[4.1.x] Fixed #33719 -- Fixed test command crash when running in parallel.

Thanks Pēteris Caune for the report.

Regression in 3b3f38b3b09b0f2373e51406ecb8c9c45d36aebc.
Backport of 41c4cb253c137edf5a96b7408ea55d57d6e0602a from main

2 files changed, 44 insertions, 7 deletions

diff --git a/tests/runtests.py b/tests/runtests.py
index e5adb902c3..bf8c79b231 100755
--- a/tests/runtests.py
+++ b/tests/runtests.py
@@ -11,7 +11,6 @@ import subprocess
 import sys
 import tempfile
 import warnings
-from functools import partial
 from pathlib import Path
 
 try:
@@ -26,7 +25,7 @@ else:
     from django.core.exceptions import ImproperlyConfigured
     from django.db import connection, connections
     from django.test import TestCase, TransactionTestCase
-    from django.test.runner import _init_worker, get_max_test_processes, parallel_type
+    from django.test.runner import get_max_test_processes, parallel_type
     from django.test.selenium import SeleniumTestCaseBase
     from django.test.utils import NullTimeKeeper, TimeKeeper, get_runner
     from django.utils.deprecation import RemovedInDjango50Warning
@@ -401,11 +400,8 @@ def django_tests(
             parallel = 1
 
     TestRunner = get_runner(settings)
-    TestRunner.parallel_test_suite.init_worker = partial(
-        _init_worker,
-        process_setup=setup_run_tests,
-        process_setup_args=process_setup_args,
-    )
+    TestRunner.parallel_test_suite.process_setup = setup_run_tests
+    TestRunner.parallel_test_suite.process_setup_args = process_setup_args
     test_runner = TestRunner(
         verbosity=verbosity,
         interactive=interactive,
diff --git a/tests/test_runner/tests.py b/tests/test_runner/tests.py
index c022a6fb86..b3c7cc5a55 100644
--- a/tests/test_runner/tests.py
+++ b/tests/test_runner/tests.py
@@ -19,6 +19,7 @@ from django.test import SimpleTestCase, TransactionTestCase, skipUnlessDBFeature
 from django.test.runner import (
     DiscoverRunner,
     Shuffler,
+    _init_worker,
     reorder_test_bin,
     reorder_tests,
     shuffle_tests,
@@ -684,6 +685,46 @@ class NoInitializeSuiteTestRunnerTests(SimpleTestCase):
             )
 
 
+class TestRunnerInitializerTests(SimpleTestCase):
+
+    # Raise an exception to don't actually run tests.
+    @mock.patch.object(
+        multiprocessing, "Pool", side_effect=Exception("multiprocessing.Pool()")
+    )
+    def test_no_initialize_suite_test_runner(self, mocked_pool):
+        class StubTestRunner(DiscoverRunner):
+            def setup_test_environment(self, **kwargs):
+                return
+
+            def setup_databases(self, **kwargs):
+                return
+
+            def run_checks(self, databases):
+                return
+
+            def teardown_databases(self, old_config, **kwargs):
+                return
+
+            def teardown_test_environment(self, **kwargs):
+                return
+
+            def run_suite(self, suite, **kwargs):
+                kwargs = self.get_test_runner_kwargs()
+                runner = self.test_runner(**kwargs)
+                return runner.run(suite)
+
+        runner = StubTestRunner(verbosity=0, interactive=False, parallel=2)
+        with self.assertRaisesMessage(Exception, "multiprocessing.Pool()"):
+            runner.run_tests(
+                [
+                    "test_runner_apps.sample.tests_sample.TestDjangoTestCase",
+                    "test_runner_apps.simple.tests",
+                ]
+            )
+        # Initializer must be a function.
+        self.assertIs(mocked_pool.call_args.kwargs["initializer"], _init_worker)
+
+
 class Ticket17477RegressionTests(AdminScriptTestCase):
     def setUp(self):
         super().setUp()