Handle IntegrityError in tag update code

Fixes https://sentry.io/coreos/backend-production/issues/173470565/events/4938537230/

data/model/__init__.py

@@ -96,6 +96,9 @@ class ServiceNameInvalid(DataModelException):
 class TagAlreadyCreatedException(DataModelException):
   pass
 
+class StaleTagException(DataModelException):
+  pass
+
 
 class TooManyLoginAttemptsException(Exception):
   def __init__(self, message, retry_after):

data/model/tag.py

@@ -4,7 +4,7 @@ from uuid import uuid4
 
 from peewee import IntegrityError
 from data.model import (image, db_transaction, DataModelException, _basequery,
-                        InvalidManifestException, TagAlreadyCreatedException)
+                        InvalidManifestException, TagAlreadyCreatedException, StaleTagException)
 from data.database import (RepositoryTag, Repository, Image, ImageStorage, Namespace, TagManifest,
                            RepositoryNotification, Label, TagManifestLabel, get_epoch_timestamp,
                            db_for_update)
@@ -92,6 +92,9 @@ def create_or_update_tag(namespace_name, repository_name, tag_name, tag_docker_i
       tag.save()
     except RepositoryTag.DoesNotExist:
       pass
+    except IntegrityError:
+      msg = 'Tag with name %s was stale when we tried to update it; Please retry the push'
+      raise StaleTagException(msg % tag_name)
 
     try:
       image_obj = Image.get(Image.docker_image_id == tag_docker_image_id, Image.repository == repo)