Ignore 404 on swift object delete

We occasionally see cases where the swift backend gets out of sync
with the object list.  In that case, issuing a delete to an object
will return a 404.  It should be fine to ignore that case.

Change-Id: I3c6cc8f1fdfd5ad286cf85aa4657cbaf4499e1a1

zuul_registry/swift.py

@@ -152,9 +152,18 @@ class SwiftDriver(storageutils.StorageDriver):
         return size, ret.iter_content(chunk_size=SWIFT_CHUNK_SIZE)
 
     def delete_object(self, path):
-        retry_function(
-            lambda: self.conn.session.delete(
-                self.get_url(path)))
+        try:
+            retry_function(
+                lambda: self.conn.session.delete(
+                    self.get_url(path)))
+        except keystoneauth1.exceptions.http.NotFound:
+            # We have seen instances where the swift object list is
+            # apparently not in sync with the backend.  If we get a
+            # 404 when deleting an object, it will typically no longer
+            # show up in object lists after that, so we can safely
+            # ignore the error.  Log it here so we have some
+            # visibility.
+            self.log.debug("NotFound error when deleting %s", path)
 
     def move_object(self, src_path, dst_path, uuid=None):
         dst = os.path.join(self.container_name, dst_path)