Prevent altering certain fields on Instance

- Prevents changing hostname, listener_port, or node_type for instances
that already exist
- API default node_type is execution
- API default node_state is installed

awx/api/serializers.py

@@ -4884,12 +4884,12 @@ class InstanceSerializer(BaseSerializer):
         read_only_fields = ('ip_address', 'uuid', 'version')
         fields = (
             'id',
+            'hostname',
             'type',
             'url',
             'related',
             'summary_fields',
             'uuid',
-            'hostname',
             'created',
             'modified',
             'last_seen',
@@ -4913,6 +4913,7 @@ class InstanceSerializer(BaseSerializer):
             'ip_address',
             'listener_port',
         )
+        extra_kwargs = {'node_type': {'default': 'execution'}, 'node_state': {'default': 'installed'}}
 
     def get_related(self, obj):
         res = super(InstanceSerializer, self).get_related(obj)
@@ -4974,6 +4975,18 @@ class InstanceSerializer(BaseSerializer):
 
         return value
 
+    def validate_hostname(self, value):
+        if self.instance and self.instance.hostname != value:
+            raise serializers.ValidationError("Cannot change hostname.")
+
+        return value
+
+    def validate_listener_port(self, value):
+        if self.instance and self.instance.listener_port != value:
+            raise serializers.ValidationError("Cannot change listener port.")
+
+        return value
+
 
 class InstanceHealthCheckSerializer(BaseSerializer):
     class Meta: