Fixed #36531 -- Added forkserver support to parallel test runner.

Author: felixxm

django/db/backends/sqlite3/creation.py

@@ -62,7 +62,7 @@ def get_test_db_clone_settings(self, suffix):
         start_method = multiprocessing.get_start_method()
         if start_method == "fork":
             return orig_settings_dict
-        if start_method == "spawn":
+        if start_method in {"forkserver", "spawn"}:
             return {
                 **orig_settings_dict,
                 "NAME": f"{self.connection.alias}_{suffix}.sqlite3",
@@ -99,9 +99,9 @@ def _clone_test_db(self, suffix, verbosity, keepdb=False):
                 self.log("Got an error cloning the test database: %s" % e)
                 sys.exit(2)
         # Forking automatically makes a copy of an in-memory database.
-        # Spawn requires migrating to disk which will be re-opened in
-        # setup_worker_connection.
-        elif multiprocessing.get_start_method() == "spawn":
+        # Forkserver and spawn require migrating to disk which will be
+        # re-opened in setup_worker_connection.
+        elif multiprocessing.get_start_method() in {"forkserver", "spawn"}:
             ondisk_db = sqlite3.connect(target_database_name, uri=True)
             self.connection.connection.backup(ondisk_db)
             ondisk_db.close()
@@ -137,7 +137,7 @@ def setup_worker_connection(self, _worker_id):
             # Update settings_dict in place.
             self.connection.settings_dict.update(settings_dict)
             self.connection.close()
-        elif start_method == "spawn":
+        elif start_method in {"forkserver", "spawn"}:
             alias = self.connection.alias
             connection_str = (
                 f"file:memorydb_{alias}_{_worker_id}?mode=memory&cache=shared"

django/test/runner.py

@@ -404,8 +404,9 @@ def get_max_test_processes():
     The maximum number of test processes when using the --parallel option.
     """
     # The current implementation of the parallel test runner requires
-    # multiprocessing to start subprocesses with fork() or spawn().
-    if multiprocessing.get_start_method() not in {"fork", "spawn"}:
+    # multiprocessing to start subprocesses with fork(), forkserver(), or
+    # spawn().
+    if multiprocessing.get_start_method() not in {"fork", "spawn", "forkserver"}:
         return 1
     try:
         return int(os.environ["DJANGO_TEST_PROCESSES"])
@@ -450,9 +451,12 @@ def _init_worker(
         counter.value += 1
         _worker_id = counter.value
 
-    start_method = multiprocessing.get_start_method()
+    is_spawn_or_forkserver = multiprocessing.get_start_method() in {
+        "forkserver",
+        "spawn",
+    }
 
-    if start_method == "spawn":
+    if is_spawn_or_forkserver:
         if process_setup and callable(process_setup):
             if process_setup_args is None:
                 process_setup_args = ()
@@ -463,7 +467,7 @@ def _init_worker(
     db_aliases = used_aliases if used_aliases is not None else connections
     for alias in db_aliases:
         connection = connections[alias]
-        if start_method == "spawn":
+        if is_spawn_or_forkserver:
             # Restore initial settings in spawned processes.
             connection.settings_dict.update(initial_settings[alias])
             if value := serialized_contents.get(alias):
@@ -606,7 +610,7 @@ def __iter__(self):
         return iter(self.subsuites)
 
     def initialize_suite(self):
-        if multiprocessing.get_start_method() == "spawn":
+        if multiprocessing.get_start_method() in {"forkserver", "spawn"}:
             self.initial_settings = {
                 alias: connections[alias].settings_dict for alias in connections
             }

docs/releases/6.0.txt

@@ -348,7 +348,8 @@ Templates
 Tests
 ~~~~~
 
-* ...
+* The :class:`.DiscoverRunner` now supports parallel test execution on systems
+  using the ``forkserver`` :mod:`multiprocessing` start method.
 
 URLs
 ~~~~

tests/backends/sqlite/test_creation.py

@@ -36,8 +36,8 @@ def test_get_test_db_clone_settings_name(self):
                 clone_settings_dict = creation_class.get_test_db_clone_settings("1")
                 self.assertEqual(clone_settings_dict["NAME"], expected_clone_name)
 
-    @mock.patch.object(multiprocessing, "get_start_method", return_value="forkserver")
+    @mock.patch.object(multiprocessing, "get_start_method", return_value="unsupported")
     def test_get_test_db_clone_settings_not_supported(self, *mocked_objects):
-        msg = "Cloning with start method 'forkserver' is not supported."
+        msg = "Cloning with start method 'unsupported' is not supported."
         with self.assertRaisesMessage(NotSupportedError, msg):
             connection.creation.get_test_db_clone_settings(1)

tests/test_runner/test_discover_runner.py

@@ -97,6 +97,16 @@ def test_get_max_test_processes_forkserver(
         mocked_cpu_count,
     ):
         mocked_get_start_method.return_value = "forkserver"
+        self.assertEqual(get_max_test_processes(), 12)
+        with mock.patch.dict(os.environ, {"DJANGO_TEST_PROCESSES": "7"}):
+            self.assertEqual(get_max_test_processes(), 7)
+
+    def test_get_max_test_processes_other(
+        self,
+        mocked_get_start_method,
+        mocked_cpu_count,
+    ):
+        mocked_get_start_method.return_value = "other"
         self.assertEqual(get_max_test_processes(), 1)
         with mock.patch.dict(os.environ, {"DJANGO_TEST_PROCESSES": "7"}):
             self.assertEqual(get_max_test_processes(), 1)