUNNING_DJANGOS_TEST_SUITE': ''})
+    @mock.patch.object(BaseDatabaseCreation, 'mark_expected_failures_and_skips')
+    def test_mark_expected_failures_and_skips_call(self, mark_expected_failures_and_skips, *mocked_objects):
+        """
+        mark_expected_failures_and_skips() isn't called unless
+        RUNNING_DJANGOS_TEST_SUITE is 'true'.
+        """
+        test_connection = get_connection_copy()
+        creation = test_connection.creation_class(test_connection)
+        if connection.vendor == 'oracle':
+            # Don't close connection on Oracle.
+            creation.connection.close = mock.Mock()
+        with mock.patch.object(creation, '_create_test_db'):
+            creation.create_test_db(verbosity=0, autoclobber=True, serialize=False)
+        self.assertIs(mark_expected_failures_and_skips.called, False)
+
 
 class TestDeserializeDbFromString(TransactionTestCase):
     available_apps = ['backends']
@@ -188,3 +205,48 @@ class TestDeserializeDbFromString(TransactionTestCase):
             data = connection.creation.serialize_db_to_string()
         self.assertIn('"model": "backends.schoolclass"', data)
         self.assertIn('"year": 1000', data)
+
+
+class SkipTestClass:
+    def skip_function(self):
+        pass
+
+
+def skip_test_function():
+    pass
+
+
+def expected_failure_test_function():
+    pass
+
+
+class TestMarkTests(SimpleTestCase):
+    def test_mark_expected_failures_and_skips(self):
+        test_connection = get_connection_copy()
+        creation = BaseDatabaseCreation(test_connection)
+        creation.connection.features.django_test_expected_failures = {
+            'backends.base.test_creation.expected_failure_test_function',
+        }
+        creation.connection.features.django_test_skips = {
+            'skip test class': {
+                'backends.base.test_creation.SkipTestClass',
+            },
+            'skip test function': {
+                'backends.base.test_creation.skip_test_function',
+            },
+        }
+        creation.mark_expected_failures_and_skips()
+        self.assertIs(
+            expected_failure_test_function.__unittest_expecting_failure__,
+            True,
+        )
+        self.assertIs(SkipTestClass.__unittest_skip__, True)
+        self.assertEqual(
+            SkipTestClass.__unittest_skip_why__,
+            'skip test class',
+        )
+        self.assertIs(skip_test_function.__unittest_skip__, True)
+        self.assertEqual(
+            skip_test_function.__unittest_skip_why__,
+            'skip test function',
+        )
diff --git a/tests/db_functions/comparison/test_greatest.py b/tests/db_functions/comparison/test_greatest.py
index ef93d808c2..f11e7b824c 100644
--- a/tests/db_functions/comparison/test_greatest.py
+++ b/tests/db_functions/comparison/test_greatest.py
@@ -1,6 +1,6 @@
 from datetime import datetime, timedelta
 from decimal import Decimal
-from unittest import skipIf, skipUnless
+from unittest import skipUnless
 
 from django.db import connection
 from django.db.models.expressions import RawSQL
@@ -33,7 +33,6 @@ class GreatestTests(TestCase):
         articles = Article.objects.annotate(last_updated=Greatest('written', 'published'))
         self.assertIsNone(articles.first().last_updated)
 
-    @skipIf(connection.vendor == 'mysql', "This doesn't work on MySQL")
     def test_coalesce_workaround(self):
         past = datetime(1900, 1, 1)
         now = timezone.now()
diff --git a/tests/db_functions/comparison/test_least.py b/tests/db_functions/comparison/test_least.py
index de2c543f0b..e0318e25c6 100644
--- a/tests/db_functions/comparison/test_least.py
+++ b/tests/db_functions/comparison/test_least.py
@@ -1,6 +1,6 @@
 from datetime import datetime, timedelta
 from decimal import Decimal
-from unittest import skipIf, skipUnless
+from unittest import skipUnless
 
 from django.db import connection
 from django.db.models.expressions import RawSQL
@@ -35,7 +35,6 @@ class LeastTests(TestCase):
         articles = Article.objects.annotate(first_updated=Least('written', 'published'))
         self.assertIsNone(articles.first().first_updated)
 
-    @skipIf(connection.vendor == 'mysql', "This doesn't work on MySQL")
     def test_coalesce_workaround(self):
         future = datetime(2100, 1, 1)
         now = timezone.now()
diff --git a/tests/db_functions/text/test_sha224.py b/tests/db_functions/text/test_sha224.py
index a8291d416c..d2f1d538dc 100644
--- a/tests/db_functions/text/test_sha224.py
+++ b/tests/db_functions/text/test_sha224.py
@@ -20,7 +20,6 @@ class SHA224Tests(TestCase):
             Author(alias=None),
         ])
 
-    @unittest.skipIf(connection.vendor == 'oracle', "Oracle doesn't support SHA224.")
     def test_basic(self):
         authors = Author.objects.annotate(
             sha224_alias=SHA224('alias'),
@@ -37,7 +36,6 @@ class SHA224Tests(TestCase):
             ],
         )
 
-    @unittest.skipIf(connection.vendor == 'oracle', "Oracle doesn't support SHA224.")
     def test_transform(self):
         with register_lookup(CharField, SHA224):
             authors = Author.objects.filter(
diff --git a/tests/expressions/tests.py b/tests/expressions/tests.py
index d8d8370cb3..463150f8a1 100644
--- a/tests/expressions/tests.py
+++ b/tests/expressions/tests.py
@@ -1230,13 +1230,11 @@ class ExpressionOperatorTests(TestCase):
         self.assertEqual(Number.objects.get(pk=self.n.pk).integer, 1764)
         self.assertEqual(Number.objects.get(pk=self.n.pk).float, Approximate(61.02, places=2))
 
-    @unittest.skipIf(connection.vendor == 'oracle', "Oracle doesn't support bitwise XOR.")
     def test_lefthand_bitwise_xor(self):
         Number.objects.update(integer=F('integer').bitxor(48))
         self.assertEqual(Number.objects.get(pk=self.n.pk).integer, 26)
         self.assertEqual(Number.objects.get(pk=self.n1.pk).integer, -26)
 
-    @unittest.skipIf(connection.vendor == 'oracle', "Oracle doesn't support bitwise XOR.")
     def test_lefthand_bitwise_xor_null(self):
         employee = Employee.objects.create(firstname='John', lastname='Doe')
         Employee.objects.update(salary=F('salary').bitxor(48))
diff --git a/tests/expressions_window/tests.py b/tests/expressions_window/tests.py
index 08f215215b..8b1b4a8a3f 100644
--- a/tests/expressions_window/tests.py
+++ b/tests/expressions_window/tests.py
@@ -1,6 +1,6 @@
 import datetime
 from decimal import Decimal
-from unittest import mock, skipIf
+from unittest import mock
 
 from django.core.exceptions import FieldError
 from django.db import NotSupportedError, connection
@@ -150,7 +150,6 @@ class WindowFunctionTests(TestCase):
             ('Johnson', 'Management', 12),
         ], lambda entry: (entry.name, entry.department, entry.row_number))
 
-    @skipIf(connection.vendor == 'oracle', "Oracle requires ORDER BY in row_number, ANSI:SQL doesn't")
     def test_row_number_no_ordering(self):
         """
         The row number window function computes the number based on the order
@@ -633,10 +632,6 @@ class WindowFunctionTests(TestCase):
             ('Brown', 'Sales', 53000, datetime.date(2009, 9, 1), 148000)
         ], transform=lambda row: (row.name, row.department, row.salary, row.hire_date, row.sum))
 
-    @skipIf(
-        connection.vendor == 'sqlite' and connection.Database.sqlite_version_info < (3, 27),
-        'Nondeterministic failure on SQLite < 3.27.'
-    )
     def test_subquery_row_range_rank(self):
         qs = Employee.objects.annotate(
             highest_avg_salary_date=Subquery(
diff --git a/tests/gis_tests/geoapp/tests.py b/tests/gis_tests/geoapp/tests.py
index 350bd04629..94cb4b10de 100644
--- a/tests/gis_tests/geoapp/tests.py
+++ b/tests/gis_tests/geoapp/tests.py
@@ -1,5 +1,4 @@
 import tempfile
-import unittest
 from io import StringIO
 
 from django.contrib.gis import gdal
@@ -229,8 +228,6 @@ class GeoLookupTest(TestCase):
 
     def test_disjoint_lookup(self):
         "Testing the `disjoint` lookup type."
-        if mysql and not mariadb and connection.mysql_version < (8, 0, 0):
-            raise unittest.SkipTest('MySQL < 8 gives different results.')
         ptown = City.objects.get(name='Pueblo')
         qs1 = City.objects.filter(point__disjoint=ptown.point)
         self.assertEqual(7, qs1.count())
diff --git a/tests/gis_tests/geogapp/tests.py b/tests/gis_tests/geogapp/tests.py
index 507d8a970e..0faf749e14 100644
--- a/tests/gis_tests/geogapp/tests.py
+++ b/tests/gis_tests/geogapp/tests.py
@@ -2,7 +2,7 @@
 Tests for geography support in PostGIS
 """
 import os
-from unittest import skipIf, skipUnless
+from unittest import skipUnless
 
 from django.contrib.gis.db import models
 from django.contrib.gis.db.models.functions import Area, Distance
@@ -22,7 +22,6 @@ class GeographyTest(TestCase):
         "Ensure geography features loaded properly."
         self.assertEqual(8, City.objects.count())
 
-    @skipIf(spatialite, "SpatiaLite doesn't support distance lookups with Distance objects.")
     @skipUnlessDBFeature("supports_distances_lookups", "supports_distance_geodetic")
     def test02_distance_lookup(self):
         "Testing distance lookup support on non-point geography fields."
diff --git a/tests/indexes/tests.py b/tests/indexes/tests.py
index 3c4541f684..6d01e3b52f 100644
--- a/tests/indexes/tests.py
+++ b/tests/indexes/tests.py
@@ -1,5 +1,5 @@
 import datetime
-from unittest import skipIf, skipUnless
+from unittest import skipUnless
 
 from django.db import connection
 from django.db.models import CASCADE, ForeignKey, Index, Q
@@ -89,7 +89,6 @@ class SchemaIndexesTests(TestCase):
         )
 
 
-@skipIf(connection.vendor == 'postgresql', 'opclasses are PostgreSQL only')
 class SchemaIndexesNotPostgreSQLTests(TransactionTestCase):
     available_apps = ['indexes']
 
diff --git a/tests/model_fields/test_charfield.py b/tests/model_fields/test_charfield.py
index 0e6109c860..17c230c776 100644
--- a/tests/model_fields/test_charfield.py
+++ b/tests/model_fields/test_charfield.py
@@ -1,7 +1,5 @@
-from unittest import skipIf
-
 from django.core.exceptions import ValidationError
-from django.db import connection, models
+from django.db import models
 from django.test import SimpleTestCase, TestCase
 
 from .models import Post
@@ -22,7 +20,6 @@ class TestCharField(TestCase):
     def test_lookup_integer_in_charfield(self):
         self.assertEqual(Post.objects.filter(title=9).count(), 0)
 
-    @skipIf(connection.vendor == 'mysql', 'Running on MySQL requires utf8mb4 encoding (#18392)')
     def test_emoji(self):
         p = Post.objects.create(title='Smile 😀', body='Whatever.')
         p.refresh_from_db()
diff --git a/tests/model_fields/test_decimalfield.py b/tests/model_fields/test_decimalfield.py
index 729b5e570a..2d96cfd437 100644
--- a/tests/model_fields/test_decimalfield.py
+++ b/tests/model_fields/test_decimalfield.py
@@ -1,9 +1,8 @@
-import unittest
 from decimal import Decimal
 
 from django.core import validators
 from django.core.exceptions import ValidationError
-from django.db import connection, models
+from django.db import models
 from django.test import TestCase
 
 from .models import BigD, Foo
@@ -66,7 +65,6 @@ class DecimalFieldTests(TestCase):
         bd = BigD.objects.get(pk=bd.pk)
         self.assertEqual(bd.d, Decimal('12.9'))
 
-    @unittest.skipIf(connection.vendor == 'sqlite', 'SQLite stores values rounded to 15 significant digits.')
     def test_fetch_from_db_without_float_rounding(self):
         big_decimal = BigD.objects.create(d=Decimal('.100000000000000000000000000005'))
         big_decimal.refresh_from_db()
diff --git a/tests/model_fields/test_integerfield.py b/tests/model_fields/test_integerfield.py
index 13b18d967c..af5a9f2e35 100644
--- a/tests/model_fields/test_integerfield.py
+++ b/tests/model_fields/test_integerfield.py
@@ -1,5 +1,3 @@
-import unittest
-
 from django.core import validators
 from django.core.exceptions import ValidationError
 from django.db import IntegrityError, connection, models
@@ -192,7 +190,6 @@ class PositiveIntegerFieldTests(IntegerFieldTests):
         else models.IntegerField
     )
 
-    @unittest.skipIf(connection.vendor == 'sqlite', "SQLite doesn't have a constraint.")
     def test_negative_values(self):
         p = PositiveIntegerModel.objects.create(value=0)
         p.value = models.F('value') - 1
diff --git a/tests/model_fields/test_jsonfield.py b/tests/model_fields/test_jsonfield.py
index 321826814b..f71efcff61 100644
--- a/tests/model_fields/test_jsonfield.py
+++ b/tests/model_fields/test_jsonfield.py
@@ -1,6 +1,6 @@
 import operator
 import uuid
-from unittest import mock, skipIf
+from unittest import mock
 
 from django import forms
 from django.core import serializers
@@ -719,10 +719,6 @@ class TestQuerying(TestCase):
             objs_with_value,
         )
 
-    @skipIf(
-        connection.vendor == 'oracle',
-        'Raises ORA-00600: internal error code on Oracle 18.',
-    )
     def test_usage_in_subquery(self):
         self.assertSequenceEqual(
             NullableJSONModel.objects.filter(
diff --git a/tests/model_fields/test_textfield.py b/tests/model_fields/test_textfield.py
index f0bce822a4..dd5f5a5280 100644
--- a/tests/model_fields/test_textfield.py
+++ b/tests/model_fields/test_textfield.py
@@ -1,7 +1,5 @@
-from unittest import skipIf
-
 from django import forms
-from django.db import connection, models
+from django.db import models
 from django.test import SimpleTestCase, TestCase
 
 from .models import Post
@@ -32,7 +30,6 @@ class TextFieldTests(TestCase):
     def test_lookup_integer_in_textfield(self):
         self.assertEqual(Post.objects.filter(body=24).count(), 0)
 
-    @skipIf(connection.vendor == 'mysql', 'Running on MySQL requires utf8mb4 encoding (#18392)')
     def test_emoji(self):
         p = Post.objects.create(title='Whatever', body='Smile 😀.')
         p.refresh_from_db()
diff --git a/tests/queries/test_explain.py b/tests/queries/test_explain.py
index a8ecd752cd..ba835afb74 100644
--- a/tests/queries/test_explain.py
+++ b/tests/queries/test_explain.py
@@ -80,9 +80,6 @@ class ExplainTests(TestCase):
 
     @unittest.skipUnless(connection.vendor == 'mysql', 'MariaDB and MySQL >= 8.0.18 specific.')
     def test_mysql_analyze(self):
-        # Inner skip to avoid module level query for MySQL version.
-        if not connection.features.supports_explain_analyze:
-            raise unittest.SkipTest('MariaDB and MySQL >= 8.0.18 specific.')
         qs = Tag.objects.filter(name='test')
         with CaptureQueriesContext(connection) as captured_queries:
             qs.explain(analyze=True)
diff --git a/tests/runtests.py b/tests/runtests.py
index ad282ce782..ba7c163bf6 100755
--- a/tests/runtests.py
+++ b/tests/runtests.py
@@ -254,6 +254,10 @@ def setup(verbosity, test_labels, parallel, start_at, start_after):
 
     apps.set_installed_apps(settings.INSTALLED_APPS)
 
+    # Set an environment variable that other code may consult to see if
+    # Django's own test suite is running.
+    os.environ['RUNNING_DJANGOS_TEST_SUITE'] = 'true'
+
     return state
 
 
@@ -267,6 +271,7 @@ def teardown(state):
     # FileNotFoundError at the end of a test run (#27890).
     from multiprocessing.util import _finalizer_registry
     _finalizer_registry.pop((-100, 0), None)
+    del os.environ['RUNNING_DJANGOS_TEST_SUITE']
 
 
 def actual_test_processes(parallel):
diff --git a/tests/schema/tests.py b/tests/schema/tests.py
index 9b8e27e207..7740065d31 100644
--- a/tests/schema/tests.py
+++ b/tests/schema/tests.py
@@ -729,12 +729,6 @@ class SchemaTests(TransactionTestCase):
         Foo.objects.create()
 
     def test_alter_not_unique_field_to_primary_key(self):
-        if (
-            connection.vendor == 'mysql' and
-            connection.mysql_is_mariadb and
-            (10, 4, 3) < connection.mysql_version < (10, 5, 2)
-        ):
-            self.skipTest('https://jira.mariadb.org/browse/MDEV-19598')
         # Create the table.
         with connection.schema_editor() as editor:
             editor.create_model(Author)
@@ -1957,7 +1951,6 @@ class SchemaTests(TransactionTestCase):
             TagUniqueRename._meta.db_table = old_table_name
 
     @isolate_apps('schema')
-    @unittest.skipIf(connection.vendor == 'sqlite', 'SQLite naively remakes the table on field alteration.')
     @skipUnlessDBFeature('supports_foreign_keys')
     def test_unique_no_unnecessary_fk_drops(self):
         """
@@ -1991,7 +1984,6 @@ class SchemaTests(TransactionTestCase):
         self.assertEqual(len(cm.records), 1)
 
     @isolate_apps('schema')
-    @unittest.skipIf(connection.vendor == 'sqlite', 'SQLite remakes the table on field alteration.')
     def test_unique_and_reverse_m2m(self):
         """
         AlterField can modify a unique field when there's a reverse M2M
@@ -2759,7 +2751,6 @@ class SchemaTests(TransactionTestCase):
             if connection.features.can_introspect_default:
                 self.assertIsNone(field.default)
 
-    @unittest.skipIf(connection.vendor == 'sqlite', 'SQLite naively remakes the table on field alteration.')
     def test_alter_field_default_doesnt_perform_queries(self):
         """
         No queries are performed if a field default changes and the field's
@@ -3041,12 +3032,6 @@ class SchemaTests(TransactionTestCase):
         Changing the primary key field name of a model with a self-referential
         foreign key (#26384).
         """
-        if (
-            connection.vendor == 'mysql' and
-            connection.mysql_is_mariadb and
-            (10, 4, 12) < connection.mysql_version < (10, 5)
-        ):
-            self.skipTest('https://jira.mariadb.org/browse/MDEV-22775')
         with connection.schema_editor() as editor:
             editor.create_model(Node)
         old_field = Node._meta.get_field('node_id')
@@ -3203,7 +3188,6 @@ class SchemaTests(TransactionTestCase):
                 self.assertIs(statement.references_table('schema_author'), False)
                 self.assertIs(statement.references_table('schema_book'), False)
 
-    @unittest.skipIf(connection.vendor == 'sqlite', 'SQLite naively remakes the table on field alteration.')
     def test_rename_column_renames_deferred_sql_references(self):
         with connection.schema_editor() as editor:
             editor.create_model(Author)
diff --git a/tests/servers/test_liveserverthread.py b/tests/servers/test_liveserverthread.py
index 9762b53791..a2af459edc 100644
--- a/tests/servers/test_liveserverthread.py
+++ b/tests/servers/test_liveserverthread.py
@@ -13,8 +13,6 @@ class LiveServerThreadTest(TestCase):
 
     def test_closes_connections(self):
         conn = connections[DEFAULT_DB_ALIAS]
-        if conn.vendor == 'sqlite' and conn.is_in_memory_db():
-            self.skipTest("the sqlite backend's close() method is a no-op when using an in-memory database")
         # Pass a connection to the thread to check they are being closed.
         connections_override = {DEFAULT_DB_ALIAS: conn}