Fixed #36083 -- Ran system checks in ParallelTestSuite workers

Workers created by ParallelTestSuite were not running system checks.
In general this is fine, but system checks can have side effects that
the tests expect.

This patch runs system checks inside of _init_worker, which is only
called by ParallelTestSuite.

django/test/runner.py

@@ -417,7 +417,7 @@ def _init_worker(
     used_aliases=None,
 ):
     """
-    Switch to databases dedicated to this worker.
+    Switch to databases dedicated to this worker and run system checks.
 
     This helper lives at module-level because of the multiprocessing module's
     requirements.
@@ -438,6 +438,7 @@ def _init_worker(
             process_setup(*process_setup_args)
         django.setup()
         setup_test_environment(debug=debug_mode)
+        call_command("check", verbosity=0, databases=used_aliases)
 
     db_aliases = used_aliases if used_aliases is not None else connections
     for alias in db_aliases:

tests/test_runner/test_parallel.py

@@ -1,12 +1,15 @@
+import ctypes
+import multiprocessing
 import pickle
 import sys
 import unittest
+from unittest import mock
 from unittest.case import TestCase
 from unittest.result import TestResult
 from unittest.suite import TestSuite, _ErrorHolder
 
 from django.test import SimpleTestCase
-from django.test.runner import ParallelTestSuite, RemoteTestResult
+from django.test.runner import ParallelTestSuite, RemoteTestResult, _init_worker
 
 try:
     import tblib.pickling_support
@@ -213,6 +216,47 @@ class RemoteTestResultTest(SimpleTestCase):
         self.assertEqual(result.collectedDurations, [("None", 2.3)])
 
 
+@mock.patch("django.test.runner.connections")
+@mock.patch("django.test.runner.call_command")
+@mock.patch("django.test.runner.setup_test_environment")
+@mock.patch("django.setup")
+@mock.patch("multiprocessing.get_start_method")
+class InitWorkerTests(SimpleTestCase):
+    def test_calls_setup_functions_when_start_method_is_spawn(self, *mocks):
+        mocks[0].return_value = "spawn"
+
+        counter = multiprocessing.Value(ctypes.c_int, 0)
+
+        _init_worker(counter, initial_settings={"default": {}}, serialized_contents={})
+
+        mocks[1].assert_called_once()
+        mocks[2].assert_called_once()
+        mocks[3].assert_called_once_with(
+            "check", verbosity=mock.ANY, databases=mock.ANY
+        )
+
+    def test_does_not_call_setup_functions_when_start_method_is_not_spawn(self, *mocks):
+        mocks[0].return_value = "fork"
+
+        _init_worker(multiprocessing.Value(ctypes.c_int, 0))
+
+        mocks[1].assert_not_called()
+        mocks[2].assert_not_called()
+        mocks[3].assert_not_called()
+
+    def test_sets_up_worker_connection_with_incremented_worker_id(self, *mocks):
+        mocks[0].return_value = "fork"
+
+        connection_mock = mock.MagicMock()
+
+        mocks[4].__iter__.return_value = {"default": connection_mock}
+        mocks[4].__getitem__.return_value = connection_mock
+
+        _init_worker(multiprocessing.Value(ctypes.c_int, 0))
+
+        connection_mock.creation.setup_worker_connection.assert_called_once_with(1)
+
+
 class ParallelTestSuiteTest(SimpleTestCase):
     @unittest.skipUnless(tblib is not None, "requires tblib to be installed")
     def test_handle_add_error_before_first_test(self):