ticket 38558: add unit test, deepcopy() only if necessary

salt/modules/pillar.py

@@ -91,10 +91,10 @@ def get(key,
             raise SaltInvocationError(
                 'default must be a dictionary when merge=True'
             )
-        default = copy.deepcopy(default)
         ret = salt.utils.traverse_dict_and_list(pillar_dict, key, {}, delimiter)
         if isinstance(ret, collections.Mapping) and \
                 isinstance(default, collections.Mapping):
+            default = copy.deepcopy(default)
             return salt.utils.dictupdate.update(default, ret)
 
     ret = salt.utils.traverse_dict_and_list(pillar_dict,

tests/unit/modules/pillar_test.py

@@ -54,6 +54,30 @@ class PillarModuleTestCase(TestCase):
     def test_ls(self):
         self.assertEqual(pillarmod.ls(), ['a', 'b'])
 
+    def test_pillar_get_default_merge_regression_38558(self):
+        """Test for pillar.get(key=..., default=..., merge=True)
+
+        Do not update the ``default`` value when using ``merge=True``.
+
+        See: https://github.com/saltstack/salt/issues/38558
+        """
+
+        pillarmod.__opts__ = {}
+        pillarmod.__pillar__ = {'l1': {'l2': {'l3': 42}}}
+
+        res = pillarmod.get(key='l1')
+        self.assertEqual({'l2': {'l3': 42}}, res)
+
+        default = {'l2': {'l3': 43}}
+
+        res = pillarmod.get(key='l1', default=default)
+        self.assertEqual({'l2': {'l3': 42}}, res)
+        self.assertEqual({'l2': {'l3': 43}}, default)
+
+        res = pillarmod.get(key='l1', default=default, merge=True)
+        self.assertEqual({'l2': {'l3': 42}}, res)
+        self.assertEqual({'l2': {'l3': 43}}, default)
+
 
 # gracinet: not sure this is really useful, but other test modules have this as well
 if __name__ == '__main__':