Handle GC constraint failures in a nicer way

Fixes #1739

data/model/repository.py

@@ -2,7 +2,7 @@ import logging
 import random
 
 from datetime import timedelta, datetime
-from peewee import JOIN_LEFT_OUTER, fn, SQL
+from peewee import JOIN_LEFT_OUTER, fn, SQL, IntegrityError
 from cachetools import ttl_cache
 
 from data.model import (DataModelException, tag, db_transaction, storage, permission,
@@ -175,10 +175,20 @@ def garbage_collect_repo(repo):
 
       # Delete any derived images and the images themselves.
       if has_derived:
-        DerivedStorageForImage.delete().where(
-          DerivedStorageForImage.source_image << to_remove).execute()
+        try:
+          (DerivedStorageForImage
+           .delete()
+           .where(DerivedStorageForImage.source_image << to_remove)
+           .execute())
+        except IntegrityError:
+          logger.info('Could not GC derived images %s; will try again soon', to_remove)
+          return
 
-      Image.delete().where(Image.id << to_remove).execute()
+      try:
+        Image.delete().where(Image.id << to_remove).execute()
+      except IntegrityError:
+        logger.info('Could not GC images %s; will try again soon', to_remove)
+        return
 
   if len(to_remove) > 0:
     logger.info('Garbage collecting storage for images: %s', to_remove)