Eliminate a race condition on instance deletes.

- Add constraint and equality conditions to nova.db[.sqlalchemy].api
- Use host constraints to ensure the compute api doesn't simply delete
  an instance from the database that a compute manager has already
  started to run.

This race condition is associated with bug #998117

Change-Id: Id74192d3e66bea073327342f57ce0f26987efd2d

1 files changed, 9 insertions, 3 deletions

diff --git a/nova/compute/api.py b/nova/compute/api.py
index 9f96b8e6b..a3bd93bac 100644
--- a/nova/compute/api.py
+++ b/nova/compute/api.py
@@ -980,9 +980,15 @@ class API(base.Base):
         try:
             if not instance['host']:
                 # Just update database, nothing else we can do
-                result = self.db.instance_destroy(context, instance['id'])
-                QUOTAS.commit(context, reservations)
-                return result
+                constraint = self.db.constraint(host=self.db.equal_any(host))
+                try:
+                    result = self.db.instance_destroy(
+                            context, instance['uuid'], constraint)
+                    QUOTAS.commit(context, reservations)
+                    return result
+                except exception.ConstraintNotMet:
+                    # Refresh to get new host information
+                    instance = self.get(context, instance['uuid'])
 
             self.update(context,
                         instance,