serializer: fix and test for KeyError '$type'

subiquity/common/serialize.py

@@ -194,6 +194,13 @@ class Serializer:
             args = {}
             fields = {field.name: field for field in attr.fields(annotation)}
             for key in value.keys():
+                if key not in fields and key == '$type':
+                    # Union types can contain a '$type' field that is not
+                    # actually one of the keys.  This happens if a object is
+                    # serialized as part of a Union, sent to an API caller,
+                    # then received back on a different endpoint that isn't a
+                    # Union.
+                    continue
                 args.update(self._deserialize_field(
                     fields[key], value[key], path))
             return annotation(**args)

subiquity/common/tests/test_serialization.py

@@ -156,6 +156,19 @@ class TestSerializer(CommonSerializerTests, unittest.TestCase):
             }
         self.assertSerialization(typing.Union[Data, Container], data, expected)
 
+    def test_arbitrary_types_may_have_type_field(self):
+        # The serializer will add a $type field to data elements in a Union.
+        # If we then take that serialized value and fling it back to another
+        # API entrypoint, one that isn't taking a Union, it must be cool with
+        # the excess $type field.
+        data = {
+            '$type': 'Data',
+            'field1': '1',
+            'field2': 2,
+        }
+        expected = Data(field1='1', field2=2)
+        self.assertDeserializesTo(Data, data, expected)
+
 
 class TestCompactSerializer(CommonSerializerTests, unittest.TestCase):