gh-71052: Enable test_concurrent_futures on platforms that lack multiprocessing (gh-115917)

Enable test_concurrent_futures on platforms that support threading but not multiprocessing.

Lib/test/test_concurrent_futures/__init__.py

@@ -3,8 +3,6 @@ import unittest
 from test import support
 from test.support import import_helper
 
-# Skip tests if _multiprocessing wasn't built.
-import_helper.import_module('_multiprocessing')
 
 if support.check_sanitizer(address=True, memory=True):
     # gh-90791: Skip the test because it is too slow when Python is built

Lib/test/test_concurrent_futures/test_init.py

@@ -5,6 +5,8 @@ import time
 import unittest
 import sys
 from concurrent.futures._base import BrokenExecutor
+from concurrent.futures.process import _check_system_limits
+
 from logging.handlers import QueueHandler
 
 from test import support
@@ -117,6 +119,11 @@ class FailingInitializerResourcesTest(unittest.TestCase):
     """
 
     def _test(self, test_class):
+        try:
+            _check_system_limits()
+        except NotImplementedError:
+            self.skipTest("ProcessPoolExecutor unavailable on this system")
+
         runner = unittest.TextTestRunner()
         runner.run(test_class('test_initializer'))
 

Lib/test/test_concurrent_futures/util.py

@@ -136,6 +136,12 @@ def create_executor_tests(remote_globals, mixin, bases=(BaseTestCase,),
 
 
 def setup_module():
-    unittest.addModuleCleanup(multiprocessing.util._cleanup_tests)
+    try:
+        _check_system_limits()
+    except NotImplementedError:
+        pass
+    else:
+        unittest.addModuleCleanup(multiprocessing.util._cleanup_tests)
+
     thread_info = threading_helper.threading_setup()
     unittest.addModuleCleanup(threading_helper.threading_cleanup, *thread_info)