Fixed #27430 -- Added -b/--buffer option to DiscoverRunner.

tests/test_runner/test_discover_runner.py

@@ -6,7 +6,7 @@ from unittest import TestSuite, TextTestRunner, defaultTestLoader, skipUnless
 from django.db import connections
 from django.test import SimpleTestCase
 from django.test.runner import DiscoverRunner
-from django.test.utils import captured_stdout
+from django.test.utils import captured_stderr, captured_stdout
 from django.utils.version import PY37
 
 
@@ -269,6 +269,34 @@ class DiscoverRunnerTests(SimpleTestCase):
         with self.assertRaisesMessage(ValueError, msg):
             DiscoverRunner(pdb=True, parallel=2)
 
+    def test_buffer_with_parallel(self):
+        msg = (
+            'You cannot use -b/--buffer with parallel tests; pass '
+            '--parallel=1 to use it.'
+        )
+        with self.assertRaisesMessage(ValueError, msg):
+            DiscoverRunner(buffer=True, parallel=2)
+
+    def test_buffer_mode_test_pass(self):
+        runner = DiscoverRunner(buffer=True, verbose=0)
+        with captured_stdout() as stdout, captured_stderr() as stderr:
+            suite = runner.build_suite([
+                'test_runner_apps.buffer.tests_buffer.WriteToStdoutStderrTestCase.test_pass',
+            ])
+            runner.run_suite(suite)
+        self.assertNotIn('Write to stderr.', stderr.getvalue())
+        self.assertNotIn('Write to stdout.', stdout.getvalue())
+
+    def test_buffer_mode_test_fail(self):
+        runner = DiscoverRunner(buffer=True, verbose=0)
+        with captured_stdout() as stdout, captured_stderr() as stderr:
+            suite = runner.build_suite([
+                'test_runner_apps.buffer.tests_buffer.WriteToStdoutStderrTestCase.test_fail',
+            ])
+            runner.run_suite(suite)
+        self.assertIn('Write to stderr.', stderr.getvalue())
+        self.assertIn('Write to stdout.', stdout.getvalue())
+
 
 class DiscoverRunnerGetDatabasesTests(SimpleTestCase):
     runner = DiscoverRunner(verbosity=2)