Fix 500 error on parallel same-object creates

fixes: pulp#4183

CHANGES/4183.bugfix

@@ -0,0 +1 @@
+Fixed a 500 server error when two concurrent requests try to create an object with the same unique fields.

pulpcore/app/serializers/base.py

@@ -9,7 +9,7 @@
 
 from django.conf import settings
 from django.core.validators import URLValidator
-from django.core.exceptions import ObjectDoesNotExist
+from django.core.exceptions import ObjectDoesNotExist, ValidationError
 from django.urls.exceptions import NoReverseMatch
 from django.db import IntegrityError
 from django.db.models import Model
@@ -439,6 +439,16 @@ def _validate_relative_path(self, path):
 
         return path
 
+    def save(self, **kwargs):
+        try:
+            super().save(**kwargs)
+        except IntegrityError as e:
+            # Concurrent request got by the unique validator on the serializer
+            if "unique" in str(e):
+                # Run validation again to properly raise an unique-ValidationError
+                self.run_validation(self.initial_data)
+            raise e
+
     def __init_subclass__(cls, **kwargs):
         """Set default attributes in subclasses.