Allow ignoring ECS Immutable fields if they aren't supplied (#463)

* Allow ignoring ECS Immutable fields if they aren't supplied in the first place.

* Bump version

kingpin/actors/aws/ecs.py

@@ -701,7 +701,11 @@ def _execute(self):
 class Service(ECSBaseActor):
     """Register and run a service on ECS.
 
-    This actor will loop indefinitely until the task is complete.
+    This actor will loop indefinitely until the task is complete. If any
+    Service parameters are not supplied, then Amazon supplies the defaults and
+    manages them. If these are immutable in Amazon, then you cannot change them
+    in the ECS Service Definition down in a future update, and Kingpin will
+    error out.
 
     If the service already exists, it is upgraded.
 
@@ -1129,6 +1133,16 @@ def _check_immutable_field_errors(self, old_params, new_params,
             new_field = new_params.get(immutable_field_name)
             old_field = old_params.get(immutable_field_name)
 
+            # If the newly supplied fields are None, then they aren't defined
+            # in the service definition at all. Its OK to ignore them.
+            if new_field is None:
+                continue
+
+            # If the supplied fields are an empty list, treat them also as an
+            # null field and move on.
+            if isinstance(new_field, list) and len(new_field) == 0:
+                continue
+
             if new_field != old_field:
                 has_error = True
                 self.log.error(

kingpin/actors/aws/test/test_ecs.py

@@ -1577,6 +1577,12 @@ def test_same_no_immutable(self):
                    new={'a': 1},
                    immutable=[], expected_error_count=0)
 
+    @testing.gen_test
+    def test_not_set_but_different(self):
+        self._test(old={'a': 1, 'b': [1, 2, 3]},
+                   new={'a': None, 'b': []},
+                   immutable=['a', 'b'], expected_error_count=0)
+
     @testing.gen_test
     def test_different_no_immutable(self):
         self._test(old={'a': 1},
@@ -1681,6 +1687,8 @@ def test_seen_all_two_events(self):
         self.assertEqual(events, [])
 
     def test_timestamp_filtering(self):
+                    # If the supplied fields are an empty list, treat them also
+                    # as an null field and move on.
         events_after = [{
             'id': 0,
             'message': 'message1',

kingpin/version.py

@@ -13,4 +13,4 @@
 # Copyright 2018 Nextdoor.com, Inc
 
 
-__version__ = '0.5.2'
+__version__ = '0.5.3'