Refs #27734 -- Prevented creation of more parallel workers than TestCases.

The parallel test runner uses multiple workers to distribute the
workload. These workers are assigned a worker ID using a globally
incremented variable, which determines what test database to connect
to. When the worker ID surpasses the test database IDs Django will
crash.

This reduce likelihood of crashing parallel tests because
ParallelTestSuite will no longer create more workers than TestCases.

It won't eliminate the problem completely though because there are
other circumstances in which new workers can be created which can then
be assigned an "illegal" worker ID.

django/test/runner.py

@@ -404,8 +404,8 @@ class ParallelTestSuite(unittest.TestSuite):
     run_subsuite = _run_subsuite
     runner_class = RemoteTestRunner
 
-    def __init__(self, suite, processes, failfast=False, buffer=False):
-        self.subsuites = partition_suite_by_case(suite)
+    def __init__(self, subsuites, processes, failfast=False, buffer=False):
+        self.subsuites = subsuites
         self.processes = processes
         self.failfast = failfast
         self.buffer = buffer
@@ -685,22 +685,17 @@ class DiscoverRunner:
         suite = self.test_suite(all_tests)
 
         if self.parallel > 1:
-            parallel_suite = self.parallel_test_suite(
-                suite,
-                self.parallel,
-                self.failfast,
-                self.buffer,
-            )
-
+            subsuites = partition_suite_by_case(suite)
             # Since tests are distributed across processes on a per-TestCase
             # basis, there's no need for more processes than TestCases.
-            parallel_units = len(parallel_suite.subsuites)
-            self.parallel = min(self.parallel, parallel_units)
-
-            # If there's only one TestCase, parallelization isn't needed.
-            if self.parallel > 1:
-                suite = parallel_suite
-
+            processes = min(self.parallel, len(subsuites))
+            if processes > 1:
+                suite = self.parallel_test_suite(
+                    subsuites,
+                    processes,
+                    self.failfast,
+                    self.buffer,
+                )
         return suite
 
     def setup_databases(self, **kwargs):

tests/test_runner/test_discover_runner.py

@@ -348,6 +348,12 @@ class DiscoverRunnerTests(SimpleTestCase):
         with self.assertRaisesMessage(ValueError, msg):
             DiscoverRunner(pdb=True, parallel=2)
 
+    def test_number_of_parallel_workers(self):
+        """Number of processes doesn't exceed the number of TestCases."""
+        runner = DiscoverRunner(parallel=5, verbosity=0)
+        suite = runner.build_suite(['test_runner_apps.tagged'])
+        self.assertEqual(suite.processes, len(suite.subsuites))
+
     def test_buffer_mode_test_pass(self):
         runner = DiscoverRunner(buffer=True, verbose=0)
         with captured_stdout() as stdout, captured_stderr() as stderr: