[Resolve #683] Fixing diff command to handle (and not create) invalid…

… templates (#1159)

Some testing on the recent diff command made a couple things evident:
* The way we were generating the placeholder values for sceptre_user_data was not fully safe for rendering templates. A safer approach is to only use alphanumeric values.
* When retrieving a local template summary, if the template is invalid (such as invalid yaml or having invalid characters), we were currently interpreting that as the stack not existing... which wasn't true. This would happen specifically when getting a template summary for the LOCALLY generated template, so that didn't make sense.

sceptre/diffing/stack_differ.py

@@ -327,7 +327,9 @@ def resolve_or_replace(attr, key, value: Resolver):
             try:
                 attr[key] = value.resolve()
             except Exception:
-                attr[key] = self._represent_unresolvable_resolver(value)
+                explicit_resolver_repr = self._represent_unresolvable_resolver(value)
+                only_alphanumeric = ''.join(c for c in explicit_resolver_repr if c.isalnum())
+                attr[key] = only_alphanumeric
 
         # Because the name is mangled, we cannot access the user data normally, so we need to access
         # it directly out of the __dict__.

sceptre/plan/actions.py

@@ -979,8 +979,11 @@ def _get_template_summary(self, **kwargs) -> Optional[dict]:
             )
             return template_summary
         except botocore.exceptions.ClientError as e:
-            # AWS returns a ValidationError if the stack doesn't exist
-            if e.response['Error']['Code'] == 'ValidationError':
+            error_response = e.response['Error']
+            if (
+                error_response['Code'] == 'ValidationError'
+                and 'does not exist' in error_response['Message']
+            ):
                 return None
             raise
 

tests/test_actions.py

@@ -1217,7 +1217,21 @@ def get_template_summary(service, command, kwargs):
         result = self.actions.fetch_local_template_summary()
         assert result == {'template': 'summary'}
 
-    def test_fetch_remote_template_summary__cloudformation_returns_validation_error__returns_none(self):
+    def test_fetch_local_template_summary__cloudformation_returns_validation_error_invalid_stack__raises_it(self):
+        self.actions.connection_manager.call.side_effect = ClientError(
+            {
+                "Error": {
+                    "Code": "ValidationError",
+                    "Message": "Template format error: Resource name {Invalid::Resource} is "
+                               "non alphanumeric.'"
+                }
+            },
+            sentinel.operation
+        )
+        with pytest.raises(ClientError):
+            self.actions.fetch_local_template_summary()
+
+    def test_fetch_remote_template_summary__cloudformation_returns_validation_error_for_no_stack__returns_none(self):
         self.actions.connection_manager.call.side_effect = ClientError(
             {
                 "Error": {

tests/test_diffing/test_stack_differ.py

@@ -511,7 +511,7 @@ def generate():
 
         self.differ.diff(self.actions)
 
-        assert self.sceptre_user_data['unresolvable'] == '{ !Mock(test) }'
+        assert self.sceptre_user_data['unresolvable'] == 'Mocktest'
 
     def test_diff__local_generation_raises_an_error__resolves_resolvable_sceptre_user_data(self):
         has_raised = False