Fixed #32808 -- Prevented DiscoverRunner.build_suite() from mutating test loader patterns.

Thanks Chris Jerdonek for the report and reviews.

django/test/runner.py

@@ -9,6 +9,7 @@ import pickle
 import sys
 import textwrap
 import unittest
+from contextlib import contextmanager
 from importlib import import_module
 from io import StringIO
 
@@ -598,12 +599,23 @@ class DiscoverRunner:
         setup_test_environment(debug=self.debug_mode)
         unittest.installHandler()
 
+    @contextmanager
+    def load_with_patterns(self):
+        original_test_name_patterns = self.test_loader.testNamePatterns
+        self.test_loader.testNamePatterns = self.test_name_patterns
+        try:
+            yield
+        finally:
+            # Restore the original patterns.
+            self.test_loader.testNamePatterns = original_test_name_patterns
+
     def load_tests_for_label(self, label, discover_kwargs):
         label_as_path = os.path.abspath(label)
         tests = None
 
         # If a module, or "module.ClassName[.method_name]", just run those.
         if not os.path.exists(label_as_path):
+            with self.load_with_patterns():
                 tests = self.test_loader.loadTestsFromName(label)
             if tests.countTestCases():
                 return tests
@@ -626,6 +638,7 @@ class DiscoverRunner:
         if os.path.isdir(label_as_path) and not self.top_level:
             kwargs['top_level_dir'] = find_top_level(label_as_path)
 
+        with self.load_with_patterns():
             tests = self.test_loader.discover(start_dir=label, **kwargs)
 
         # Make unittest forget the top-level dir it calculated from this run,
@@ -636,7 +649,6 @@ class DiscoverRunner:
     def build_suite(self, test_labels=None, extra_tests=None, **kwargs):
         test_labels = test_labels or ['.']
         extra_tests = extra_tests or []
-        self.test_loader.testNamePatterns = self.test_name_patterns
 
         discover_kwargs = {}
         if self.pattern is not None:

tests/test_runner/test_discover_runner.py

@@ -26,6 +26,16 @@ def change_cwd(directory):
         os.chdir(old_cwd)
 
 
+@contextmanager
+def change_loader_patterns(patterns):
+    original_patterns = DiscoverRunner.test_loader.testNamePatterns
+    DiscoverRunner.test_loader.testNamePatterns = patterns
+    try:
+        yield
+    finally:
+        DiscoverRunner.test_loader.testNamePatterns = original_patterns
+
+
 class DiscoverRunnerTests(SimpleTestCase):
 
     @staticmethod
@@ -122,6 +132,28 @@ class DiscoverRunnerTests(SimpleTestCase):
                 ).build_suite(['test_runner_apps.simple'])
                 self.assertEqual(expected, self.get_test_methods_names(suite))
 
+    def test_loader_patterns_not_mutated(self):
+        runner = DiscoverRunner(test_name_patterns=['test_sample'], verbosity=0)
+        tests = [
+            ('test_runner_apps.sample.tests', 1),
+            ('test_runner_apps.sample.tests.Test.test_sample', 1),
+            ('test_runner_apps.sample.empty', 0),
+            ('test_runner_apps.sample.tests_sample.EmptyTestCase', 0),
+        ]
+        for test_labels, tests_count in tests:
+            with self.subTest(test_labels=test_labels):
+                with change_loader_patterns(['UnittestCase1']):
+                    count = runner.build_suite([test_labels]).countTestCases()
+                    self.assertEqual(count, tests_count)
+                    self.assertEqual(runner.test_loader.testNamePatterns, ['UnittestCase1'])
+
+    def test_loader_patterns_not_mutated_when_test_label_is_file_path(self):
+        runner = DiscoverRunner(test_name_patterns=['test_sample'], verbosity=0)
+        with change_cwd('.'), change_loader_patterns(['UnittestCase1']):
+            with self.assertRaises(RuntimeError):
+                runner.build_suite(['test_discover_runner.py'])
+            self.assertEqual(runner.test_loader.testNamePatterns, ['UnittestCase1'])
+
     def test_file_path(self):
         with change_cwd(".."):
             count = DiscoverRunner(verbosity=0).build_suite(