[6.0.x] Fixed #36491 -- Fixed crash in ParallelTestRunner with --buffer.

Thanks Javier Buzzi and Adam Johnson for reviews.

Co-authored-by: Simon Charette <charette.s@gmail.com>

Backport of be581ff473e8ade6365975db2df602f295a4cb4b from main.

django/test/runner.py

@@ -566,6 +566,9 @@ class ParallelTestSuite(unittest.TestSuite):
             (self.runner_class, index, subsuite, self.failfast, self.buffer)
             for index, subsuite in enumerate(self.subsuites)
         ]
+        # Don't buffer in the main process to avoid error propagation issues.
+        result.buffer = False
+
         test_results = pool.imap_unordered(self.run_subsuite.__func__, args)
 
         while True:

tests/test_runner/test_parallel.py

@@ -282,3 +282,27 @@ class ParallelTestSuiteTest(SimpleTestCase):
 
         self.assertEqual(len(result.errors), 0)
         self.assertEqual(len(result.failures), 0)
+
+    def test_buffer_mode_reports_setupclass_failure(self):
+        test = SampleErrorTest("dummy_test")
+        remote_result = RemoteTestResult()
+        suite = TestSuite([test])
+        suite.run(remote_result)
+
+        pts = ParallelTestSuite([suite], processes=2, buffer=True)
+        pts.serialized_aliases = set()
+        test_result = TestResult()
+        test_result.buffer = True
+
+        with unittest.mock.patch("multiprocessing.Pool") as mock_pool:
+
+            def fake_next(*args, **kwargs):
+                test_result.shouldStop = True
+                return (0, remote_result.events)
+
+            mock_pool.return_value.imap_unordered.return_value = unittest.mock.Mock(
+                next=fake_next
+            )
+            pts.run(test_result)
+
+        self.assertIn("ValueError: woops", test_result.errors[0][1])