Fix validation in AttributeSerializer

The current implementation of `to_internal_value` doesn't call the base
class method, which is supposed to carry out generic validation of
serializer fields. This means that we essentially don't validate attribute
fields beyond what is done incidentally by database contraints.

Fix it by removing the overridden method entirely, and adding a special
field class for the `values` field.

CHANGELOG.md

@@ -22,6 +22,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - \[SDK\] Ability to create attributes with blank default values
   (<https://github.com/opencv/cvat/pull/6454>)
 - \[SDK\] SDK should not change input data in models (<https://github.com/opencv/cvat/pull/6455>)
+- Server-side validation for attribute specifications
+  (<https://github.com/opencv/cvat/pull/6447>)
 
 ### Security
 - TDB

cvat/apps/engine/serializers.py

@@ -212,30 +212,22 @@ class Meta:
             'last_login': { 'allow_null': True }
         }
 
+class DelimitedStringListField(serializers.ListField):
+    def to_representation(self, value):
+        return super().to_representation(value.split('\n'))
+
+    def to_internal_value(self, data):
+        return '\n'.join(super().to_internal_value(data))
+
 class AttributeSerializer(serializers.ModelSerializer):
-    values = serializers.ListField(allow_empty=True,
+    values = DelimitedStringListField(allow_empty=True,
         child=serializers.CharField(allow_blank=True, max_length=200),
     )
 
     class Meta:
         model = models.AttributeSpec
         fields = ('id', 'name', 'mutable', 'input_type', 'default_value', 'values')
 
-    # pylint: disable=no-self-use
-    def to_internal_value(self, data):
-        attribute = data.copy()
-        attribute['values'] = '\n'.join(data.get('values', []))
-        return attribute
-
-    def to_representation(self, instance):
-        if instance:
-            rep = super().to_representation(instance)
-            rep['values'] = instance.values.split('\n')
-        else:
-            rep = instance
-
-        return rep
-
 class SublabelSerializer(serializers.ModelSerializer):
     id = serializers.IntegerField(required=False)
     attributes = AttributeSerializer(many=True, source='attributespec_set', default=[],