python · vstinner · May 18, 2026
@@ -50,7 +50,7 @@
 default_family = 'AF_INET'
 families = ['AF_INET']
 
-if hasattr(socket, 'AF_UNIX'):
+if hasattr(socket, 'AF_UNIX') and sys.platform != 'cygwin':
     default_family = 'AF_UNIX'
     families += ['AF_UNIX']
 

@@ -324,14 +324,15 @@ def _check_available(self):
                 raise ValueError('forkserver start method not available')
 
     _concrete_contexts = {
-        'fork': ForkContext(),
         'spawn': SpawnContext(),
-        'forkserver': ForkServerContext(),
     }
+    if sys.platform != 'cygwin':
+        _concrete_contexts['fork'] = ForkContext()
+        _concrete_contexts['forkserver'] = ForkServerContext()
     # bpo-33725: running arbitrary code after fork() is no longer reliable
     # on macOS since macOS 10.14 (Mojave). Use spawn by default instead.
     # gh-84559: We changed everyones default to a thread safeish one in 3.14.
-    if reduction.HAVE_SEND_HANDLE and sys.platform != 'darwin':
+    if reduction.HAVE_SEND_HANDLE and sys.platform not in ('darwin', 'cygwin'):
         _default_context = DefaultContext(_concrete_contexts['forkserver'])
     else:
         _default_context = DefaultContext(_concrete_contexts['spawn'])

@@ -142,6 +142,7 @@ def _resource_unlink(name, rtype):
                             'HAVE_BROKEN_SEM_GETVALUE', False)
 
 WIN32 = (sys.platform == "win32")
+ALL_START_METHODS = multiprocessing.get_all_start_methods()
 
 def wait_for_handle(handle, timeout):
     if timeout is not None and timeout < 0.0:
@@ -239,6 +240,8 @@ class TestInternalDecorators(unittest.TestCase):
     """Logic within a test suite that could errantly skip tests? Test it!"""
 
     @support.requires_fork()
+    # Cygwin has fork() but lacks start method 'fork'
+    @unittest.skipUnless('fork' in ALL_START_METHODS, 'need fork start method')
     def test_only_run_in_spawn_testsuite(self):
         if multiprocessing.get_start_method() != "spawn":
             raise unittest.SkipTest("only run in test_multiprocessing_spawn.")
@@ -6104,7 +6107,7 @@ def test_set_get(self):
     def test_get_all_start_methods(self):
         methods = multiprocessing.get_all_start_methods()
         self.assertIn('spawn', methods)
-        if sys.platform == 'win32':
+        if sys.platform in ('win32', 'cygwin'):
             self.assertEqual(methods, ['spawn'])
         elif sys.platform == 'darwin':
             self.assertEqual(methods[0], 'spawn')  # The default is first.
@@ -6138,7 +6141,7 @@ def test_preload_resources(self):
             self.fail("failed spawning forkserver or grandchild")
 
     @warnings_helper.ignore_fork_in_thread_deprecation_warnings()
-    @unittest.skipIf(sys.platform == "win32",
+    @unittest.skipIf(sys.platform in ("win32", "cygwin"),
                      "Only Spawn on windows so no risk of mixing")
     @only_run_in_spawn_testsuite("avoids redundant testing.")
     def test_mixed_startmethod(self):

diff --git a/Lib/test/test_concurrent_futures/test_init.py b/Lib/test/test_concurrent_futures/test_init.py
@@ -120,7 +120,8 @@ def _assert_logged(self, msg):
 create_executor_tests(globals(), FailingInitializerMixin)
 
 
-@unittest.skipIf(sys.platform == "win32", "Resource Tracker doesn't run on Windows")
+@unittest.skipIf(sys.platform in ("win32", "cygwin"),
+                 "Resource Tracker doesn't run on Windows")
 class FailingInitializerResourcesTest(unittest.TestCase):
     """
     Source: https://github.com/python/cpython/issues/104090

diff --git a/Lib/test/test_concurrent_futures/util.py b/Lib/test/test_concurrent_futures/util.py
@@ -101,8 +101,8 @@ def get_context(self):
             _check_system_limits()
         except NotImplementedError:
             self.skipTest("ProcessPoolExecutor unavailable on this system")
-        if sys.platform == "win32":
-            self.skipTest("require unix system")
+        if sys.platform in ("win32", "cygwin"):
+            self.skipTest("require Unix system")
         if support.check_sanitizer(thread=True):
             self.skipTest("TSAN doesn't support threads after fork")
         return super().get_context()
@@ -135,8 +135,8 @@ def get_context(self):
             _check_system_limits()
         except NotImplementedError:
             self.skipTest("ProcessPoolExecutor unavailable on this system")
-        if sys.platform == "win32":
-            self.skipTest("require unix system")
+        if sys.platform in ("win32", "cygwin"):
+            self.skipTest("require Unix system")
         if support.check_sanitizer(thread=True):
             self.skipTest("TSAN doesn't support threads after fork")
         return super().get_context()

@@ -4066,7 +4066,7 @@ def test_config_reject_simple_queue_handler_multiprocessing_context(self):
 
         import multiprocessing
 
-        if support.MS_WINDOWS:
+        if support.MS_WINDOWS or sys.platform == 'cygwin':
             start_methods = ['spawn']
         else:
             start_methods = ['spawn', 'fork', 'forkserver']
@@ -4085,7 +4085,7 @@ def test_config_reject_simple_queue_handler_multiprocessing_context(self):
                                            " assertions in multiprocessing")
     def test_config_queue_handler_multiprocessing_context(self):
         # regression test for gh-121723
-        if support.MS_WINDOWS:
+        if support.MS_WINDOWS or sys.platform == 'cygwin':
             start_methods = ['spawn']
         else:
             start_methods = ['spawn', 'fork', 'forkserver']

@@ -6,7 +6,7 @@
 if support.PGO:
     raise unittest.SkipTest("test is not helpful for PGO")
 
-if sys.platform == "win32":
+if sys.platform in ("win32", "cygwin"):
     raise unittest.SkipTest("fork is not available on Windows")
 
 if sys.platform == 'darwin':

@@ -6,7 +6,7 @@
 if support.PGO:
     raise unittest.SkipTest("test is not helpful for PGO")
 
-if sys.platform == "win32":
+if sys.platform in ("win32", "cygwin"):
     raise unittest.SkipTest("forkserver is not available on Windows")
 
 if not support.has_fork_support: