gh-116322: Add Py_mod_gil module slot (#116882)

This PR adds the ability to enable the GIL if it was disabled at
interpreter startup, and modifies the multi-phase module initialization
path to enable the GIL when loading a module, unless that module's spec
includes a slot indicating it can run safely without the GIL.

PEP 703 called the constant for the slot `Py_mod_gil_not_used`; I went
with `Py_MOD_GIL_NOT_USED` for consistency with gh-104148.

A warning will be issued up to once per interpreter for the first
GIL-using module that is loaded. If `-v` is given, a shorter message
will be printed to stderr every time a GIL-using module is loaded
(including the first one that issues a warning).

Lib/test/test_importlib/extension/_test_nonmodule_cases.py

@@ -0,0 +1,44 @@
+import types
+import unittest
+from test.test_importlib import util
+
+machinery = util.import_importlib('importlib.machinery')
+
+from test.test_importlib.extension.test_loader import MultiPhaseExtensionModuleTests
+
+
+class NonModuleExtensionTests:
+    setUp = MultiPhaseExtensionModuleTests.setUp
+    load_module_by_name = MultiPhaseExtensionModuleTests.load_module_by_name
+
+    def _test_nonmodule(self):
+        # Test returning a non-module object from create works.
+        name = self.name + '_nonmodule'
+        mod = self.load_module_by_name(name)
+        self.assertNotEqual(type(mod), type(unittest))
+        self.assertEqual(mod.three, 3)
+
+    # issue 27782
+    def test_nonmodule_with_methods(self):
+        # Test creating a non-module object with methods defined.
+        name = self.name + '_nonmodule_with_methods'
+        mod = self.load_module_by_name(name)
+        self.assertNotEqual(type(mod), type(unittest))
+        self.assertEqual(mod.three, 3)
+        self.assertEqual(mod.bar(10, 1), 9)
+
+    def test_null_slots(self):
+        # Test that NULL slots aren't a problem.
+        name = self.name + '_null_slots'
+        module = self.load_module_by_name(name)
+        self.assertIsInstance(module, types.ModuleType)
+        self.assertEqual(module.__name__, name)
+
+
+(Frozen_NonModuleExtensionTests,
+ Source_NonModuleExtensionTests
+ ) = util.test_both(NonModuleExtensionTests, machinery=machinery)
+
+
+if __name__ == '__main__':
+    unittest.main()

Lib/test/test_importlib/extension/test_loader.py

@@ -10,7 +10,8 @@ import unittest
 import warnings
 import importlib.util
 import importlib
-from test.support import MISSING_C_DOCSTRINGS
+from test import support
+from test.support import MISSING_C_DOCSTRINGS, script_helper
 
 
 class LoaderTests:
@@ -325,29 +326,6 @@ class MultiPhaseExtensionModuleTests(abc.LoaderTests):
             self.load_module_by_name(name)
         self.assertEqual(cm.exception.name, name)
 
-    def test_nonmodule(self):
-        # Test returning a non-module object from create works.
-        name = self.name + '_nonmodule'
-        mod = self.load_module_by_name(name)
-        self.assertNotEqual(type(mod), type(unittest))
-        self.assertEqual(mod.three, 3)
-
-    # issue 27782
-    def test_nonmodule_with_methods(self):
-        # Test creating a non-module object with methods defined.
-        name = self.name + '_nonmodule_with_methods'
-        mod = self.load_module_by_name(name)
-        self.assertNotEqual(type(mod), type(unittest))
-        self.assertEqual(mod.three, 3)
-        self.assertEqual(mod.bar(10, 1), 9)
-
-    def test_null_slots(self):
-        # Test that NULL slots aren't a problem.
-        name = self.name + '_null_slots'
-        module = self.load_module_by_name(name)
-        self.assertIsInstance(module, types.ModuleType)
-        self.assertEqual(module.__name__, name)
-
     def test_bad_modules(self):
         # Test SystemError is raised for misbehaving extensions.
         for name_base in [
@@ -401,5 +379,14 @@ class MultiPhaseExtensionModuleTests(abc.LoaderTests):
  ) = util.test_both(MultiPhaseExtensionModuleTests, machinery=machinery)
 
 
+class NonModuleExtensionTests(unittest.TestCase):
+    def test_nonmodule_cases(self):
+        # The test cases in this file cause the GIL to be enabled permanently
+        # in free-threaded builds, so they are run in a subprocess to isolate
+        # this effect.
+        script = support.findfile("test_importlib/extension/_test_nonmodule_cases.py")
+        script_helper.run_test_script(script)
+
+
 if __name__ == '__main__':
     unittest.main()