Create replacement resource atomically

Use a single transaction to create the replacement resource and set it as
the replaced_by link in the old resource. Also, ensure that no other
traversal has taken a lock on the old resource before we modify it.

If we end up bailing out and not creating a replacement or sending an RPC
message to check it, make sure we retrigger any new traversal.

Change-Id: I23db4f06a4060f3d26a78f7b26700de426f355e3
Closes-Bug: #1727128

1 files changed, 15 insertions, 0 deletions

diff --git a/heat/objects/resource.py b/heat/objects/resource.py
index 1f2e2bbdf..67cc24a0c 100644
--- a/heat/objects/resource.py
+++ b/heat/objects/resource.py
@@ -200,6 +200,21 @@ class Resource(
                                    db_api.resource_create(context, values))
 
     @classmethod
+    def replacement(cls, context,
+                    existing_res_id, existing_res_values,
+                    new_res_values,
+                    atomic_key=0, expected_engine_id=None):
+        replacement = db_api.resource_create_replacement(context,
+                                                         existing_res_id,
+                                                         existing_res_values,
+                                                         new_res_values,
+                                                         atomic_key,
+                                                         expected_engine_id)
+        if replacement is None:
+            return None
+        return cls._from_db_object(cls(context), context, replacement)
+
+    @classmethod
     def delete(cls, context, resource_id):
         db_api.resource_delete(context, resource_id)