Fix mutable default argument in multiprocessing.dummy.DummyProcess

Change the default value of the 'kwargs' parameter in
DummyProcess.__init__() from {} to None to avoid mutable default
argument issues. This makes DummyProcess consistent with
threading.Thread and threading.Timer which already use this pattern.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Reported by: Dmitrii Chuprov <cheese@altlinux.org>

Signed-off-by: Denis Sergeev <zeff@altlinux.org>

Author: zeff-ir

Lib/multiprocessing/dummy/__init__.py

@@ -33,7 +33,9 @@
 
 class DummyProcess(threading.Thread):
 
-    def __init__(self, group=None, target=None, name=None, args=(), kwargs={}):
+    def __init__(self, group=None, target=None, name=None, args=(), kwargs=None):
+        if kwargs is None:
+            kwargs = {}
         threading.Thread.__init__(self, group, target, name, args, kwargs)
         self._pid = None
         self._children = weakref.WeakKeyDictionary()

Lib/test/_test_multiprocessing.py

@@ -7347,3 +7347,15 @@ def test_forkpty(self):
         res = assert_python_failure("-c", code, PYTHONWARNINGS='error')
         self.assertIn(b'DeprecationWarning', res.err)
         self.assertIn(b'is multi-threaded, use of forkpty() may lead to deadlocks in the child', res.err)
+
+class _TestDummyProcessKwargs(BaseTestCase):
+    ALLOWED_TYPES = ('threads',)
+
+    def test_dummyprocess_kwargs_default_not_shared(self):
+        p1 = self.Process()
+        p2 = self.Process()
+        self.assertIsInstance(p1._kwargs, dict)
+        self.assertIsInstance(p2._kwargs, dict)
+        self.assertIsNot(p1._kwargs, p2._kwargs)
+        p1._kwargs['sentinel'] = 42
+        self.assertNotIn('sentinel', p2._kwargs)